How Shober Organs Work

How Schober Organs Work 4th EDITION

by: Richard H. Dorf Transcribed by: Frank Towle, Schober Orphan

THE SCHOBER ORGAN CORPORATION 43 WEST 61ST STREET * NEW YORK, NEW YOR, 10023

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Table of Contents How.........................................................................................................................1 Schober....................................................................................................................1 Organs Work...........................................................................................................1 4th EDITION..........................................................................................................1 by: Richard H. Dorf................................................................................................1 Table of Contents...........................................................................................................2 Introduction.............................................................................................................5 Chapter 1 SCHOBER ORGAN PRINCIPLES...........................................................................6 Tone Generation......................................................................................................7 Keying.....................................................................................................................9 Couplers................................................................................................................12 Tone Coloring.......................................................................................................14 Woodwind Tone....................................................................................................18 The Schober Vibrato.............................................................................................18 Chapter 2 THE SCHOBER RECITAL ORGAN......................................................................22 Tone Generators....................................................................................................22 Key Switching.......................................................................................................24 Registration Circuits.............................................................................................24 Stop Filters............................................................................................................26 Preamplification and Controls..............................................................................28 Regulated Power Supply.......................................................................................30 Combination Action..............................................................................................31 Chapter 3 THE SCHOBER CONSOLETTE II ORGAN.........................................................33 Tone Generation....................................................................................................34 Manual Keying......................................................................................................35 Bus Amplification.................................................................................................35 Stop Filters............................................................................................................36 Preamplification and Control................................................................................36 Chapter 4 THE SCHOBER THEATRE ORGAN.....................................................................37 Main Tone Generators...........................................................................................37 Pedal Generator.....................................................................................................42 Keying and Coupling............................................................................................42 Bus Amplifiers......................................................................................................44 Stop Filters............................................................................................................45

Preamplification, Vibrato, and Control.................................................................................................................47 Vibrato Circuits.....................................................................................................48 Sound Production..................................................................................................49 Power Supply........................................................................................................49 Chapter 5 THE SCHOBER SPINET ORGAN.........................................................................51 Maine Tone Generators.........................................................................................52 Pedal Generator.....................................................................................................53 Manual Keying......................................................................................................54 Bus Amplification.................................................................................................54 Stop Filters............................................................................................................55 Preamplifier-Vibrato Unit.....................................................................................55 Chapter 6 AMPLIFIERS AND SPEAKER SYSTEMS............................................................58 TR-2 Power Amplifier..........................................................................................59 The LSS-10A Speaker System........................................................................................................................60 The LSS-100 Speaker System........................................................................................................................61 Built-In Speaker Systems......................................................................................62 Leslie Organ Speaker............................................................................................62 Headphones...........................................................................................................63 Chapter 7 THE SCHOBER REVERBATAPETM UNIT.........................................................64 A Bird’s-Eye View................................................................................................65 Tape Drive.............................................................................................................66 Circuitry................................................................................................................67 Bias & Erase..........................................................................................................68 Playback Amplifiers..............................................................................................69 Mixing and Output................................................................................................69 System Noise.........................................................................................................69 Control System......................................................................................................70 Reverbatape Installation........................................................................................72 Chapter 8 THE SCHOBER PERCUSSION GROUP...............................................................73 What it Does..........................................................................................................73 Key Switches.........................................................................................................74 Keyer Circuits.......................................................................................................74 Keyer Outputs.......................................................................................................77 Piano and Shortener Control.................................................................................77 Keying and Repeating...........................................................................................79

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Bus Amplifiers......................................................................................................82 Gates and Filters....................................................................................................83 Output Connection................................................................................................83 Power Supply........................................................................................................83 Chapter 9 THE MIXER-COMPRESSOR.................................................................................84 Audio Channel......................................................................................................86 Compression..........................................................................................................87 Adjustment............................................................................................................88 THE SCHOBER ORGAN CORPORATION.............................................................89

Introduction Of the millions upon millions of people who drive cars in the world, only a very small number have the slightest idea how an internal combustion engine works – and many don’t even know what the term means. The large majority of these unknowledgeable people are operating a highly complex and expensive piece of precision machinery with complete satisfaction.

In the same way, you can play an organ – and even assemble one yourself from kits – without being able to tell an electron from a piece of green cheese. Playing is a purely musical matter and whether or not you know what’s under the hood of your organ makes absolutely no difference. And, while you will become very familiar with the organ’s innards when you assemble it, Schober instructions tell you exactly what to do and every move to make in layman’s terms which require no prior knowledge or experience. So you’ll install the components and make the connections correctly and easily without any necessity to know what the

component is there for or what would happen if you didn’t make the connection.

What we are doing, in essence, is starting off this booklet by telling you that you don’t need to read it, a strange thing for any author to do, but appropriate in this case. The thousands of words that follow are the nuts and bolts story of Schober Organs – the detailed technical facts that tell how Schober Organs work, that spread out before you the thoughts that were in the design engineers’ minds and exactly how those thoughts resulted in a series of fine musical instruments. You do not need to know any of this to assemble and play a Schober organ with complete ease and success.

If you are an electronic technician or engineer and you are equipped to understand all the technicalities we shall discuss, and you are the specific person whose interest and curiosity this booklet was written to satisfy. If you know a little about electronics, there may be parts you will not absorb thoroughly; and if you know nothing of tech-

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nical matters, most (though not all) of the material will probably leave you cold.

Since the inception of our company many years ago, it has been firm Schober policy never to put you in the position of having to buy a pig in a poke.

We believe you can buy more intelligently when you have full information, and we have been dedicated to presenting all available information to you, factually and completely. Our catalog tries to give all the facts the usual organ purchaser wants and needs. This booklet, the latest up-to-date version of a series which began the day Schober

opened its doors, rounds out the Schober information program with complete data on the theory and design of Schober products.

The information here is necessarily confined to Schober Organs and there is a minimum of background material on electronic organs in general, to keep the booklet to a feasible size. Technical people who are interested in exploring the field more deeply, both as to principles and for data on other kinds of organs, may well profit by reading the writer’s, “Electronic Musical Instruments,” which has become the standard text on electronic organs. It is available in many libraries and can be purchased from local bookstores or from Schober (see the end of the Schober catalog).

If you are totally or largely unfamiliar with organs, I strongly suggest you read “What Is an Organ?,” which will tell you a good deal about organs from the functional and operational standpoint. (See the end of the Schober catalog.)

Knowing what an organ should do, you will be well equipped to understand the following material on how it is done.

- Richard H. Dorf

Chapter 1 SCHOBER ORGAN PRINCIPLES Electronic organs are unlike most other commercial products in that, while they are all intended to be musical instruments and have keys and pedals, there are very few other similarities between the organs of one manufactures and those of another. All automobiles are alike in operating principles – internal combustion engines employing the same fuel in the same way, four wheels with steering linkages and differentials, brakes, electrical

systems, and tires of some kind. Understanding one make of car enables one to understand all the others almost perfectly.

Several different sets of principles, however, can govern organ design.

Different circuitry and philosophies of operation are employed in the vital tasks of producing the initial tone and in altering it to afford the different voices. Indeed, the voices themselves, although thy may have similar names,

vary enormously from one organ to another in sound. Control functions of various sorts may be present or not, and are carried out differently. And so on.

In describing Schober Organs, therefore, we must begin with a brief trip through the organs, from the source of tones to the loudspeaker system, describing on the way the general nature of the mechanisms used to produce the result. As is usual with any one manufacturer, all Schober Organs operate on the same general principles, which we shall describe in this chapter. All of them use the same types of amplifiers and speaker systems, which are discussed in Chapter 6. Aside from these similarities, each model has more or less different detailed circuitry; and that you will find in the chapters on the individual Schober models.

Tone Generation Every organ must contain within itself at least one set of tone generators. These provide a signal for each of the fundamental pitches which are to be available to the player. They are analogous to the strings of a piano, which may be considered as the tone generators of the piano – or, of course, the pipes of a pipe organ, each one of which is tuned to just one pitch and is blown whenever a tone of that pitch is desired. And, just as in the piano or pipe organ, the pitches for which generators are provided are those of the tempered scale common to all keyboard instruments.

It is a matter of the designer’s choice how many generators and pitches are provided. The piano has a little over seven octaves of pitches available. A pipe organ may have six to nine octaves. A fullsize electronic organ capable of playing the full pipe-organ literature usually provides seven octaves of tone, while a “home-size” electronic organ may have as few as four to keep costs down. Obviously, the larger the number of available pitches, the wider the scope of playing. All Schober Organs provide seven octaves except the Theatre Organ, which supplies eight.

Tone generators can be designed in either of two principal forms. Obviously, separate oscillators can be employed. That is, if 84 pitches are desired

(representing seven octaves – 12 notes to the octave, including both black and white key notes), 84 separate and distinct oscillators can be used, each individually tuned to the right frequency. Schober generators use a different principle, known as the frequency-divider system.

In this scheme, outlined in Fig. 1-1, the 84 notes are divided into 12 groups of seven notes each.

Each group is called a Tone Generator. Each Tone Generator, usually on a separate printed-circuit board, produces seven notes of the same name. One generator, for instance, produces all the C’s, and other all the C#’s and so on. Each generator contains only one self-controlled oscillator which can be fine-tuned to exactly the right pitch. This master oscillator operates at the highest desired pitch for the note. In the Recital Organ, for instance, the master oscillator for the A generator operates at 3520 cycles per second, the frequency of the highest A note available in the instrument. This master oscillator is a highly stable L-C type, but it is tunable over a range of abut two notes either way so that it can be set exactly to meet the usual tuning standard of A = 440 or any different standard the owner may wish to use.

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Following the master oscillator, each Tone Generator contains a string of frequency dividers, each of which accepts tone of one frequency and produces an output tone at half that frequency. In the A generator, for example, the 3520-cps tone is fed to the first frequency divider, which then produces a second tone at half of 3520, or 1760, the frequency of note A an octave lower. The 1760-cps

ator is always exactly octavely related to the pitch to which the master oscillator has been tuned.

The advantages of this system are at least twofold. First, since there are only 12 frequencydetermining oscillators, one for each generator, C through B, each can employ the best components and the most stable possible design without imposing an economic burden. Second, when the

tone, in turn, is fed to a second divider, which produces a new tone of 1760/2, or 880. And in the same way, additional dividers produce the remaining desired A notes, the last one at 55 cps, the lowest A of the organ.

None of the frequency dividers are tuned or even (except in the Spinet) adjustable. Each of them will divide in half any note fed to it (within a reasonable range), not just a note of exactly the design frequency. In this way, the entire sequence of pitches is locked to the master oscillator, and every one of the notes produced by a given gener-

organ’s tuning is to be touched up, as it usually is about once a year, it is not necessary to tune each of 84 notes. Instead a single adjustment to the D# oscillator, for example, tunes all D# notes in the organ, and so on, so that a total of 12 adjustments suffice for the entire instrument. And with only 12 frequency governing elements for the whole organ rather than 84, the organ can never present a situation of complete out-of-tune chaos, since at least all octavely related notes must always be in perfect tune In Fig. 1-1, we have shown the outline of the A

generator from the Recital Organ, with the output

frequencies from top to bottom. This is, of course, bare bones, for in the actual generator, as you will read in Chapter 2, the circuitry contains not only frequency dividers, but also means for isolating the master oscillator and for obtaining output signals of sawtooth waveform. In any generator, all seven frequencies are being produced continuously as long as the organ is turned on.

Generators for other Schober models differ in actual circuitry and in fact the fact that the lowest frequency may not be produced by the main generator. It may instead be produced by a “pedal generator,” which is a divider stage whose input is obtained from the lowest available main generator and switched by a pedal, whereupon this “floating” frequency divider produces tone and octave lower.

Such an economy is feasible without musical sacrifice because, especially in popular music, which the Theatre, Consolette II, and Spinet Organs are principally used for, only one pedal note is ever played at a time, and only that one need be divided in half. The Theatre Organ, however, contains a seven-octave main generator, like the Recital (on which you can play 32 pedal notes at a time if you have 32 feet), plus a pedal generator, for a total of eight octaves of available pitches. On the Theatre Organ, the notes provided by the main A generator

are an octave higher than shown in Fig. 1-1, going from 7040 cps down to 110.

Keying Each of the two keyboards (or manuals, to use organ terminology) is in reality a group of electrical switches “dressed up” to look, feel, and move like typical playing keys. The same is true of almost all pipe-organ keyboards as well. However, Schober keyboards are made of metal and plastic, rather than of wood.

Fig 1-2 is a top view of a Consolette II keyboard.

The action itself is made entirely of heavy steel to avoid the warpage and changes in dimensions which occur with wood or plastic. A keyboard is a precision mechanism, and the steel construction assures that the precision will be maintained indefinitely. The white and black keys, which are the only visible parts in the finished organ, are actually merely easily removable caps of plastic, and in Fig. 1-2 a few of them have been removed to show the real keys, which are steel levers operating on knife-edge pivots.

Fig 1-3 shows part of the underside of a keyboard. At the lower right can be seen the carefully calibrated steel springs which counterbalance the keys. Along the underside appear the switches themselves. These are tightly wound springs clad with gold and set into a long printed circuit attached to the keyboard’s steel chassis base

with resilient mounts. There are three contact springs for each key, and the upper ends of all three 9

springs in each group are held by a plastic actuator bar. When a key is pressed, the bar is brought forward (toward the top left corner of the photo) and each of the gold springs is wrapped around one of three gold bus bars which run the length of the keyboard. Under the printed circuit, copper-foil “wiring” connects each of the gold spring contacts to a numbered terminal, to which wires are connected from the tone generator outputs.

In the Consolette II each keyboard produces three pitch registers. The switches and generator outputs are connected in such a way that when you press a key – for example middle A – the gold spring which comes in contact with the center gold bus-bar places on that bus the normal tone of middle A at 440 cycles. This is known as the 8-foot (abbreviated 8’) bus, because the largest pipe of a rank of pipes which produces these “normal” or

unison pitches is 8 feet long. At 8’ pitch any key produces the same pitch as the corresponding key would produce on a piano.

At the same time, the gold spring nearest the rear of the keyboard transfers to its gold bus the A note which sounds one octave higher than middle A. This is the 4′ bus, and 4′ tone is always 1 octave higher that 8′ tone. And the spring nearest the front of the keyboard transfers to its gold bus an A note one octave below middle A; this is the 16′ bus.

In this way, pressing any key causes three tones to be connected to the three gold busses – and, of course, pressing several keys at a time causes the same action with all the notes concerned. So if you connected an amplifier directly to each of the busses in turn, you would hear whatever selection you play in each of three different, octavely related pitch registers. Or if you connected the amplifier to two or all three busses at the same time, you would actually hear all three pitch registers at the same time – amounting to as much as three times as many pitches as keys being held down!

Fig. 1-4 presents this idea in schematic form.

The diagram shows the three transverse busses, 16′, 8′, and 4′, and fire groups of triple switch fingers, one for each of the five G keys of a 5-octave (61note) keyboard such as is used for all Schober Organs except the Spinet. The seven octavely related G tones available from a G generator are shown as numbers, along with the colors of the wires used for connection, which correspond with the resistor color code to keep things easily in mind.

Note that each switch finger is connected to the appropriate tone source through an isolating resistor, so that two or more fingers can take output from the same generator divider without causing any loading of the generator or interaction. The view, by the way, is from the back of the organ, with the bass end of the keyboard at your right.

Tone 1 is the lowest G generated and tone 7 the highest.

The same scheme and the same keyboard construction are used for all Schober Organs, except that the number of pitch registers varies, and with it the number of transverse busses and the number of gold contact fingers for each key.

An important element of key-switch design in any organ is elimination of clicks and pops. If you will connect an audio sine-wave signal generator to an amplifier through a switch, you will find that opening and closing the switch causes transient noises. Fig. 1-5 shows what creates the undesirable transient.

Assuming that, for instance, a sine wave is being switched from its source to the grid of a tube or the base of a transistor, the switch may be closed during some part of the time when the wave is not at its zero axis. (The statistical probability of this is extremely high, since the wave passes through zero only at two brief instants.) Grid or base voltage then changes from its quiescent value instantaneously to some other value, and of course plate or collector current does the same. The almost infinitely steep rise time of this sudden change is in effect a portion of a wave containing an infinity of high-frequency components. These components are heard as a click. In other organ designs it has been

of the sawtooth itself, and the filters treat it the same. To say it another way, each wave of a good sawtooth has a built-in key click. When you add another by closing a switch, neither the circuitry nor the ear can tell the difference.

The mechanical design of the keyboard and switches leas to extremely long and reliable life.

Contact material is always a vital consideration when audio is keyed; only the best materials will maintain good enough surface conductivity to make and break positively over many years. The usual silver contacts found in pipe organs or in ordinary relays will usually acquire a coating of sulfide with weeks and cause intermittent, scratchy contact or none at all. In Schober Organs all switches have gold or palladium contacts. Gold and palladium do not tarnish and the cladding is heavy enough so that it will not wear off in the foreseeable future. A special advantage of the use of coil springs as switch fingers is that thy cannot take a set and lose their springiness, as straight-wire contacts can; and

found necessary or desirable to place capacitive low-pass filters across the switching system, use gradual resistive keying, or to key plate or collector voltage on oscillator to eliminate or reduce the clicks.

The secret of Schober’s clickless keying is simply that the generator outputs are sawtooth waves with very fast flyback. The filter system is designed to take care of this – it can take advantage of the high-frequency components in imparting brilliance to stops that require it and can roll off the highs for less brilliant stops. When the sawtooth tone is keyed at some point in its rise, the vertical rise added by the keying is just like the vertical part

not amount of abuse other than a tight bend at the point of attachment or an endwise pull can harm them. The reason is, of course, that in a spring of this type, any individual tiny section of the wire undergoes only the most minute movement and flexure. The steel key action itself is virtually indestructible by anything short of a steamroller, and the plastic key caps can easily be removed should they be accidentally chipped or stained.

Each keyboard is furnished completely assembled and permanently adjusted.

The constructor connects wires to a terminal board within the organ and thence to the keyboards, one wire per available generator pitch.

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On all Schober Organs except the Recital, the lowest octave of the pedal 16′ register is produced by the pedal generator, a “floating” frequency divider. Fig. 1-6 shows the connections of the pedal switches for the Spinet. There are 13 pedals, each with a double-throw switch. With no pedals pressed (switches as shown in the diagram), the line to the pedal generator is grounded. When any pedal is pressed, the lower spring of its flat-blade switch (right contact in the diagram) carries the lowest tone from a main generator to the pedal generator, which thereupon produces a tone one octave lower.

Because of the switching scheme, the pedals are effectively interlocked, so that only one generator tone at a time can go to the pedal divider even if two or more pedals are held down. If two tones

went to the divider the result would be hash, since the divider would not be able to make up its mind which frequency to divide. The interlocking also helps the inexpert player who often has difficulty pressing only one pedal at a time.

Fig 1-7 shows the Spinet pedals, a rugged assembly of hardwood and heavy steel. The sharps, like all Schober pedal sharps, are of heavy plastic, which is black all the way through and thus shows no wear. The Consolette II has 17 pedals of exactly this same construction, while the Theatre Organ has 25 pedals of the large-organ type, the front ends of

which press down on flat-blade switches mounted near the console floor. All of these work in the same way electrically. The recital pedals actuate specials switches made of gold springs like those on the manuals. They draw directly on the main generators for all pitches, as some classic music requires more than one pedal note at a time.

Couplers Each organ stop or voice has not only its own peculiar timbre or tone color, but also sounds in a particular one of the available pitch registers. The three pitch registers produced, for example, in the Consolette II key-switching system on the transverse gold busses (as described in the previous section) are used to produce voices of three different pitch registers. If tone taken from the 4′ bus is passed through voicing filters (see next section) and thence to the amplifier and speakers, the corresponding voices are at 4′ pitch; and of course the same is true for as many other pitch registers as may be built into the switching system, and for each of the two manuals. Thus, any stop is produced from signal obtained from the bus of a particular manual at a particular pitch register.

Looking at it from the other end, any voice normally sounds at only one pitch register and as the result of playing the keys of only one keyboard (it may be a manual or pedal keyboard). However, the organ acquires much enhanced flexibility if it can be made possible to sound a stop in pitch registers or from a manual other than its own. The organist might, for example, wish to play with both hands on the lower manual with an ensemble of stops, in which he might wish to include some of those normally assigned to the upper manual. Or if using the upper manual, alone or along with the lower, he might wish to have all stops sound both at their normal pitches and also an octave higher, to add brilliance to the sound.

These functions are made feasible by couplers, which to the organist operate just like those of a pipe organ, but are designed electronically quite differently. From the engineering standpoint, all couplers work in the same way and can be explained with the aid of Fig 1-8.

The diagram assumes an organ with two manual keyboards, each of which has three keying busses and thus three pitch registers, 4′, 8′, and 16′. These six keying buses appear at the upper left of the drawing and are the sources for keyed tones. Ignore the triple switch labeled SUO for the moment and just consider it closed as shown. Each of the six tone sources is connected through a resistor to the input of a corresponding bus amplifier. The bus amplifiers are simple voltage amplifiers and may not even amplify; their principal function is isolation and impedance conversion. Each bus amplifier feeds tone to a group of voicing filters which, when switched on, sound at the pitch register carried by the bus amplifier. (We are ignoring the pedal keyboard for simplicity.)

Normally, as shown, then, the Swell (upper) keyboard’s 4′ keying bus, carrying tones selected by the organist’s playing at 4′ pitch – one octave above normal piano pitch – feeds the 4′ Swell bus amplifier, which sends tone to the 4′ Swell stops.

Each of the six groups of stops is fed tone in just the same way. Everything is normal.

Now suppose we wish to play the Great (lower) manual, hearing any Great stops we have selected

as usual but also hearing any Swell stops which are switched on. In other words, we wish to make all the organ’s manual stops available on the Great manual not just those which normally sound when

Great keys are played. To do this we switch on the Swell to Great 8′ coupler.

From the organist’s standpoint, the term Swell to Great means “Swell stops to Great keys.” The terminology is hard to keep track of in the abstract, though this little mnemonic line, “ --- stops to --keys,” is helpful. But on the organ, a coupler control tablet is always found among the stops of the manual whose keys produce the sound. In other words, the Swell to Great coupler we are discussing is controlled by a tablet in the Great stop group; so the organist knows that the tablet has an effect only on what he hears when playing the Great keys.

To make Swell stops sound when Great keys are pressed means in electrical terms that the signals appearing on the Great keying busses must go not only to the Great bus amplifies, as usual, but also to the Swell bus amplifiers. The SG8 switch shown in Fig. 1-8 takes care of this when it is closed. The uppermost of its three gangs switches Great 4′ tone to the 4′ Swell bus amplifier; the center switch brings Great 8′ tone to the Swell 8′ amplifier; and the lower contact connects Great 16′ tone to the Swell 16′ amplifier. Now, when Great keys are played, the tones produced on the Great keying busses feed the bus amplifiers and voicing circuits of the Great division, as usual, and also those of the Swell division. As a result, both sets of stops are heard even though only the Great manual is played.

Notice that all sops continue to sound at their normal pitches, 4′, 8′, and 16′. This is why the 8′ figure is added to the name of the coupler. It does not mean that only 8′ stops are coupled; it means that no change in the normal pitch register of any stop takes place. If the coupler were labeled Swell to Great 4′ for example, then all Swell stops heard when playing Great keys would be an octave higher than normal. This could easily be accomplished by feeding the 8′ Swell bus amplifier with tone from the Great 4′ keying bus, and so. It should be obvious that all sorts of finagling is possible and many different kinds of couplers can be had.

The coupler we have just discussed is an intermanual coupler because it transfers voices between keyboards. A second type is the intramanual coupler, which works only on one

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keyboard. The Swell To Swell 4′ coupler labeled SS4 in Fig. 1-8 is an example.

The purpose of the Swell to Swell 4′ is to make the Swell stops sound at the usual pitches in the normal manner when the Swell keyboard is played but also to sound an octave higher than normal.

With this coupler on, any key and stop produce two tones simultaneously – the normal one determined by the pitch register of the stop, plus one an octave higher.

The switching which does this follows exactly the same logic as that of the first coupler we described. The three Swell keying busses remain connected to the three Swell bus amplifiers. In addition, when the SS4 switch is closed, 4′ Swell tone goes to the 8′ Swell amplifier and stops; and 8′ Swell tone goes to the 16′ Swell amplifier and stops.

Ideally, there should also be 2′ tone to switch to the 4′ stops. However, 2′ tone is not provided, so the 4′ stops do not produce the extra octave tone even with the SS4 coupler on. This is why only a 2-gang switch is needed.

Here again, various kinds of intramanual stops can be envisioned for each manual. Swell To Swell 16′, Great to Great 4′, and Great To Great 16′ are about the limit possible for this organ. But other models having more than three pitch registers for each manual widen the choices. The actual final choices are a matter of designer’s preference.

Standards do not exist and every organ has a different complement of couplers.

From the design angle a couple of points should be noted. First, back-coupling must be avoided. For example, looking at the SS4 switch and assuming it closed, Swell 4′ tone goes to the Swell 8′ bus amplifier. But the Swell 8′ tone already connected to the bus amplifier must not get back to the 4′ bus amplifier through the switch. This can be taken care of by impedance selection.

All resistors shown are of the same value and they are for isolation. They are large compared to both the impedance of the bus amplifier inputs and the terminations for the keying busses (not shown).

In passing from a keying bus to an amplifier, through either a coupler switch or permanent connection, the signal is attenuated many decibels

by the voltage-divider action of the resistor shown, as the series leg, and the amplifier input impedance, as the shunt leg. In trying to get back to another keying bus from an amplifier input through a switch, the signal is attenuated again enormously by the isolating resistor as the series leg and the lowvalue keying bus termination as the shunt leg of another divider. Any back-coupling, therefore, goes through two large attenuations, while a forwardcoupled signal goes through only one.

Second, signals permanently connected to a bus amplifier must not be affected by additional ones switched in by a coupler, as would happen if the coupling loaded down the amplifier input changing its impedance appreciably. This problem is avoided by the fact that the resistors are large compared to the amplifier input impedance, so that any reasonable number of them switched across the amplifier have no audible effect.

One more coupler remains to be described – the simplest of them all. It is the Swell Unison Off, shown as switch SUO. It simply interrupts the normal connections. When it is opened (it is normally closed, as shown) and the SS4 coupler is on (switches closed), only the added higher octave sounds through each stop, while the normal pitch is removed. The effect, therefore, is to raise the pitch register of each stop (except the 4′ stops, which are silenced) by an octave, rather than to hear it at both its normal and octave pitches.

The couplers of the Recital and Consolette II Organs work just as described. Those of the Theatre Organ employ transistor-diode switching rather than multi-gang audio switches, but this difference is superficial and the scheme is actually just the same as in Fig. 1-8.

Tone Coloring Among the ways in which organs vary is the method of producing from the generated waveforms the different voices or stops. In the pipe organ a separate complete rank of anything from 61 to 85 pipes is provided for each voice, the pipes of each rank being so proportioned and shaped as to yield the particular timbre required. This duplication of tone sources is not required in electronics, because

it is possible to have a single rank of generators and to modify the basic waveforms as desired to produce any number of different kinds of sounds, which may imitate the sounds of pipes and orchestral instruments or may be entirely new. It is even entirely feasible to “split up” the outputs of a single rank of generators and produce simultaneously as many different modified voicing characteristics as one wishes. This is one of the principal reasons for the existence of the electronic organ – its economy of cost and space.

The basic tones generated in Schober Organs are of sawtooth shape. However, they are converted to spikes for use in string voices, and they are modified in many ways to produce various voices, as discussed below.

We have said that pure tones like the sine wave (Fig. 1-9C) are too dull for musical interest. But it is equally true that sawtooth and spike tones are too raucous and brilliant for practical use. So every instrument and pipe includes a means of modifying these basic sounds. In fact, you could hear the basic sounds only if you removed the body from a read, brass, or string instrument; the results would be very unpleasant. For it is the body of the instrument which modifies the basic tones in a particular way to produce the tone which is characteristic of the instrument.

An instrument body is an acoustical filter. It allows certain of the overtones or harmonics to come through, and reduces the strengths of others.

The action of this filter depends on the material of which the instrument is made – its softness and resiliency – the shape of the bore (conical or cylindrical or a combination of the two) and the general shape, especially in certain instruments the shape and size of the bell at the end. All these factors determine which of the overtones of a particular note are allowed to come through

strongly, softly, or otherwise. And since every instrument is built differently (and each kind of organ pipe shaped differently), the tone color or timbre of each instrument is different.

One fact about this filtering is very important.

The filter characteristics of any one instrument do not depend on which note is being played. If, for example, a certain instrument body tends to emphasize tones around 1,000 cycles per second, it will emphasize the fundamental pitch of a 1,000cycle tone, the second harmonic of a 500-cycle tone, the third harmonic of a 300-cycle tone, and so on. This means that the recognizable sound of a given instrument does not depend on which harmonics have what strength – for example a prominent third and sixth harmonic at all pitches – but instead on which part of the audible spectrum is prominent. Such emphasized parts of the spectrum are known as the formants of the instrument. They “form” the nature of the tone.

In Schober Organs, these is a selection of electrical filters, which have the same effects on electronic oscillations as instrument formants have on air oscillations. An electrical filter can be designed to correspond to any acoustic filter. The tone produced by any voice of a Schober Organ can therefore be made to correspond almost exactly with that produced by almost any orchestral instrument or pipe. A simple switching system in the organ, operated by the stop tablets, enables the player to select one or many voices, simply by switching the desired imitative filters.

The possible numbers of types of filters (and corresponding tone colors) in pipes and instruments – and in Schober Organs – is very large. Some emphasize particular parts of the audio spectrum.

Where the emphasis is comparatively great over a small part of the spectrum, the sounds produced are those of reed instruments and reed pipes. Various emphasis frequencies and degrees of emphasis produce the sounds of different reed instruments.

There are low-pass filters, which gradually reduce the volume of the higher overtones; in general these are flute sounds. There are high-pass filters which reduce the volume of the lower frequencies; these are usually string sounds. And there is an infinite

15

variety of combinations of low-pass, high-pass, and resonance to produce the complex sounds of any imaginable pipes or instruments – or even create new sounds.

In fact, one of the fascinating facets of formanttype design is that an amateur, with only a slight knowledge of electronics, can produce almost any kind of tone color simply by experimenting with the values of the resistors and capacitors employed in the stop filters. To take advantage of this possibility, the Schober Recital Organ is designed with plug-in voicing filters. The owner can himself design any number of additional or modified stops and simply plug them in when wanted. This is a unique Schober Library Of Stops™ feature, for which a special kit is available containing a large variety of components and plug-in boards for experimental use.

In each Schober Organ, a bus amplifier stage is provided for each pitch register of each keyboard, as outlined in the section on couplers. All the stop filters of one pitch register and keyboards are connected to a single bus amplifier, which is their source of tone.

Fig. 1-10 show four typical stop filters to illustrate how they work. Each filter begins with an input resistor R1, whose value is high enough so that when the inputs of all filters of one pitch register and keyboard are connected in common to the output of a bus amplifier, the combined impedance will not load the amplifier stage. These input resistors also play a part in determining the volume level of the tone emerging from the filter,

through the capacitance of an open switch; and these is no change in loading on either bus amplifier or the common filter output line when the switch is operated, so the switch does not affect the volumes of other stops.

The filters shown are typical of those employed for the four families of organ tone, though within each family many variations are made in both values and configurations to obtain different voices.

and in setting the characteristics of the filter.

Following each input resistor a wire is run to the table-operated switch with which the organist turns the stop on and off. With a stop off (tablet up) the arrow shown is connected to ground, shorting out the filter. When the stop is desired, the ground short is removed. This type of shunt switching has two advantages over a not uncommon system which has a normally open switch in series with the filter output. It kills any possibility of tone leaking

The flute filter is a severe low-pass. The diapason filter, imitating the basic sound of open organ pipes, has less severe low-pass action and also deliberately introduces a measured amount of second harmonic by having an extra input (with large resistor) from the bus amplifier of pitch register an octave higher.

The diapason shown, for instance is nominally an 8′ stop; a certain amount of 4′ tone is added, and this duplicates the characteristic of diapason pipes. (The Greek prefix di in diapason indicates two or double

17

and almost undoubtedly refers to the prominent octave component, often called generally in musicology the double.) The reed filter contains an L-C tuned circuit to produce a relatively strong emphasis or formant at some part of the spectrum, and it may have a series resistor to reduce the Q of the coil so that the emphasis band will not be undesirably narrow or too severe. And the string filter has a series capacitor to taper off the lower harmonics, plus a small shunt capacitor to lop off the extreme high harmonics and prevent too buzzy a tone.

In the usual Schober scheme, the outputs of all filters associated with one keyboard are connected together and through a balance switch to the Preamplifier-Vibrato Unit. The balance switch (present on Theatre and Consolette II Organs) is a simple resistor network designed to bring together all organ signals at a preselected normal volume relationship with the switch knob at center, or to emphasize the volume of the filters of either keyboard and cut down that of the other in the two side positions.

Woodwind Tone One type of tone color in both organs and orchestral instruments is characterized by a sort of “hollow” tone, which is obtained by virtual elimination of even harmonics. Stopped organ pipes, such as stopped flutes are typical; and among instruments of the orchestra the clarinet is the most obvious example. Schober Organs contain stops of this kind and the tone fed to the filters for those stops is obtained by a special combination of pitch registers which produces what we call a woodwind tone.

The waveshape characteristic of a stopped or woodwind tone containing no even harmonics is symmetry. A symmetrical wave contains no even harmonics. Fig. 1-11 shows how a symmetrical wave (square in this case) is derived from sawtooth waves.

The trick requires two sawtooth waves separated by one octave. Since there are always octavely related keying busses in the organ, tones with this characteristic are always available. The diagram

assumes that we wish to obtain a square wave at 16′ pitch.

Looking first at the left drawing, we have first (solid line) a 16′ sawtooth wave at original amplitude, obtained from the output of a bus amplifier. With it we combine 8′ sawtooth (dashed line) which we have reduced to 50% of the amplitude of the 16′ wave, and which we have also inverted in phase by passing it through an additional amplifier. When these two components are combined linearly, we obtain the resultant in the right drawing (as you can prove if you are versed in making graphical constructions), which is a square – symmetrical – wave. This tone used for the inputs of selected stop filters, responds to filtering of various kinds a well as does sawtooth, but the result always has a hollow, woody quality. In the actual organ, the sawtooth waves do not have the linearity depicted in the drawing and the conditions are not so idealized. Nonetheless, the effect is as complete as necessary. It is interesting for the experimenter to make up a circuit which does this job and then vary the resistance which controls the mixing proportions. The nulling effect on the even harmonics is astonishingly obvious and complete.

The Schober Vibrato Each Schober Organ contains a PreamplifierVibrato Unit, whose functions are (a) to collect all organ signals from all keyboards and other sources, (b) to amplify the collected signals to the voltage necessary to feed a power amplifier, (c) to provide an output with low enough impedance to make possible a lossless line of any desired length between organ and amplifier, (d) to provide a way of inserting electrically the volume-controlling effects of one or more swell shoes (foot-operated volume controls), and (e) to insert vibrato when and in the quantity desired by the organist.

The amplification, output impedance, signal collection, and volume-control functions will be abundantly clear when you read the descriptions in the individual organ chapters. The vibrato, however, is produced by a unique method with significant special advantages and merits a special explanation.

Vibrato is the slight rhythmic variation of pitch which the orchestral-instrument player produces in various ways (for instance, the violinist does it by moving his fingers at a rate of 4 to 6 times per second, alternately slightly lengthening and shortening the string). Vibrato lends a certain kind of warmth and movement to a tone. However, it is not appropriate for all kinds of music, and different degrees of intensity are needed for different occasions.

In most electronic organs vibrato is provided by modulating the master oscillator (in frequencydivider systems) or each of the oscillators (in individual-oscillator organs) with a low-frequency signal of 4 to 7 cycles per second. This causes the oscillators to change pitch smoothly and regularly over a small range about the center frequency. For most of these systems three disadvantages exist.

First, almost any oscillator whose frequency can thus be varied in response to an external signal must be designed for less than maximum stability, usually be the selection of the time constant and/or tuned-circuit Q. Second, at least in frequencydivider organs, the fundamentals of all notes are “vibrated” by about the same percentage and in both frequency-divider and individual-oscillator organs all overtones or harmonics of each note have the same percentage frequency swing as the fundamental. The subjective aural effect of large vibrato on high harmonics is a sort of squeal.

A third and highly important disadvantage in frequency-divider systems is that there is no way of having vibrato on the manuals without having it on pedal notes as well. The tremulant never operates on pip-organ pedals, even in theatres, for the simple reason that low-pitched notes with vibrato sound

“sick,” possibly because the audio frequency is not sufficiently higher than the modulation frequency.

The latter is indicated by the observable fact that when an orchestral double bass player has occasion to use vibrato, his rate is very much slower than that used for higher-pitched instruments.

It is obvious, too, that where vibrato is applied to the master oscillators of frequency dividers, no such things are possible as vibrato on one manual only or vibrato for the organ stops without vibrato on percussion tones.

For all these reasons – but principally because it simply sounds better –Schober Organs have always employed a system of adding vibrato in the preamplifier section rather than at the generators.

This trick of taking tones of fixed pitches and varying their frequencies is performed by phasemodulating them. A continuing change of phase is equivalent to a change of frequency; and indeed most FM transmitters operate on this principle. The apparent amount of frequency change is proportional to the degree of phase change and the velocity of phase change. The whole idea is in essence just the same as the well known Doppler effect typified by the old example of the train whistle which, emitted by a moving train, appears to the stationary observer to change pitch.

Phase shifters are, of course, old hat, and the circuit of Fig. 1-12 is the classic one. Varying the value of either the resistor or the capacitor changes the phase of the output signal, if the capacitive reactance is equal to or greater than the resistance at the frequencies involved. However, for the frequencies affected amplitude changes with frequency and so does degree of phase shift; the circuit acts as a high-pass filter with an attenuation of 6 db per octave. Such a circuit is obviously unsuitable of an instrument which must pass the entire audio band of something like nine octaves, including harmonics.

The circuit of Fig. 1-13, however, does not suffer from these disadvantages. The input signal is in push-pull form – two signals of equal amplitude and opposite phase with respect to the center point.

(Such a signal is easily provided by a centertapped transformer winding or a phase splitter.) With connections as shown, variation of either impedance

19

component causes changes in relative output phase over a maximum possible angle of about 175 degrees. (The circuit of Fig. 1-12 – with its disadvantages – is capable of less than 90 degrees.)

While the absolute phases of outputs of different frequencies with respect to either half of the input signal are different, the phase of a signal of any frequency within a wide band can be varied over a considerable range with respect to its resting phase.

Most important, the circuit is nonfrequencydiscriminating in amplitude terms and does not act as a filter.

The action of the circuit can be seen superficially with the aid of the theory of extremes.

If the resistor value is reduced to zero, the output is taken directly from GEN 2 and it has the phase of that half of the input. If the capacitor is at zero reactance, the output phase is somewhere between the possible extremes. A vector analysis shows a swinging vector without change of length. At frequencies where the change in reactance-resistance relationship begins to become less significant, phase change is reduced, though amplitude is unaffected.

Thus a set of values can be chosen which will give maximum phase change (vibrato) around the center keyboard frequencies, with progressively smaller amounts at the much higher and lower frequencies.

This reduces vibrato at the low keyboard frequencies and in the higher harmonics where it would be unpleasant.

To reduce this basic circuit to practice for vibrato purposes requires two main changes. First, the signal from the stop filters must be converted to a push-pull signal. And second, one of the elements in the phase-shift network must be made variable electronically at a selected rate.

The practical circuitry in the Schober transistor organs is depicted in Fig. 1-14. The transistor stage is the familiar split-load phase splitter so often used in audio amplifiers for converting a single-ended signal to feed a push-pull output stage. The emitterground and collector-B- resistors are identical.

Signals at emitter and collector are of identical amplitude and 180 degrees out of phase, so that these two points correspond electrically to the top output of GEN 1 and the lower output line of GEN

2 in Fig. 1-13. Since ground and B- are essentially the same for audio due to the low impedance of the power supply, either one (ground in practice) may be considered to be the common point connecting the two generators in Fig. 1-13.

Just as in Fig. 1-13, the opposing phase signals are connected together through a capacitor and a resistor, and output taken from the junction. The capacitor in Fig. 1-14 is an ordinary one and is fixed. The resistor, however, is an LDR, a lightdependant resistor composed of a photo resistor enclosed in a light-tight tube with a small lamp.

The lamp (not shown) is driven by a vibratofrequency oscillator which causes its brilliance to vary at vibrato rate. This in turn causes the resistance of the photoresistor which it illuminates to vary at a vibrato rate and the phase of the output signal varies correspondingly, producing frequency modulation of the organ signals.

Two such stages in cascade are used in the Consolette II and in the Recital Organ; three appear in the Theatre Organ to produce the extra-wide vibrato needed for the best theatrical effects.

Vibrato for the organ as a whole can be turned on or off by switching on and off the oscillator which drives the lamps or by switching the lamp itself; variations in vibrato intensity and speed can be made by adjusting the oscillator or circuitry between it and the lamps. And signals which are not to undergo vibrato at all, such as pedal notes, percussion, chimes, and so on, are simply introduced into the organ’s output circuitry at a

point following the vibrato stages.

The Spinet

Organ has a vibrato circuit employing the same principle, but using varistors.

21

Chapter 2 THE SCHOBER RECITAL ORGAN The Recital Organ is designed basically as a modern classic instrument. Because its playing facilities comply with American Guild of Organists (AGO) specifications, and because its sound is

extra eighth C, so that the total number of notes for the organ is 85 and includes the top C for the 4′register.

Fig. 2-2 is a schematic diagram of a Recital

extremely pipe-like, the Recital is a direct substitute for a pipe organ, though no electronic organ has yet been marketed which is the musical equal of a firstclass pipe instrument in all its subtleties and shadings of sound.

Aside from tone quality, the judgment of which must be subjective, certain very specific and objective criteria must be satisfied to qualify an organ such as the Recital for serious musical work.

There must be at least two manuals, each of 61 keys; the keys must have overhanging fronts rather than the vertical fronts common to piano keys. This places the manuals closer together and makes it easier to go from one to the other. The pedals must number 32 – two and a half octaves – and must be both radiating and concave to conform to the natural attitude and swing of the feet as they go from center to either side. Pedals, bench, and manuals must be at the specified correct relative heights.

generator, all 12 of which are identical except for capacitors shown.

The first stage, which includes transistor 5, is a grounded-collector Hartley oscillator, which corresponds directly to the grounded-plate tube version. Like any sine-wave oscillator, the greatest frequency stability is obtained when feedback is just enough to sustain oscillation. The emitter resistor 3, which goes to the centertap of the coil, is variable.

It is adjusted so that the circuit just oscillates. The adjustment is simple and permanent. By using a variable resistor here (and by using a coil of excellent Q, tunable with a powdered-iron core) an enormous degree of stability is obtained, despite the tendency of transistors to change characteristics with temperature and supply voltage. In the kits, each coil and capacitor combination is supplied pretuned at the factory. I many cases it is not necessary to tune the organ after construction if the coil adjustment has not been altered.

The second stage is a simple clipper which transforms the output of the master oscillator to a square wave. Transistor 9 is a basic amplifier and the signal from the oscillator is sufficient to overload it severely. Is has a low input impedance, and resistor 6 is sufficient to isolate it from the oscillator so that its effect on frequency is minimal.

Tone Generators The recital Organ contains 12 tone generators, each generating seven octavely related notes, according to the scheme described on page 4. Each generator is on a separate printed circuit, and all 12 are mounted in a row at the rear of the console. Fig. 2-1 shows one generator. The C generator has an additional small printed circuit board attached to its copper side (not visible in the photo) to furnish an

The square wave is fed into transistor 14, a sawtooth waveshaper. Since the base of the transistor is

from the effects of load resistances. A permanent resistor 17 is provided to prevent capacitor 16 from

unbiased, it is normally almost nonconducting and its collector voltage is approximately equal to the supply voltage. Thus capacitors 15-16 are charged negative. The base-ground impedance is quite low, and capacitor 11 is quite small. When the negativegoing portion of the square wave is applied to the base of 14 through 11, a narrow negative pulse appears at the base. This causes the transistor suddenly to conduct heavily. The collector goes positive due to the voltage drop across 13, and the resulting low resistance of the transistor acts as a short circuit across the combined capacitors 15 and 16 quickly discharging them.

When the pulse disappears, the transistor again ceases to conduct and a new negative charge is built up in capacitors 15-16. It is built up slowly because of the time-constant circuit which includes resistor 13. When the next pulse comes along, the action repeats.

This fast discharge and slow charge cause a voltage at the top of 15 which is abruptly positive, slowly going to negative, then abruptly positive again – and so on. This description fits a sawtooth wave, as shown in Fig. 2-3. The actual tone output is taken from the junction of 15 and 16, which act as a nonfrequency-sensitive capacitive voltage divider, so that the sawtooth waveshape and all its harmonics are preserved. Such a divider is necessary to prevent passage of a d.c. component to the keying circuits and to give some isolation for the stage

acquiring a cumulative charge.

The output of this stage is, of course, at the same frequency as the master oscillator. The master oscillators of the 12 generators operate in the band between 2217 cps (the frequency of the highest C# note in the organ) and 4186 cps (the highest C).

This octave is known as octave 7, shown on the diagram.

The next stage, transistors 21 and 27, is a flipflop. It is similar to a multivibrator, but it will not oscillate of itself.

In such a stage, one transistor is always nonconducting and the other conducts fully. When a negative pulse is applied to

both bases through differentiators 22 and 23 from

squarer 9, the base of the transistor which was off is turned on. Due to normal phase reversal in an amplifier, its collector produces a positive pulse which is applied to the base of the “on” transistor, turning it off. The next pulse produces exactly the same effect in the other transistor. Since a single cycle of square-wave output taken from the collector of either 21 or 27 requires both a positive and a negative excursion, two input pulses are needed to produce a single output cycle – and the output is thus a square wave at half the frequency of the input.

We have, therefore, a frequency divider which always divides incoming frequency exactly by 2.

Since the tone an octave below any given note is at a frequency exactly half the original, we have in this way produced a second tone exactly one octave below that of output 7.

The square-wave output of the flipflop is undesirable, since it would make all tones sound hollow and woody; it contains only odd harmonics.

It is therefore fed to a sawtooth waveshaper 31, exactly like that of transistor 14. Output is produced in the same way as 14 and appears at the junction of capacitors 34 and 35 as output 6, somewhere in the range between 1109 and 2093 cps, depending on the note. Output 7 provides the topmost octave of tones for the organ’s 4′ register; output 6 is the highest 8′ octave.

The same sequence is followed for the remaining stages. Each flipflop is fed pulses from the previous one. Amplitude of the output wave depends on frequency and capacitance. The capacitor values in any one generator are multiplied by approximately 2 for each succeeding stage in which frequency is divided by 2, maintaining an almost constant frequency-capacitance product and constant output amplitude. The table indicates that the capacitors are also graded for four groups of 3 generators each for the same reason.

Key Switching All key switching in the Recital Organ conforms to the method described on page 6, including the pedal keying. Each keyboard has five busses and thus each key closes five switches. The pitch registers for both manuals are 16′, 8′, 4′, 2-2/3′ and 2’; those for the pedals are the same with the exception of the 2-2/3′. (The 2-2/3′ register, which

provides the note an octave and a fifth above the 8′, is one of a number of mutation pitches often used in pipe organs because of the peculiar color the fifth adds to organ tone.)

As indicated in Chapter 1, there is an isolation resistor between each generator output and each key contact to which it is connected. These resistors are graded in value with the largest resistance at the bass end of the keyboards and the smallest at the treble. The purpose is deliberately to emphasize the volume of treble tones compared to bass, so that when the tones are eventually passed through the voicing filters, most of which have at least some low-pass action, the filters will depress the harmonics of any note as they should with respect to the fundamental, but in acting on trebleemphasized tones will reduce the treble fundamentals only to about the same amplitudes as the bass tones. The resistors, which are built into the keyboards at the factory, change every six or nine notes by one standard value.

Registration Circuits The keyed tones pass from the five pitch register busses of each manual and the four of the pedal clavier to three printed circuit boards which contain bus amplifiers, woodwind circuits, stop and coupler switches, and printed-circuit edge connectors into which the changeable stop filters are plugged. Each of these three large boards is known as a registration circuit, and each is mounted behind the organ’s stop board so that the stop tablets can be linked mechanically with the switches.

While the three registration circuits are not identical because of the requirements for different numbers of stop filters, switches, and woodwind circuits in each of the organ’s three divisions, they all have the same type of circuitry. They are diagrammed in full in the Recital instruction manuals, but it suffices here to show the Pedal Registration Circuit, Fig. 2-4. The rear of the one for the Swell is shown in the photo of Fig 2-5.

In Fig. 2-4, P16, P8, P4, and P2 are the inputs to which tone from the pedal keying busses is connected. A 1500-ohm resistor across each input (45, 46, 47, and 48) terminates each keying bus.

A look at one bus amplifier and one woodwind circuit will show how all of them work. Let us examine those for 16′ tone in Fig. 2-4. The 16′ bus amplifier is transistor 44. This is a simple stabilized voltage amplifier whose input (to the base) is taken from the 16′ keying bus through blocking capacitor 39 and series isolating resistor 38. You will recall that these isolating resistors were discussed in Chapter 1 (see page 10) in connecting with the coupler system. The collector output of transistor 44 (shown as a terminal marked 16′ in the diagram) is suitably d.c.-blocked by capacitor 3 and connected to all pedal 16′ stop filter inputs.

The stop complement of the organ requires that a second type of pedal 16′ tone be made available – woodwind tone, from which the even harmonics have been removed (see page 17). this is a function of transistors 30 and 36. As described in Chapter 1, tone of the basic pitch register - 16′ in this case – must be provided at the output in the same phase and amplitude as any other tone. To accomplish this, 16′ sawtooth tone is taken from the output of bus amplifier 44 to the base of 30, and from the collector of 30 to the base of 36, the collector of which is the 16′ woodwind output. In going through both these stages in cascade, the 16′ sawtooth tone has been phase-reversed twice, so it appears at the 16W terminal in the same phase as at the 16′ terminal. Series resistors 37 and 31 have offset the

gains of the two stages, so its amplitude at 16W is also the same as at 16′.

At the base of 36, tone is also introduced from the output of the 8′ bus amplifier 21, and it also passes through 36 to 16W. Having gone through only one stage, however, its phase is reversed from that of a normal bus amplifier output. And having reached the base of 36 through a larger resistor 23, its amplitude has been halved. Output 16W, therefore, contains 16′ tone at normal phase and amplitude, plus 8′ tone phase-reversed and amplitude-halved, satisfying the requirements for a woodwind circuit and producing symmetrical waves which are fed to selected stops which require them.

There are two pedal couplers, Great To Pedal 8′ (GP8) and Swell to Pedal 8′ (SP8). Each of these consists of the four switch fingers shown in a vertical row.

When, for instance the Great To Pedal 8′ coupler tablet is moved down, all the switch fingers (identified by arrows) effectively move upward in the drawing and contact gold busses (the short, heavy horizontal lines). The top finger carries pedal 2′ tone from the pedal 2′ keying bus through terminal D to the 2′ bus amplifier on the Great Registration Circuit, through the standard 56,000ohm isolating resistor, thus causing any Great stop filters switched on to emit 2′ pedal tone as well as Great 2′ tone. The same connections are made between the other fingers and Great bus amplifiers of corresponding pitches. And the other coupler works the same way, sending pedal tones to the Swell bus amplifiers and stop filters.

Stop Filters The output of all the bus amplifiers and woodwind circuits go, not to single terminations as might be indicated by Fig. 2-4, but to horizontal copper lines on each Registration Board. These horizontal lines can be seen in Fig. 2-5, which shows the Swell board. On the Pedal board, the diagram of Fig. 2-4 can be used to determine the number of parallel lines. There is one for each of the five pedal signals: 16′, 8′, 4′, and 2′, plus the 16′ woodwind (16W). There is a sixth line for ground, and a seventh for the combined stop filter outputs.

On the Swell board of Fig. 2-5, there are seven signal lines: 16′, 8′, 4′, 2⅔′, 2′, and 16′ and 8′ woodwinds. And added to these seven for a total of nine are a ground and an output line.

On each board, all these lines run almost the full length. Spaced at intervals along he lines are printed-circuit edge connectors. Each of these is a strip of phenolic carrying 10 spring-metal contacts whose function is both to hold and to make contact to copper lines on a small printed board inserted into the connector.

Each stop filter is made up on such a small board, which is known as a Universal Filter Card.

The copper side of one appears in Fig. 2-6. The card is so designed that holes can be found in it for the components of a filter of any reasonable configuration and complexity. Once the components are installed, the card is inserted into a connector on the appropriate Registration Circuit and all needed connections are made by the edge connector to the heavy copper strips which appear at right in the photograph.

The labels printed in copper indicate the function of each connector strip. Those labeled 16′, 8′, 4′, Q, and 2′ pick up sawtooth tone of the indicated pitch register from the parallel lines on the large board (Q means quint or 2⅔′). WL and AWH pick up woodwind tone in the one or two pitches available for each division (L and H meaning low and high, as, for instance, the 16′ and 8′ woodwinds available on the Swell).

Tone of the selected pitch or pitches is introduced into the filter through a resistor between the hole connected by copper to the appropriate strip and one of the IN holes, three of which are provided to allow a filter to be fed by tones of as many as three pitch registers, in varying quantities.

The copper line labeled SW connects the filter end of the input resistor(s) to a connector contact leading to a gold switch spring for the individual stop on the front face of the Registration Circuit.

This spring is mechanically connected to a stop tablet mechanism and contacts a grounded transverse gold bus when the tablet is up, thereby shorting the stop filter input to ground. When the tablet is down, the gold spring lifts from the bus and the stop sounds.

Ground enters the card through the connector strip one down from the top in the photo, and is available at many holes to accommodate filters of different kinds. The holes labeled OR accommodate the output resistor, and a line leads from it over the top of the card to the top connector strip, which

is connected to the big-board line carrying all filter outputs for the keyboard involved.

This card, the edge connectors, and the parallel lines are the elements of the Schober Library of Stops system. They enable the owner to insert a stop filter which creates any desired voice, which will be controlled by any desired stop tablet and may be of any pitch register available on the keyboard. To complete the system, the plastic stop tablets themselves can be removed, simply by pulling, and different ones, engraved to correspond with the filter cards, can be inserted in their places.

The organist can thus have available a total of 32 speaking stops actually on the organ at any one time, but can have in addition a “library” of any additional number ready to be inserted and used on a few seconds’ notice.

The Library Of Stops idea has many benefits.

First, while the Recital Organ was basically designed as a classic instrument, it can be purchased with a complete theatre registration instead, so that no compromises are necessary.

The organist interested entirely in classic music need not put up with some stops inserted by the designer to make the instrument acceptable to pop music players.

Conversely, the pop player does not have to have stops of purely classic usefulness. And those who want to play both kinds of music can have both kinds of stops available.

Schober designers and executives have also been able to reduce their aspirin purchases. A common headache is caused by the fact that if 19 different organists were asked to specify the registration for an organ, 19 different registrations would result.

Inevitably, an organist likes some voices provided him by the designer in the usual instrument, but isn’t entirely happy with others. This problem disappears with the Recital Organ; if you don’t like a voice you can change it or substitute another! The Library of Stops Kit makes this possible. It is a large collection of components, plus very detailed and enlightening instructions on how to experiment to make a stop exactly as you would like it to sound.

It also includes many suggested laboratory-brewed stops which can be made up and tried without any experimentation.

The combined outputs of all stop filters for each division go to an amplifier such as transistor 51 in Fig. 2-4. This amplifier has a volume control only on the Pedal Registration Circuit, so that the organ’s

over-all pedal volume can be adjusted to suit the room, speaker equipment, and organist’s taste. The outputs of all three Registration Circuits – Swell, Great, and Pedal – go to the Preamplifier-Vibrato Unit and the pedal control switches.

The final section of the organ is the PreamplifierVibrato Unit, plus the vibrato and pedal control switches. These switches are supplied as a separate kit (CRA-1), but they are explained more easily as part of the preamplifier. They are therefore shown in Fig. 2-7 in dashed boxes.

The recital Organ actually has two complete, separate preamplifiers on a single printed circuit, so

amplifier-speaker systems, one manual sounding through each. As you will see, the two outputs can also be combined for use with a single sound system. In the diagram, the complete unit is shown.

However, we will speak mainly about the one for the Swell, and you will immediately see that exactly the same functions are duplicated I the other section.

Output of the amplifier stage on the Swell Registration Circuit is connected to terminal SO in Fig. 2-7. This causes all tones from the Swell stop filters to pass through two stages of phase-shift vibrato of the type described on page 14, transistors 6 and 15. Each transistor is used as a split-load phase splitter, the two phases of which are

that the organ can be played through two separate

connected together through a capacitor 9 or 17 and

Preamplification and Controls

a variable resistance 10 or 18 which is a light dependent resistor. The LDR lamps are energized by multivibrator 24-29, whose speed can be set by a potentiometer 20 on the printed board. While the rate of phase-frequency change must be approximately sine so that it will sound smooth, the square-wave multivibrator is a satisfactory drive because most of the harmonics are eliminated by the LDR. The lamp cannot respond to frequencies much above about 6 cps because of thermal inertia. The square-wave drive is automatically converted to near-sine waves and the vibrato sounds right.

The vibrato system is controlled by one rotary switch and the vibrato stop tablet. The switch, S2 in a dashed box at left, places vibrato on either manual or both by controlling a connection between the base of transistor 24 and ground. When this connection is made the multivibrator stops and so does vibrato. With the 2-gang switch in center position as shown, the base connection from transistor 24 goes to terminal VSS on the Swell Registration Circuit. At the same time, the base connection of transistor 67 in the Great multivibrator goes to terminal VSG. Both VSS and VSG are gold switch springs operated by the vibrato tablet. When the tablet is up (off), both these springs are grounded. This stops both multivibrators and there is no vibrato in the organ. When the tablet is down, the ground is removed from the springs and both vibratos operate.

If the switch arm is moved upward in the drawing, the arm of S2A connects the Swell multivibrator transistor base to ground so that there can be no Swell vibrato even if the tablet is down. S2B, however, keeps things as they were for the Great. In this switch position, therefore, there is Great vibrato when the tablet is down, but no Swell vibrato. With the switch arm at bottom, the situation is reversed and vibrato is placed on the Swell only.

A second rotary switch controls the vibrato depth. This is S1, part of which is shown in the large dashed box at center and part in the smaller one at bottom center. The two sections are ganged.

With the arms at the top contact, nothing is done and both vibratos are full. With the arms at center, a certain steady d.c. voltage from the -20 source through 220-ohm and 390-ohm resistors is placed on the series LDR lamps, so that the excursions of the multivibrator are less effective in causing the lamp brilliance to vary. With the switch arms at

bottom, still more steady voltage is added and the lights vary comparatively little in intensity.

Following the two vibrato stages is a simple amplifier stage 34. The input to this stage includes not only the “vibrated” manual signal, but also some or all pedal signal, depending on the pedal switch system.

Output of the Pedal Registration Circuit is fed to point PO, the arms of switch S3. S3 is the Pedal Balance switch, a stop-board control allowing the organist to vary overall pedal volume in three steps.

The variation is brought about simply by passing the pedal signal through one resistor, two resistors, or no resistance.

Switch S4 (2-gang) determines whether the pedal signal shall be passed through the Swell preamplifier, the Great preamplifier, or partially through both. In the center position as shown, pedal signal from S3 goes through the arm of S4A and a 220,000-ohm resistor to each preamplifier (through permanent resistors 45 and 46). With the S4A and S4B arms in the up position, the full pedal signal goes to the Swell preamplifier through S4A, and the Great input is shorted to ground for a.c. by the arm of S4B and capacitor 117. Resistors 115 and 116 simply keep the capacitor charged to the d.c. present at the preamplifier input points so it will not charge with a thump when the switch is moved to either side of center.

Output of the optional ChimeAtron is also injected at this point in the Great preamplifier (terminals CS and CH), and percussion (Chapter 8) may be injected at terminal PN (Swell preamplifier) if a REVERBATAPE unit (Chapter 7) is not also being used and the Mixer-Compressor (page 75) has not been added to the organ.

Each of the swell shoes consists of a pedal mechanism which operates a 10,000-ohm potentiometer through a rack and pinion arrangement. The potentiometer is in the output circuit of transistor 34, as shown in the dashed box. It is in series with a 1,000-ohm resistor so that volume cannot be reduced to zero, and with a 1-mfd capacitor 37, which tends to emphasize the bass as the potentiometer slider moves toward ground, to compensate for the ear’s loss of sensitivity for bass at lower volumes. The .47-mfd capacitor on the swell shoe compensates for the similar loss of treble sensitivity by having some high-cut action when the shoe is fully open (which forms part of the over-all voicing

of the organ) and reducing its effect as the shoe closes.

The output from the swell shoe arm normally goes through resistor 38, variable resistor 94, and blocking capacitor 95 to the base of the final amplifier 100. Control 94 is on the printed board and is the overall volume preset adjustment. The amplifier includes a series variable resistor 96 and capacitor 97 in a collector-base feedback loop. This is a preset brilliance control; brilliance is normal and maximum with maximum resistance of 96.

Output is taken from the collector of 100 through a phone jack on the swell shoe, and fed directly to the power amplifier or through REVERBATAPE Unit and/or the optional Mixer-Compressor. All this circuitry is duplicated in the Great preamplifier below.

Resistor 38 has one peculiarity, however. Its right end can be placed in either of two holes on the printed board. If placed as shown, each manual has a separate output. But if placed in the optional hole, labeled 38S on the diagram, the final stage if the Swell preamplifier is not used. Instead, the complete Swell signal goes through 38 down to the Great preamplifier where it joins the output of resistor 83, the counterpart of 38. The entire organ signal then goes through the final stage of the Great preamplifier and the organ becomes a singlechannel instrument – though the separate swell shoes are still effective. This connection of resistor 38 can, of course, be changed at any time; so the organ can be played single-channel, but the owner may change to dual-channel any time he likes.

Regulated Power Supply The same Power Supply is used for the Recital and Consolette II Organs. It is pictured in Fig. 2-8 and diagrammed in Fig. 2-9. The supply uses a silicon-rectifier bridge circuit and is regulated by two transistors with a zener diode to produce about 18.5 volts of filtered d.c. over loads ranging from zero to its rated maximum of about 2 amperes. The transformer primary is fused, and a resettable circuit protector is used in series with the output to protect the power transistor in case of a B- to ground short circuit anywhere in the organ.

The regulator circuit is very similar to series-type tube regulators which were common for years. The 2N277 power transistor acts as a variable resistor in series with the high side of the d.c. line emanating from the rectifier bridge.

To see how it actually works, the regulator section has been redrawn in Fig 2-10. B- in this drawing represents the voltage supplied by the bridge rectifier. Filter capacitors have been omitted for clarity. The battery from the base of Q2 to ground represents the constant voltage placed on this base by the zener diode. The load resistor in Fig. 2-10 represents the equivalent resistance of the power-consuming circuits in the organ. The 1500ohm resistor in Fig. 2-9 is omitted in Fig. 2-10, as it is merely a minimum load useful only when no external load is connected to the supply.

The circuit of Fig. 2-10 is obviously that of two cascaded emitter followers. (R3 is merely a small current limited to protect the small transistor).

The voltage gain from base to emitter of an emitter follower is slightly less than unity. Thus, if a fixed 20 volts appears between the base of Q2 and

This completes the electronic circuitry of the Recital Organ proper. All that remains is to connect the output or outputs to suitable amplifiers and speaker systems, discussed in Chapter 6. There are optional accessories which can be – but do not need to be – added, such as Percussion, REVERBATAPE Unit and Mixer-Compressor.

These are discussed in later chapters, as they apply to almost all Schober Organs.

Combination Action

ground, slightly less than 20 volts will always appear from its emitter to ground, regardless, over wide ranges, of the value of the emitter resistor.

This Q2 emitter voltage is applied to the base of the second emitter follower Q1. Here again, the gain of the Q1 emitter follower is slightly less than 1, so the voltage appearing across its emitter resistor must always be slightly less than that applied to the base. In this case, after the original 20 volts from the battery or zener diode has gone through both emitter followers the Q1 emitter voltage is just about 18.5 volts.

The emitter resister of Q1 is the load – the organ circuits. Suppose the load resistance becomes lower – caused by adding circuitry to the organ or perhaps by using transistors some of which draw more current than in the original prototype. Without regulation, this would cause the voltage across the load to decrease. The emitter, in other words, goes more positive. Since the voltage determining the conduction of Q1 (emitter-collector current) is that between base and emitter, which in turn is made up of the base-ground and emitter-ground voltages in series, this causes the base voltage to become more negative with respect to emitter. Since negative voltage on the base increases transistor conduction, the transistor then becomes a lower resistance, sending more current through the load, and restoring the original 18.5-volt drop across the reduced value of load resistance. The emitter follower is a degenerative circuit, which by definition is a circuit which tends to compensate for changes.

When an organ is as large musically as the Recital with its 39 stop tablets, the player very often wishes to make changes in the settings of the tablets while playing, but has not the time to do so without interrupting the music. In a pipe organ of any size, he has a Combination Action to help him make registration changes, and one of the most modern of these devices is available as an option for the Recital.

From the player’s standpoint, operation is very simple. Fig. 2-11 shows the numbered combination buttons (or pistons, as they are often called). While

these buttons can be designed in any combination, the Schober action has five for the Swell manual, four for the Great, and four “generals.”

When one of the five buttons under the Swell manual is pressed a preset combination of stop

tablets automatically comes down in both the Swell and Pedal sections. Pressing one of the four buttons under the center of the Great manual does the same for the Great and Pedal stops. And pressing one of the four general buttons under the left end of the lower manual does the same for stops in all three groups. (A group of four toe studs slightly above the pedals at the left end of the organ duplicates the action of the general buttons when the hands are too busy.) Thus it is possible to set up 13 different combinations of stops and call on any one of them by pressing a single one of the piston buttons.

The most intriguing facet of the Combination

tablets, and each jack has two possible positions, corresponding to stop tablets u or down. When the slider moves, the jack causes one of the fork-like rockers to rotate in one direction or the other; and the end of the fork, which is linked to the rear of the tablet mechanism in the organ, makes the tablet move up or down. If the slider remains pulled (when the button is held in), the same linkage transmits movement of the tablet back to the jack, so that moving the tablet by hand will reset the jack to its other position if the combination is to be changed. Detailed explanation of the action would be very complicated and has no real point.

Action is the manner in which combinations are set up – for they are changeable at a moment’s notice!

Let us assume that we wish to set up button #1 on the Swell for a flute chorus on the upper manual and a Bourdon 16′ pedal bass. We press button #1 and hold it in. While holding it in, we push down the four flute tablets in the Swell group and push up all others; we also push down the Pedal Bourdon 16′ tablet and push up all others. Now we release the button. From that moment on, whenever we press button #1 the Swell Flute and Pedal Bourdon stops will come down, and all others will go up or remain up. In this same way, any conceivable combination can be set up in about 5 seconds without leaving the organ bench!

Fig. 2-12 shows the combination Action before installation in the organ. It has 13 lengthwise sliders, one for each of the available combinations.

The slider selected by a button is pulled slightly by a powerful electromagnet whose action is cushioned by an aircheck to avoid excessive noise. Each slider has a small metallic “jack” fro each of the stop

This action, which is known in organ terminology as a “tripper” type, is made entirely of metal and plastic. It is easily installed and adjusted and will remain in adjustment for years. This same action is used in some of the finest pipe organs.

Because of the need to install the linkages between the stop tablet mechanisms and the action, it is easiest to install the action at the time the organ is originally being assembled. However, it can be installed at a later time, though a little disassembly of the organ is necessary. Large holes are necessary in the wood trim strips beneath the manuals for the buttons, and holes are required in the toeboard for the toe studs. If you plan to install a Combination Action at any time in the future, you will do well to order or build your console with these holes already drilled.

The Combination Action can be furnished only with the setup shown – five Swell, four Great, and four General pistons. It is not available for any models other than the Recital. The Theatre Organ has a combination action of a different type.

Chapter 3 THE SCHOBER CONSOLETTE II ORGAN

The Consolette II is the second and most up to date of an organ species Schober originated several years ago – an instrument physically of the “home size,” with all the graceful lines and the low silhouette ideal for a living room setting, but with the full 5-octave pipe-organ keyboards of a concert organ and a complement of many stops, all distinctive and authentic. The original Consolette was a trail blazer and is in daily use I homes and small churches all over the world. The modern Consolette II is fully transistorized, has 22 stops and a coupler, and provides 17 pedals. Though its specification is smaller than that of the Recital Organ – fewer stops and couplers, no interchangeable “Library of Stops” voicing, and a single swell shoe – its musical effects are produced by the same circuitry as that of the Recital and within its specification it is every bit as satisfying an instrument. Because of its big-organ design in a relatively small console, it is considered by many to be the premium instrument among those of less than full pipe-console size.

The complete specification – voicing, dimensions, appearance, and the like -–is detailed in the Schober catalog.

As you read this chapter dissecting the Consolette II from the engineering standpoint, the string resemblance you will find between the Consolette II and Recital circuits points out the fact (which is not apparent in most small organs) that the factors that make a console large (like that of the Recital Organ) are not those which really determine sound quality. Instead, they are mechanical matters such as the use of a different kind of pedal clavier, a stop board enlarged to include the tables for more stops and couplers, and the like. While these matters are of importance to many people, the point is that a small organ need not sound small. Especially since the adoption of transistors and full use of printed circuit boards, space requirements for purely electronic parts are really quite modest, and there is no reason why a conscientious designer cannot turn out a home-size organ with all the tonal splendor (though perhaps fewer voices and operating conveniences) of a giant.

Tone Generation The Consolette II provides the same seven octaves of the basic tone as the Recital, the upper

six in exactly the same way. Each of the 12 main Consolette Tone Generators is electrically identical to its Recital counterpart, with the exception that the lowest octave, 32.7 to 65 cps is missing. Each board therefore reads diagrammatically the same as that of the Recital of Fig. 2-2, Page 17, with the exception that components 108 through 126 are missing. The description beginning on page 17 is identical as well, and you should read that now if you have not already done so.

The seventh and lowest octave of tones is produced for the pedals only, by a pedal generator conforming to the general description on page 5.

The Consolette II Pedal Generator is diagrammed in Fig. 3-1.

The Consolette II pedal and pedal-switch assembly is the same as that of the Spinet, pictured in Fig. 1-7 on page 8 and shown schematically in

Fig. 1-6, except that there are 17 pedals rather than 13. As Chapter 1 indicates, the output of the pedalswitch system s always a single tone. For the first 12 pedals this is always the lowest note from the main generators, while for the upper five it is the second lowest.

Pedal-switch output is connected to terminal PS in Fig. 3-1, and amplified by transistor 5. Collector output of this transistor goes through level adjusting resistor 20 and blocking capacitor 19 to the base of transistor 28, the collector output of which is nothing but the same sawtooth tone at the same pitch as at input terminal PS. This tone is fed through blocking capacitor 31 to the pedal Bass

Flute 8′ stop filter, and also provides a certain amount of 8′ tone for the Open Diapason 16′ filter through resistor 39.

Collected filter outputs are fed to final amplifier 62 through a control with which the owner sets pedal volume at installation time.

Manual Keying

Transistors 11 and 18 and the associated components are a flipflop of the same type as those in the Tone Generators. When the flipflop is triggered by amplified pedal tones from the collector of 5, coupled through capacitors 13 and 14, the collector of 11 produces square waves at half the incoming frequency. These are fed directly to the Bourdon 16′ filter through blocking capacitor 25. The Bourdon is a low-pitched stopped flute and produces the correct sort of “hollow” tone when the tone source is square.

Two pedal stops are left, Open Diapason and Bombarde, both of which are at 16′ pitch, which requires the frequency-divided output of the flipflop, but are not “stopped” tone colors and therefore should have a sawtooth signal source. To provide the 16′ sawtooth, a mixing process is used which is approximately the reverse of the mixing which produces square waves from sawtooth in the woodwind circuits of the organ. Fig. 3-2 is a graphical construction which shows how mixing a 16′ square wave with an 8′ sawtooth (dashed lines) results in a 16′ sawtooth.

To do the mixing, the base of transistor 35 is connected (a) to the source of amplified 8′ sawtooth, transistor 5, through 21 and 22, and (b) to a source of 16′ square waves, the collector of 18, through 23 and 24. (The collector of 18 is used rather than that of 11 so that the finished sawtooth fed to the filters will be in phase with the 8′ sawtooth and prevent cancellations that would occur in the organ with phase opposition.) The collector of 35 feeds both the 16′ sawtooth stop filters. The letters A, B, C, and D in the filters indicate connection points for stop-tablet switches which short the points to ground to turn off the stops.

The keyboards and key switch of the Consolette II are identical in design and function to those of the Recital, as described on pages 6 and 19 and have 61 keys each instead of the 44 keys usual for smaller organs. Instead of the five pitch registers of the Recital, however, each Consolette manual provides three, 16′, 8′ and 4′, and the switching system consequently involves three moving gold springs for each key and three gold collector busses for each manual.

Bus Amplification As in the Recital, each of the keyboard collector busses is connected to a bus amplifier which amplifies all tones for the particular manual and pitch register and feeds them to stop filters. Since there are six manual pitch registers, three for each keyboard, and the pedal system is self contained entirely within the Pedal Generator, only six bus amplifiers are required. All of them are on a single printed board, and each of the six bus amplifiers is identical to those in the Recital, such as transistor 44, 21, 14, or 7 in Fig. 2-4 page 20. As in Fig. 2-4, each keying bus is terminated with a 1500-ohm resistor to ground and is fed to its bus amplifier through a 56,000-ohm resistor.

The Consolette II has one coupler, a Swell To Great 8′, which works in the same way as the Recital couplers (page 20), When a 3-section switch (a flat-blade switch with palladium contacts) is closed by the coupler tablet, tone from each of the Great manual keying busses is connected through a 56,000-ohm resistor to the Swell bus amplifier of corresponding pitch. Thus when the coupler is on, tones produced by keying the lower manual pass through the filters of the upper, and both Great and Swell stops are heard.

The Consolette II also includes woodwind circuits (page 14) for the Swell at 16′ and 8′ pitches to take care of those stops requiring “stopped” tone.

Each of these is identical to its Recital counterpart, such as the 16W output in Fig. 2-4.

Stop Filters The filters for the 11 Swell and 7 Great stops are all on a single printed circuit, which also includes a simple voltage amplifier for each of the two groups.

The filters are of the same type as in the Recital, though the components are soldered to the board in the usual way rather than with the plug-in Library Of Stops system. As in the Recital, each filter is electrically separate, so that any changes an experimenter might wish to make in one filter will not affect others. Discounting the inconvenience of soldering and unsoldering components, it is quite possible for the electronically knowledgeable owner to make registration changes. Anyone seriously interested in that sort of thing can purchase the Library Of Stops Kit normally sold for the Recital Organ. Its large assortment of components and its detailed instructions about tonal experimentation will be very useful.

Preamplification and Control The organist sometimes likes to change the normal volume balance between the manuals. In the Recital Organ each manual is controlled by a separate swell shoe, but there is only one shoe on the Consolette II, as is invariably the case with “home-size” instruments. To take care of the changes in balance a Manual Balance switch appears on the stop board. It is diagrammed in Fig. 3-3. The diagram also shows the outputs of the voltage amplifiers which are located on the stop filter board, each of which has an output impedance in the order of 2,000 ohms.

The resistors attached to the switch plus the output impedances of the stop filter amplifiers make up a pair of signal voltage dividers. When the switch arm is at the center normal position, as shown, signals from both amplifiers undergo equal attenuation before reaching the preamplifier input, to which the switch arm is connected. The Great signal goes through a voltage divider composed of R1 as the series leg and R2 plus the impedance of the Swell amplifier as the shunt leg, so that about 75 percent of the Great amplifier output goes to the preamplifier. The same is true for Swell signal, in the other direction. Obviously both are attenuated equally.

If the switch arm is moved down to the Great position, the preamplifier receives 100 percent of

the Great signal. However, the Swell signal goes through a voltage divider composed of R1 plus R2 as the series leg and the impedance of the Great amplifier as the shunt leg; so that only fifty percent of the Swell signal goes to the switch arm. The same action occurs, of course, when the Swell signal is emphasized by switching to the Swell position. The switch thus increases the signal from one source by about 3 db and attenuates the other by about 3 db (both referred to the normal switch position). One should note that this does not mean the actual signal levels are equal or 3 db apart.

Actual levels depend on which stops are on and the loudness values of the stops. And apparent loudness as far as human ears are concerned also depend on the nature of voices, since those which are higherpitched or have more harmonic content tend to seem louder and cut through other tones.

The Consolette II Preamplifier-Vibrato Unit is identical to either channel of the Recital (page 23), except that only one additional input appears at the base of the transistor following the vibrato stages – that for the pedals, the signal taken from the output of the Pedal Generator amplifier in Fig. 3-1. There is, it is true, also an input at that point originally designed for the older type percussion. While it can be used for the current percussions, it is preferable to use the Mixer-Compressor (page 75) to couple in the percussions, especially when a REVERBATAPE Unit (Chapter 7) is present.

The only difference between Recital and Consolette vibrato is the method of control. On the Consolette II a single 3-position rotary switch provides two vibrato depths and an off position (there is no vibrato tablet). The full and light positions of the switch look just like the top and center positions of Fig. 2-7, page 23, but there is no connection at the bottom position. There is an additional section on the switch in which the bottom

position grounds the base of one of the vibrato oscillator transistors to turn the oscillator off and kill vibrato.

The Consolette II Power Supply is the same one used for the Recital (page 25). The Consolette II also accepts several of the same optional accessories – REVERBATAPE Unit, percussions, Mixer-Compressor. The ChimeAtron can be added but is physically unwieldy, so it is almost never used with the Consolette. There is no combination

action. Unlike the Recital Organ, however, the Consolette II can accommodate a built-in speaker system, the LSS-1, (page 53) in case there is too little room space available for the preferable external speakers. The LSL-150 Leslie Organ Speaker (page 54) also makes an excellent addition to the Consolette II – along with, and never without, a standard speaker system – since this organ is primarily theatrical in voicing and takes an added authenticity with the Leslie.

Chapter 4 THE SCHOBER THEATRE ORGAN The modern resurgence of interest in the old theatre or cinema pipe organ has brought forth a number of electronic instruments designed to exploit this interest. Some of them, in addition to suggesting the theatre organ physically by having a curved stop board, actually do present sound that has some or even a great deal of the charm of the theatre organ – perhaps one of the most attractive ways ever invented of allowing a single individual to play light music.

One of the most authentic sounding and versatile of these new instruments is the Schober Theatre Organ. Its total of 35 speaking stops gives a tremendous variety of sound, each voice tailored to reproduce one of those of the original pipe organ.

While the cinema organ usually had relatively few ranks of pipes and thus depended for it flexibility on sounding those ranks at many different pitch registers and on different keyboards (a practice known as unification and duplexing), no such limitation encumbers the Schober Theatre Organ. Each of its 35 voices is quite separate from the others, and the four couplers (including a Unison Off) have the enhanced value more common with the “straight” design of the usual formal organ.

As the complete specification shown in the Schober catalog indicates, the Theatre Organ has five pitch registers in the Solo (upper) manual, including the unusual 1′ register complete to within one octave of the top. The lower manual has three pitch registers, and the pedal clavier two. The instrument as a whole encompasses eight octaves of fundamental tone, seven of which are produced by

the main Tone Generators and the lowest by the Pedal Generator. Both manuals provide the complete 61-key standard pipe-organ range. Optional features include the highly authentic Percussion Group described in Chapter 8 and a 10-piston (plus cancel) Combination Action, plus built-in refinements such as either-or-both manual selection from vibrato and manual and pedal balance controls. The organ is, of course, entirely transistor operated, with a regulated Power Supply.

Main Tone Generators One of the main reasons why the Theatre Organ can offer so much for so little cost and without an out-size console is that continuing laboratory work has turned up new ways to do things which are less costly and take less space, without in any sense compromising the audible results. The Theatre Tone Generators are a typical result – each of the 12 printed boards employing only 15 transistors to provide seven octaves of stable tone with the same effective harmonic structure as a sawtooth. Fig. 4-1 is the generator schematic. Components have the same values for all 12 notes, except for the master oscillator tuning capacitor 7C and the oscillator feedback control resistor 7F.

The master oscillator is a fairly conventional grounded-emitter, plate-tuned stage (transistor 7E).

Its stability is extremely high, the more so because, as is common to all Schober Organs, no provision is made to “shake” it for vibrato purposes, vibrato being added by dynamic phase shift in the preamplifier. The 12 master oscillators run at frequencies between 4186 and 7902 cps - the fourth octave above the middle-C octave of pitches. Oscillator output is taken through capacitor 7H and passed through two squarer stages (overdriven amplifiers), so that the final tone, taken from the collector of 7R, is fully squared and is in the correct phase. This output serves to trigger the following string of flipflop frequency dividers, each of which produces tone an octave lower. Seven octavely related tones thus appear at capacitors 1P through 7P, all of them square waves.

The final generator outputs are produced by mixing the products of the seven stages in a process called staircasing – a fairly common system in modern organs, though usually used in a rudimentary manner and rarely carried to the perfection shown here.

The ideal waveform for a formant voicing system (page 10) is the sawtooth, because of its

TABLE 4-1 HARMONICS (AND CPS) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

GENERAT OR OCT AVES 1 2 1 3 1 2 1 4 1 2 1 3 1 2 1 5 1 2

harmonic structure. The energy carried in a sawtooth for each harmonic is inversely proportional to the ordinal number of the harmonic. That is, there is half as much second harmonic as fundamental, 1/3 as much third harmonic as fundamental, 1/17 as much 17th harmonic, and so on. Both odd and even harmonics are present, and with a good, fast flyback, harmonics extend well past the 30th.

The outputs of the frequency dividers in a flipflop system are square waves. They completely lack even harmonics, but the energy of each of the odd harmonics is proportional to the fundamental in just the same way as in a sawtooth – 1/3 as much third, 1/5 as much fifth, etc. It is possible to combine octavely related square waves to produce approximately the same odd and even harmonics as in a sawtooth.

The system can be explained with the aid of Table 4-1. Let us assume, to simplify things that the lowest-frequency tone-generator stage operates at 1 cycle per second (rather than at its actual frequency of something between 65.4 and 123.5 cps). The advantage of this assumption is that the frequency and number of each harmonic is the same; that is, the fundamental is 1 cycle, third harmonic is 3 cps, 7th harmonic is 7 cps, etc. Let us assume, as is the case that this generator provides only odd harmonics 1, 3, 5, and so on; but we wish to end up with a signal containing all harmonics.

The 1 cycle stage is known as Octave 1. Octave 2 then generates a fundamental of 2 cps; Octave 3

HARMONICS (AND CPS) 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

GENERAT OR OCT AVES 1 3 1 2 1 4 1 2 1 3 1 2 1 6 1 2 1 3

generates a 4 cps fundamental; Octave 4 generates 8 cps – and so on. Our task is to fill in the harmonics missing from the square wave of Octave 1 by mixing into the output a quantity of the products of the other octaves.

Down the left vertical row of each column in the table we have listed all the harmonics. Opposite each on the right we show which octave supplies that harmonic. To begin with, we know that Octave 1 supplies a 1-cyle fundamental plus all odd harmonics. We thus find Octave 1 in the right column opposite all odd required harmonics in the left column.

Octave 2 generates a 2-cps fundamental, which is equivalent to the second harmonic of the Octave 1 fundamental. We thus place the numeral 2, representing generator octave 2, to the right of harmonic 2.

Octave 2 also produces all frequencies equal to 2 times an odd number. Two times 3 is 6; so the 6th harmonic in the left column (equivalent to 6 cps) is produced by Octave 2, and opposite 6 we show Octave 2. Similarly, 5 X 2 is 10, and opposite the 10th harmonic we show Octave 2 generator. And so on.

Octave 3 produces a fundamental of 4 cps. This is the fourth harmonic of the tone we are building up, so opposite harmonic 4 we show Octave 3.

Octave 3 also produces a harmonic of 3 X 4, or 12, so 12 in the harmonic column is produced by the Octave 3 stage. And so on.

Octave 4 produces an 8-cps fundamental, so harmonic 8 and all odd multiples of 8 are shown to be produced by Octave 4. Octave 5 produces 16 cps, so harmonic 16, and all odd multiples of 16 (though none are shown, as the table only goes up to 36 harmonics) are produced by Octave 5. Octave 6 add the 32nd harmonic (and odd numbers times 16, not shown), and Octave 7 adds the 64th harmonic and its odd multiples. (Believe it or not, in a fully voiced organ adding this last can actually be heard on a few stops!)

We can see now how mixing generator outputs – adding upper-frequency tones to those of the octave in whose fundamental we are primarily interested – builds up the full harmonic structure. But now, what about the amplitudes of the harmonics?

The answer can be easily shown just by going back to the first and simplest addition – Octave 2.

Starting with Octave 1, the fundamental is 2 cycles, to which we assign the arbitrary amplitude A. The third harmonic of Octave 1, 3 cps, has an amplitude A/3. We now need a 2-cps component with amplitude A/2. As the table showed, we filled in this second harmonic by adding output from Octave 2, with its 2-cps fundamental. Suppose, then, that in adding Octave 2, we do so through a resistor network which simply adds it at half the amplitude of Octave 1. Then indeed, the 2-cps component has amplitude A/2 – and in addition, all the harmonics of Octave 2 fall into the correct amplitude relationships with the harmonics of Octave 1, as a moments thought will confirm. And in the same way, we add the outputs of the remaining generator octaves through resistors which halve each in turn. Octave 3 is added at ¼ the amplitude of Octave 1; Octave 4 is added at ⅛ the amplitude of 1; and so on.

The resulting waveform can easily be seen from a graphical construction. In Fig. 4-2, A is square wave from Octave 1 and B is that from Octave 2 at twice the frequency and half the amplitude. C shows the resultant, called for obvious reasons a staircase. The staircase at C has rather gross steps and does not sound like a sawtooth, though it is not so hollow sounding as a square wave. No wonder, for the only even harmonics which have been filled in are 2, 6, 10, 14, etc. But as more and more octaves are added to the mix, the steps in the staircase become finer and finer, so that eventually so little even harmonic content is missing that the

ear cannot tell the difference. The result is equal to a good, linear sawtooth.

In the actual Theatre generator, the fullest mix can only be obtained for Octave 1, since six other octaves exist above it. For Octave 2, only five octaves remain to be added, and for Octave 3 only four remain – and so on. However, the tone color remains about the same throughout, because as harmonics begin to be omitted, the frequencies of those harmonics become so high that they are not missed. For example, the lowest Octave 3 fundamental is at about 261 cps. The earliest harmonic it lacks after higher outputs are added the 32nd at 8,352 cps. (Remember this does not mean there are no harmonics above the 32nd; in fact, the next harmonic this tone lacks is the 64th, all in between being present.) In Octave 5, the lowest fundamental is about 1046 cps. Its first missing harmonic is the eights – again in the neighborhood of 8,000 cps.

The staircasing system is used in a number of organs today, but usually only one or two octaves are added to fill in the harmonic series. In the Schober Theatre generators, all available octaves are added by the resistor network in the lower part of the diagram. For the lowest note the total output for which is the circled numeral 1, signal is taken from the sixth frequency divider through resistor 12, 22,000 ohms. Signal is also taken from the fifth divider through 23, which is twice the value of 12 and thus gives half the amplitude. Signal is also taken from the other stages through resistors 34, 45, 56, 67, and 78. In each case the resistor carrying the

higher-frequency signal is approximately twice the value of the previous one, to carry out the scheme

we have explained. The resistors from outputs to ground act both as low-frequency terminations to keep the output impedances down, and as parts of the voltage divider system.

Pedal Generator The pedal tones are furnished by the Pedal Generator, which includes a flipflop to divide input tones by two and the six formant filters for the pedal stops. Fig. 4-3 is a schematic diagram.

The 25 pedal switches are single-pole, doublethrow units and can be diagrammed just as in Fig. 1-6, page 8, except, of course, for the quantity. The lowest 12 input tones are taken from the last

frequency dividers of the main generators, and the remaining octave is taken from the next-to-last. The output of the switching system is therefore one tone at a time in the range of 65.41 to 261.6 cps, the pitches for the 8′ pedal stops.

The tones from the pedal switches are connected to terminal PS in Fig. 4-3, where they go through a 2-stage squarer, transistors 4 and 9. The transistors are direct-coupled and include a d.c. feedback loop, resistor 7, for stabilization against changes in temperature and variations in transistor beta. One output from 9, through blocking capacitor 12, is the 8′ bus which feeds 8′ square-wave tone to the appropriate stop filters. The same output, passed through 14 and 17 for differentiation, with the pulse further sharpened by diode 15, is the trigger for the

flipflop 25-26. The flipflop furnishes tone an octave lower that the 8′ which is the 16′ input for the filters and extends the range of the organ down to the necessary 32.7 cps.

The Diaphone 16′ is a member of the diapason family, and its accentuated second harmonic is furnished by giving it some 8′ input as well as the 16′, resulting in a 2-step staircase. The Tibia 16′ is a stopped flute, and the 16′ square wave obtained from the flipflop is ideal for it. The Dulciana 16′, like he Diaphone, also is fed some 8′ tone. The 8′ bass Flute is “stopped” tone, operating directly from the squarer.

The two reads, Tuba Profunda 16′ and Tuba 8′, are rather unusual. They could be produced simply by passing sawtooth tone through a formant filter.

Better scaling and more authentic tone colors, however, are obtained with a slightly different philosophy. In each case, the square-wave input is differentiated and the positive excursions shorted to ground by the shunt diode, so that the net input signal to the filter is a series of negative pulses at rather high level. These shock-excite the tuned circuits, which have fairly high Q, producing a very pronounced ringing, with the consequent highly reedy quality. To complete the required tone color, the input tone is also passed through a parallel channel (resistor 63 or 93), which fattens the tone and gives it body; and output goes through a lowpass section which adjusts brilliance.

The outputs of all filters are collected and passed through potentiometer 37 to ground. The potentiometer is the pedal volume control, which is set at the time of installation for best balance with the manuals; and the output taken from terminal PO is fed to the pedal balance switch, which appears on the organ stop board and is described along with the preamplifier.

Keying and Coupling The outputs of the main Tone Generators are fed to the key switches under the manuals and the switching circuits are of the same kind as in all Schober Organs (page 6). The upper (Solo) manual has five switches for each key and five output busses, 16′, 8′, 4′, 2′, and 1′. The lower (Accompaniment) has three busses, 16′, 8′, and 4′.

The coupling system in the Theatre Organ is entirely electronic. The diagram of the Coupler Switcher printed board is Fig. 4-4. Inputs from the key-switch busses are at the upper right, all indicated by initial letter K. (KS1 is Solo 1′ tone, KA8 is Accompaniment 8′ tone, etc.) Terminal G is ground. The terminals at lower right, identified by B, are outputs which o to the bus amplifiers.

Each of the Solo inputs is connected through a 1500-ohm resistor to ground, 116 through 120, which is the load resistor for the keying bus. Each of the Accompaniment inputs has a 1200-ohm load resistor, 113-115. The three Accompaniment inputs are then fed through capacitors 137, 138, and 139 directly to the output terminals. There are no couplers which require putting Solo tone into Accompaniment bus amplifiers (that is, no Accompaniment to Swell couplers), so only Accompaniment tones go to Accompaniment bus amplifiers.

There are four couplers: Swell To Accompaniment 8′ (SA8), Solo To Solo 16′ (SS16), Solo To Solo 4′ (SS4), and Solo Unison Off (SUO). Let us first explain the SUO coupler.

Solo 1′ tone goes from the KS1 input to the base of transistor 16 (through d.c. blocking capacitor 48).

This transistor, like all of those in this diagram, is an emitter follower. Its collector is connected directly to the common B- line, and it has an emitter

resistor 80. When the Swell Unison Off tablet is up, its switch is closed and negative voltage is furnished

to terminal SUO, This biases the base of 16 negative, so that the transistor conducts and operates normally. Output is taken from the emitter, through blocking capacitor 136 and isolating resistor 96, and is fed to output terminal BS1, whence it will go to the Solo 1′ bus amplifier.

Tracing the circuit will show that there is a similar transistor 15 which operates in just the same way for Solo 2′ tone, and three more, 14, 13, and 12, which do the same for Solo 4′, 8′ and 16′ tones.

All of them are biased on by the negative voltage applied by the tablet to terminal SUO. When the Swell Unison Off tablet is pressed down, its switch opens and this voltage is removed from terminal SUO. Lacking “on” bias, all five of these transistors go to cutoff, so that the tones of the five Solo pitch registers are no longer fed to their normal bus amplifiers. A turned-off transistor, however, still does transmit some leakage tone through the diode composed of the base and emitter.

It is a function of the diodes (such as 32 for transistor 16) to kill the leakage. A steady negative voltage is fed to the cathodes of all the diodes from terminal C-. When the transistors are at cutoff due to absence of base “on” voltage from the tablet, the diodes conduct. Since they are conducting, they are the equivalent of a very low resistance from emitter to ground. Since emitter is the point from which output signal is taken, the low diode resistance, forming nearly a short circuit to ground, brings leakage in the output to an extremely low point, well over 100 db below normal signal level.

When the transistor is turned on, however, by applying negative voltage to its base, the d.c. voltage drop across the emitter resistor (such as 80), which is positive at the emitter with respect to ground, overcomes the C- applied to the diodes, making the cathode negative with respect to anode.

This stops diode conduction and makes it effectively an open circuit so that it has no effect on signal transmission.

This unusual transistor-diode electronic gate circuit (which is also used in the Percussion Group, Chapter 8) gives results superior to those of more conventional ones, either transistor or diode. Pure diode circuits require considerable current and give difficulty where time constants are wanted, due to their low impedance when turned on. Transistor gates are more easily controllable but, in common with diode arrangements, have leakage problems

which sometimes cause designers deliberately to limit high frequencies where gates are involved.

The combination idea effectively eliminates leakage so that tone color and range need not be compromised. The low gate-control current needed makes gradual keying easy. Capacitor 147 is part of a timeconstant circuit (the R component is 140) for the SUO coupler, which slightly delays the rise and fall of conduction in the transistors to prevent pops and clicks. The value of the circuit from this standpoint is even more forcefully illustrated in the Percussion keyers.

The remaining couplers operate along the same lines except that operation of the control tablet is reversed. Take, for instance, the Sell To Swell 16′ coupler (SS16). When the tablet is down, negative voltage is applied to terminal SS16 and to the bases of transistors 4, 5, 6, and 7, which are thus turned on. Transistor 5 sends 8′ Solo tone to the Solo 4′ bus amplifier. And so on. The remaining couplers can easily be traced. (We suggest you use a straightedge to avoid losing your place on the closely spaced horizontal lines.)

Bus Amplifiers Fig. 4-5 is a schematic diagram of the Bus Amplifiers board. The function of this unit is to amplify each of the outputs of the Coupler Switches (one output for each pitch register of each manual, a total of 8) and to produce outputs at conveniently low impedance to feed to the stop filters.

In Fig. 4-5 the eight input terminals at the top correspond to the similarly identified outputs of Fig. 4-4. A typical bus amplifier is transistor 1 fro the Accompaniment 16′ tones (BA16). The transistor is a perfectly normal voltage amplifier stage, with an un-bypassed emitter resistor for both additional stability (the gain thrown away is not needed) and a purpose which will soon emerge. The input needs no capacitor, as d.c. blocking capacitors are present on the coupler board of Fig. 4-5. The collector output of transistor 1 is coupled directly to the base of emitter follower 79. The direct coupling stabilizes the emitter follower without any need for base-ground and base-B – resistors which would lower the input impedance. The emitter output of 79 goes to terminal A16, from which a wire carries the Accompaniment 16′ tones to their stop filters.

There is a similar bus amplifier and emitter follower for each of the remaining seven inputs.

Five transistors are added as woodwind circuits, to produce the results discussed on page 13. Transistor 2 is typical. Its base receives Accompaniment 16′ tone through resistor 48 from the emitter of 16′ bus amplifier 1; this tone is, of course, in the same phase as the BA16 input. Accompaniment 8′ tone is also fed to the base of 2 through resistor 22 from the collector of BA8 amplifier 3; this tone is reversed in phase from the BA8 input. The base of 2 thus receives 8′ and 16′ tone in opposing phases. The values of resistors 48 and 22 determine that the 8′ tone is present at half the amplitude of the 16′, satisfying the requirements explained on page 13.

The result at the base of 2, therefore, is nearly a square wave, lacking even harmonics. Transistor 2 reverses the total phase so that the 16′ component, as fed to the Accompaniment 16′ woodwind emitter follower 80 and the A16W output and stop filters, is the same as that of “straight” 16′ tone; if the two were opposed, there would be some cancellation when stops of both types were on. The remaining four woodwind outputs work the same way.

There is now a total of 13 outputs, each ready to be fed to a group of stop filters. The actual tones – their pitch registers and the manuals from which they come – are marked along the terminals at bottom only when all the couplers are off (which means tablets up, so that the Solo Unison Off transistors in Fig. 4-4 are conducting but none of the others are). When the Solo To Solo 16′ coupler is on, for instance, Solo 16′ tone appears at the S8 (and S8W) output, and so on, all in addition to the normal tones. However, the normal tones disappear when the Solo Unison Off coupler goes on, so that if the Solo To Solo 16′ coupler is also on, all Solo 8′ stops become 16′ stops only, all 4′ stops become 8V stops only, etc. Usually the Unison Off coupler is used along with one of the others, but it is also handy for simply killing all Solo organ tone quickly if a percussion solo is to be played, and restoring it without manipulating a lot of tablets when the percussion solo is finished.

Stop Filters The manual stop filters strictly follow the standard formant thinking (page 10), except that

most of the reeds are shock excited like those of the pedals.

The manual stop filters are divided into two diagrams to avoid clutter – Fig. 4-6 for the Accompaniment and Fig. 4-7 for the Solo. Each input from the Bus Amplifiers of Fig. 4-5 is passed through a d.c. blocking capacitor and to a grounded 47,000ohm terminating resistor. Each filter then draws upon the input or inputs it needs for tones of the correct pitch registers and type (straight or woodwind). In each filter diagram there is a terminal circle which is the connection to the stop-tablet switch. The switch is closed when the stop is off, short-circuiting the tone to ground. The values of the resistors are high enough to prevent this from changing the input signal level from the Bus

Amplifiers due to loading or the signal level at the

common filter outputs. Thus, all stops add as they would in a pipe organ, and there is no interaction regardless of how the stops are combined during playing. Resistor values are chosen also to grade the stops in volume in accordance with the relative volumes they would have in a pipe organ.

There is a 3-transistor output amplifier for the filters of each manual, ending in an emitter follower for low-impedance output. It was discovered during development that when the reed filters and others feed a common input to the amplifier, there is sometimes some cancellation (evidenced as a change in tone color) when a reed is used along with something else. This is because these reed filters affect signal phase quite differently. The bad effects are eliminated simply by having the two groups feed the input of the amplifier in opposing phases – the reeds connected to the base, and the flues (stops normally produced without a reed

Preamplification, Vibrato, and Control The final major section of the Theatre Organ is the Preamplifier-Vibrato Unit, which accepts tones from the stop filters of both manuals and the pedals, adds vibrato when desired, and produces an amplified output at conveniently low impedance.

The Unit itself, shown in Fig. 4-8, contains potentiometers for brilliance and maximum output level. The stop board switches controlling four other factors are also shown in Fig. 4-8 in dashed boxes.

The preamplifier is divided into two principal sections. The first, transistors 5, 17, 22, 35, 40, and 50, comprises a dynamic phase-shift system which imparts vibrato to the tones fed to its input, terminal VB. The second, transistors 54, 66, and 71, amplifies signals applied to the base of 54, provides for inserting the swell-shoe-operated potentiometer, and provides the total output signal.

The story begins with the two switches in the dashed box at lower left. The two signals from the two stop filter amplifiers of Figs. 4-6 and 4-7 are fed to the two switch arms of the Manual Balance switch, which is on the console. In the center position of the switch, both signals pass through 6800-ohm resistors. In the ACC position, the Accompaniment signal goes through a 4700-ohm resistor, while that of the Solo goes through a 10,000-ohm resistor. These resistors are in series between the signal sources and the preamplifier inputs, so in the ACC position, the Accompaniment signal is louder than normal, while that from the Solo is softer. In the SOLO position, the action is reversed.

Each signal goes to one of the arms of the Vibrato Selector switch. In the center (BOTH) position of this switch, both signals go to terminal VB at the input of the vibrato section of the preamplifier, and both signals have vibrato added.

In the SOLO position, only the Solo signal goes to VB, with a 680-ohm resistor added to maintain it at the same volume level as before. The Accompaniment signal, however, goes to terminal NV and thence to the base of 54, entirely bypassing the vibrato circuits. Thus vibrato is heard on the Solo manual but not on the Accompaniment. In the other extreme position, the switch applies vibrato only to the Accompaniment manual and not to the Solo.

The Pedal Balance switch is in the box at lower right. Pedal signal from the Pedal Generator is applied to the switch arm. The switch passes the signal selectively through different total resistances to the base of 54. This makes it possible to adjust pedal volume over a small range as part of organ registration, and to have pedal tones without vibrato, a distinct advantage as explained on page 14.

Vibrato Circuits The basic idea of the Schober phase-shift vibrato, as explained on page 14, is carried out by tree stages, each identical to transistor 5, and each a practical embodiment of Fig. 1-13, page 14. Instead of a transformer, the transistor is connected as a split-load phase splitter to provide the two signals of equal amplitude and opposite phase. Collector and emitter are equivalent to the top of GEN 1 and the bottom of GEN 2, and ground is the common point

between them. Collector and emitter are connected together through a capacitor 8 and a variable resistor 9, the latter actually a light-dependent resistor composed of a small lamp L and a photoresistive cell P facing each other inside a light-tight shell. When the light intensity is varied at the vibrato rate of around 6 times per second, the resistance of the cell varies at a similar rate and the audio signal phase at the joining point, capacitor 10, varies to produce an effective frequency modulation vibrato effect. The part of the system which causes the resistance of the lamp to vary is not connected in any way into the audio circuitry so that one of the advantages of the light system is elimination of any need to get rid of the low-frequency pulses.

Transistors 90 and 103 provide the low-frequency signal which causes the lights to vary. The former is a phase-shift oscillator whose frequency is variable with potentiometer 97 over a large enough range to satisfy the tastes of any organist in vibrato speed. The collector output is fed to emitter follower 103 through two switches, shown at right of 103 in the dashed box. The uppermost switch is the one operated by the vibrato stop tablet. It closes when the tablet is turned on, feeding oscillator signal through one of two series resistors (or no resistor at all) to the base of 103. The 3-position Vibrato Depth switch controls the amplitude of the oscillations fed to 103 and thus the depth of the vibrato in three steps. Emitter follower 103 is used to supply the low-frequency signal to the three lamps in series, which form part of the emitterground resistance. Separate B- (marked B2) and ground (G2) terminals are provided and externally decoupled from the power supply, so that no oscillator signal can couple into the audio channels through the supply.

Vibrato Stage 5 is followed by amplifier 17, with resistor 11 to satisfy the requirement of this type of phase shift circuit for a high-impedance load.

Capacitor 12 bypasses series resistor 11 to tilt up the treble slightly, compensating for unavoidable losses. Two more of these modulator-amplifier combinations succeed the first. They are the same, except that transistor 50 is an emitter follower and is also part of the remaining preamplifier complex in that the last four stages are direct-coupled, with a stabilizing d.c. feedback loop (through resistor 49) back to the base of 50.

The base of 54 is the second input point for the Unit, accepting not only the Accompaniment and Solo signals which are to be without vibrato, and the pedal tones, but also possible other signals, such as percussion, chimes, and a nameless auxiliary. In actual practice, these additional inputs are rarely used, but they are provided at nearly zero cost. The history of Schober development shows that inventiveness continues to bring forth new accessories developed after the organs to which they are to be added. The decision this time was to provide plenty of inputs in the hope of being safe rather than sorry!

The ChimeAtron can, of course, be connected here, though few are used on the Theatre Organ in view of its elaborate Percussion Group. Percussion is normally connected to the Mixer-Compressor.

The output of the Preamplifier-Vibrato Unit, terminal OT, is wired to the input of the MixerCompressor. That device standard with the Theatre Organ and optional with the Recital and Consolette II, is described beginning on page 75.

Sound Production The same high-quality amplifier and speaker system must be used with the Theatre Organ as with other Schober Organs. These are discussed in Chapter 6, and for the best results an external speaker system such as the LSS-10A, LSS-100, or a quality equivalent is recommended. Output from the amplifier is taken from the Mixer-Compressor, and if the owner wishes, that device also provides a separated output for the Percussion Group to sound through a separate system.

The Theatre Organ does, however, provide the option of a built-in speaker system, the LSS-5, for locations where there is just insufficient space for an external one. The sound quality of the LSS-5 is as good as that of the LSS-10A; it sues the same components and the enclosure, which is built into the structure of the console, is acoustically about the same. The system speaks from the rear of the organ and diffuses over the hard wall which the back of the organ should face. Its only disadvantage is the same as for all built-in speakers – they are necessarily too close to the organist to give him the full sense of spaciousness that a well located outside system can give.

Power Supply

The power Supply used for the entire organ is diagrammed in Fig. 4-9. it begins with six convenience outlets across the input line, controlled by the on-off switch. The outlets can be used for accessories like the REVERBATAPE Unit, power amplifier, etc. The primary line to the transformer is protected by a circuit breaker with a red reset button on the Power Supply chassis. The breaker protects both the power line and all the supply components against any damage from accidentally shorting

Three different outputs are derived. The 39-volt output, carrying unregulated d.c., is fed to the seven pilot lamps mounted in depressions under the front of the console lid the throw light on the keyboards in the time-honored theater-organ tradition. The lights are connected to the supply through the two loose-pin hinges holding the lid in place. The 20volt output is taken from a subsidiary 2n277 regulator for the 12 Tone Generators. This regulator is not needed to establish the right voltage, as the

anything in the supply or the organ.

The transformer secondary voltage is converted to d.c. by a silicon bridge rectifier and filtered by two 1,000-mfd capacitors. It is then regulated by two cascaded emitter-follower stages and a 30-volt zener diode, much as in the Consolette II and Recital supply, page 25. The 4-ohn power resistor in the 2N277 collector circuit prevents the transistor from being forced to dissipate too much heat.

main regulator does that with the help of the zener diode. Its purpose is to establish an extremely low supply impedance to prevent generator hash from getting back into the supply and into other circuits.

The 30-volt output of the main regulator is connected to the decoupler network of Fig. 4-10, located in the upper part of the console with the major circuitry. It provides good isolation from board to board, and gives the voltages required in each case.

Chapter 5 THE SCHOBER SPINET ORGAN The Spinet is the only Schober Organ which does not employ transistors. It is not, however, appreciably less reliable that other Schober models on that account, for in the Tone Generators, the only circuits where the exact characteristics and condition of the active components must be maintained, the essential elements are neon lamps, whose life is rated in the area of 10,000 hours – a lot of organ playing in anyone’s language! The chief and frank reason for the existence if the Spinet is that many people have a decided limit to how much money they can or want to spend for an organ. The Schober philosophy recognizes this legitimate limitation, but makes a sharp distinction between a cheap and an inexpensive organ. A cheap organ may have parts of inferior quality, circuits liable to deterioration or failure, keys and pedals of limited life and utility, and an array of gadgetry of

limited musical usefulness, designed to attract the

musically unwary but of short-lived interest. The Schober Spinet, on the contrary, contains a relatively large number (18) of individual and distinctive organ voices, the one solid-rock sine qua non which makes an instrument musically satisfying, three pitch registers, and the same seven octaves of basic tone as the Schober Consolette II and Recital. Its components are of premium commercial quality and the circuits designed with the same elaboration and safety factors as in other Schober constructional quality, save that the regular Spinet console is of walnut-stained birch rather than actual walnut. The money saving is brought about by using neon lamps rather than transistors in the generators, by the smaller console and somewhat fewer stops, keys, and pedals, and by the absence of items which are peripheral to organ enjoyment such as elaborate percussions and the like.

Maine Tone Generators The over-all design philosophy of the Spinet is the same as that of the other Schober Organs – frequency-divider generations providing sawtooth waveforms, the formant type of tone coloring, and vibrato added in the preamplifier through phase modulation, both for the pleasanter sound and to avoid pedal vibrato. Note that the Spinet has 12 Tone Generators, just as have other Schober Organs. The fact that two generators are physically mounted on each of six printed circuits has led some people to believe – erroneously – that there were only six generators, with some kind of sharing system. Since each generator requires half of a duotriode type, it makes sense to put two generators on a board, as sawing a tube in half is difficult.

Fig. 5-1 is a schematic diagram of a Spinet tone generator. Only one triode tube, half of a 12AX7 is used; it is a master oscillator, a highly stable grounded-plate Hartley.

A typical neon frequency-divider stage is the one producing output 5. This consists of a classical neon-lamp relaxation oscillator comprising a resistance (16 and 47 in series) in series with a neon lamp (actually two lamps in series) between B+ and ground, with a capacitor (19 and 20 in series) across the lamp. The frequency of free-running oscillation for a given B+ voltage and lamp type is determined by the values of R and C. Output from this stage is taken from a capacitive voltage divider 19-20, in which, from an impedance standpoint, the output lead is closer to ground than to the lamp-capacitor junction, minimizing loading effects on the oscillator.

In the specific circuit employed there are some significant deviations from the standard neon oscillator; the purpose of the deviations is to cause each stage to synchronize at half the frequency of the previous one, without causing its lower-frequency tone to be heard in the output of the previous stage, and to achieve this frequency division with great stability.

To understand this, look next at the output-4 oscillator stage, which is identical to that for output 5, except that the value of the capacitors and resistors has been chosen for a free-running frequency of about half of stage 5. A second output is taken from stage 5 through a second capacitive voltage divider 22-23. These capacitors are so much

smaller than 19-20 that they do not significantly affect the frequency of oscillator 5. When lamps 17-18 fire to create the flyback of oscillator 5, a negative pulse is applied through 22-23 to the junction of lamps 25-26 in stage 4. Because the stage-4 components purposely cause the firing of 25-26 to be delayed, 25-26 are in effect almost an open circuit, so that almost half the full output of stage 5 is applied at the junction. The negative pulse causes lamp 25 to become suddenly very negative with respect to the positive voltage at its top. This causes it to fire, transmitting positive voltage to lamp 26, which also fired. The standard neonoscillator build-up then begins again, until the next sync pulse comes atop 25 is not positive enough to allow 25 to fire, even with the negative pulse added to its bottom, so nothing happens. But when the third negative pulse comes along, the top of 25 has become positive enough, and 25 fires. The neon lamps of stage 4 thus fire once for every two pulses received from stage 5, and its controlled oscillation frequency is half of stage 5, or an octave lower.

A special point about this circuit is important. It is most undesirable when playing a particular tone to hear anything of the tone an octave lower. While something of a higher tone is not noticeable – it appears to be merely a part of the harmonic development of the tone – a lower component is extremely noticeable. If the sync-pulse path from stage 5 to stage 4 could conduct equally well in the back direction, stage 4 tone would be heard in output 5. Bout for pulses at the 25-26 junction to get into output 5, they must pass through a voltage divider consisting of a very small capacitor, 22 and 19 in series, as the series leg and a very much larger capacitor, 20, as the shunt leg. The voltage division is 5,000/22; and the back-fed tone is thus inaudible.

The neon laps used in this generator are of the NE-2 type, but reliable operation cannot be expected with the usual kind sold in parts stores for use as indicator lamps. Schober neon lamp might, quite without pretentiousness, better be called coldcathode tubes, for they are made by Signalite, Inc., the one manufacturer who has traditionally designed these devices as circuit elements rather than as lights, and are tested to two extremely tight specifications. Aging is the first. The firing and extinction voltages of a neon lamp vary considerably and erratically until the lamp has been aged by operating it at a specified current and through specified cycles

for a certain period of time, after which these parameters stabilize. Leakage resistance is the other important factor, since each lamp is presumed to be an open circuit until the gas is ionized. Actually any lamp has a leakage path between electrodes over the glass envelope. The stringent specification for leakage resistance in Schober lamps is 80,000 megohms, 8 x 1010 ohms! These specifications, rigidly adhered to, the regulated power supply, and the unusual nature of the electronic design, make a tone generator whose stability over long periods of time has been proven difficult to surpass.

Pedal Generator The main generators provide six octaves of tone, lacking the lowest octave down to 32.7 cps as with the Consolette II and most small organs. The low octave is provided by the Pedal Generator diagrammed in Fig 5-2, in accordance with the philosophy discussed briefly on page 5. Tone for the Pedal Generator is provided by the pedal switch system diagrammed and described on page 8. Only one note at a time can reach the input, point PS in Fig. 5-2 – the lowest note from one of the generators. The first dual-triode stage is simply an amplifier with negative feedback to adjust output level and retain the very short flyback of the sawtooth. The output, taken from the plate of V1-G, is fed to the Bass Flute 8′ filter. (The entire unit is

on a single printed circuit, which includes the four pedal stop filters.)

The second stage is a flipflop. This stage is bistable – it has two conditions of equilibrium, one when v2-A is conducting and V2-B is cut off, and the other when the reverse is true. A single input pulse from the first stage reverses these conditions and in the absence of further pulses the new condition is stable and will not change. The unit is therefore not an oscillator and cannot run free. It will not change condition in response to pulses, and the timing and frequency of the pulses may vary over a wide range with no change in type of operation. In this case the pulses are the steep negative flybacks of the sawtooth waves, differentiated by capacitors 41 and 42.

Since one pulse will cause a change from “condition A” to “condition B,” it takes two input pulses to cause a complete cycle – from “A” to “B” and back again to “A.” Two incoming pulses, therefore, are required to cause a complete cycle of plate-voltage change on either tube, and when output is taken from just one triode – the plate of V2-B in this case – the output frequency is exactly half the input or one octave lower. The usual flipflop is symmetrical throughout and produces square waves. That would be undesirable here, as they would all be devoid of even harmonics and would have woodwind tone. The opposing capacitors 13 and 17 are therefore made unequal, and the result is a quasi-sawtooth, which is fed to the 16′ filters. As in all Schober diapasons, a strong octave component is added to the Open Diapason 16′ filter by direct addition of some 8-foot tone.

Output of the pedal filters is controlled by a potentiometer so that the pedal balance may be adjusted to suit the room, the speakers and the organist’s taste.

Manual Keying

Bus Amplification

Spinet keying conforms to the system discussed on page 6; the manuals and the key switches are of exactly the same construction as in other Schober Organs. There are, however, 44 keys on each manual – low F to high C – rather than 61; the keying system provides the same 16′, 8′, and 4′ pitch registers on the upper manual as the Consolette II, but only two registers, 4′ and 8′, on the lower.

The manuals are offset, as the photograph in the Schober catalog indicates so that the lower manual, usually used for chords in the left hand, has the lower pitches required for accompaniment and the upper has the higher pitches commonly used for the melody.

The offset manuals frequently confuse those unfamiliar with organs, but the scheme is quite simple.

Each manual is a section of a normal 5-octave manual. The lower manual has the upper octave and lower half-octave of keys “removed.” The upper manual has the lowest octave and a half of keys “removed.” But the remaining keys are just where they would be on a full manual. So the keys with identical pitches are those which are vertically in line, and one plays on these manuals just as though they had 61 keys each, merely ignoring the missing keys. Short manuals of this type have only one advantage – lower cost. Neither they nor any other elements of an organ which make it small or inexpensive contribute to playing or learning ease.

As in other Schober Organs, each keying bus carrying the keyed tones for one pitch register is connected to a bus amplifier, the output of which feeds stop filters of that pitch register and manual.

Both 16′ and 8′ woodwind circuits are also provided for the Swell.

Fig. 5-3 shows the diagram of the Spinet Bus Amplifier kit. Each of the inputs at bottom comes from a key-switch bus and is terminated in a 12,000-ohm resistor to ground. These termination resistors are relatively large because the neon tone generator outputs are of much higher impedance than those of the other organs and the isolating resistor between generators and each key switch is much larger. Each keying bus is also connected to the grid of an ordinary triode voltage amplifier, plate output of which goes to stop filters.

The

On the contrary, a larger organ with more complete facilities always makes learning faster.

woodwind circuits are somewhat different from those in the transistor organs, but operate on the

same principle (page 13). That producing the 8′ Swell woodwind tone uses two triodes, the left triode of V2 and the right of V1. Swell 4′ tone from the keying bus is cathode-coupled from the V1 triode to the V2 triode so that it arrives at the output (S80) unchanged in phase but reduced to half the amplitude of the straight 4′ output from the left triode of V1. Swell 8′ tone goes through the V2 triode at full amplitude and with the normal phase reversal.

Only one triode is used to produce 16′ woodwind tone, the left triode of V3. Swell 8′ tone and Swell 16′ tone are mixed at the grid. The mixing resistors adjust the relative amplitudes. Since 8′ tone is taken from the plate of the 8′ bus amplifier while the 16′ tone comes from the keying bus, the phases are opposed. In both cases capacitors are used to get rid of higher even harmonics which appear because a neon-lamp oscillator does not provide as many high harmonics as a transistor divider and certain of the high even harmonics are not cancelled by the outphasing action.

Stop Filters Fig. 5-4 is a complete diagram of the Spinet Stop Filters, which illustrates that some economies have been realized by “telescoping” filters – using parts of filters for more than one voice, rather than the completely separate filter for each stop as in the other organs. In the diagram the circled numbers indicate connection points for the tablet switches which ground these points to turn the stops off, and they are also key numbers for the stop list to indicate which circuit elements are for which voices.

Note, for example, the circuitry encompassing numbers 1, 3, and 8, which are all the flute-type voices for the Swell. There is just a single flute filter – the three 47,000-ohn series resistors and the three shunt .001-mfd and .01-mfd capacitors.

Outputs from the appropriate bus amplifiers and woodwind circuits are fed to this flute filter through isolating resistors. The three flute voices are still different, however. The Tibia 16′ (1) and the Stopped Flute 8′ (3) differ in pitch register, and also in amplitude due to the differences in both the two woodwind circuits and the isolation resistors. The Orchestral Flute 4′ (8) is different in pitch register and loudness, and also has less total rolloff because

of injection of additional high harmonics into the center of the filter through capacitor 39. This same kind of thing can be observed in other stop groups.

The output of the entire filter system appears at point BS.

Preamplifier-Vibrato Unit The final electronic element of the Spinet Organ is the Preamplifier-Vibrato Unit diagrammed in Fig. 5-5. the first triode V1-A is a simple voltage amplifier which receives signal from the stop filters.

Its output is connected to the grid of V2-A, the first of two phase-shift vibrato stages. Also connected to this point is an input from the percussion circuit.

While the vibrato phase shifters in the Spinet obey the same theory as in the transistor organs (page 14), they employ varistors rather than lightdependent resistors. A varistors is a resistor whose resistance is dependant on the voltage across it.

Generally the resistance varies inversely as an exponent of the voltage. Very wide ranges of resistance change are possible.

The V2-A circuit conforms to the requirements of the phase-shift vibrato as outlined on page 14.

The two phase-opposed signals are created by using the triode as a split-load phase splitter, so that equal and opposite signals appear on plate and cathode. It remains only to join these two signals, one through a fixed capacitor – 23 in the diagram – and the other through a resistance which varies at a vibrato rate.

The variable resistance is a pair of varistors 19 and 20, whose resistance is varied by applying an oscillator-generated control voltage.

splitter V3-B, whose output is two low-frequency signals of opposite phase.

Fig. 5-6 is a rearranged diagram of the components connected to the cathode of V2-A. The transformer represents the outputs of V3-B. The generator represents the V2-A cathode circuit. Note that this is a special type of bridge circuit in which there are paths for both the audio and the low-frequency signal, but both grounds are centered at the same point by the symmetrical disposition of components.

The control voltage is developed by lowfrequency oscillator V3-A, of the standard phaseshift type. Its frequency is controlled by potentiometer 41 so that the organ owner can adjust vibrato speed to his taste. The output is taken from one of the shunt legs of the phase-shift network and the control potentiometer (in dashed box – actually located on the stop board) determines output amplitude. The low-frequency signal is fed to phase

The purpose of this section of the circuit is to place a controllably variable resistance in series with the audio signal without having the controlling low-Frequency oscillator signal appear at the point where audio output is taken. Audio from the generator passes through both capacitors 17-18, through both varistors 19-20, and output for the grid of the following tube is taken from point A and ground, assume the half-cycle of vibrato signal

when the transformer phase is as marked, with current flowing according to the arrows. At this time the phases of the voltage appearing across the two varistors are as shown; since equal current is passing through both, voltage drops across them are the same. At point A, therefore, varistors 19 is positive with respect to ground and varistors 20 is negative with respect to ground. The two cancel and

no low-frequency voltage is transferred to the next stage. However, for the audio signal, the two varistors are simply in parallel with each other and in series with the signal, so that as their resistances vary according to the applied vibrato signal, more and less resistance is placed in series with the audio path.

When the low-frequency signal is at zero, varistors resistance is at maximum; since the actuating signal is push-pull, and varistors resistance becomes smaller regardless of the phase of the applied control voltage, the resistance is reduced during each half-cycle of signal. This would cause a distorted vibrato effect at twice the frequency of the applied control signal. To permit operation of the oscillator at the actual vibrato frequency and to operate them on a linear part of their characteristic, the varistors are biased. Fig. 5-5 shows that a small d.c. from the plate and cathode of V3-B is passed through the varistors (through resistors 39 and 50). The vibrato signal then alternately adds to the d.c. voltage across the varistors and subtracts from it with each half-cycle; operation is at actual vibrato frequency. A second similar stage V1-B is added to give deeper vibrato.

The remaining stage is the output V2-B. To its grid, through the preset level potentiometer 25 with which the owner sets maximum desired level for the room, goes not only the signal from the earlier

stages, but also the signal from the Pedal Generator and the optional Percussion unit.

An output transformer is used rather than a cathode follower, since this allows loading with a low-impedance swell-shoe potentiometer, and also permits connection of the output to an external amplifier without communing organ and amplifier grounds, which may once in a while produce hum.

The large capacitor in series with the special military-grade potentiometer is a simple loudness control, raising the relative level of the bass as the shoe is closed to compensate for the deficiency of the human ear in hearing bass at low volumes; this maintains about the same tonal balance regardless of overall volume.

The Power Supply completes the standard elements of the Spinet. It is a normal high-voltage supply using silicon rectifiers, and it includes a pair of gas regulator tubes to stabilize the voltage for the Tone generators.

As with all Schober Organs, certain optional accessories are available for the Spinet. The organ requires, of course, and amplifier and speaker system. If there is space for an external system, the Schober TR-2 amplifier and LSS-10A speaker system are ideal, though any good high-fidelity system is adequate. The LSS-1 speaker system can be built into the console if space is at a premium.

The fact that the TR-2 amplifier is a transistor unit does not cause any clash with the tube design of the Spinet; it works perfectly well, and the same is true of the REVERBATAPE Unit.

The Percussion available for the spinet is less elaborate than that for the transistor organs, and of course much less expensive.

Chapter 6 AMPLIFIERS AND SPEAKER SYSTEMS It is common in electronic organs to find speaker systems which, regarded from the standpoint of conventional fidelity, are very poor indeed. These organs- and the class includes practically all homesize instruments and many larger ones – usually do not reproduce the higher harmonics and do a marginal job on the low pedal notes, though they often make some attempt to better matters by tilting up the circuitry response at the high and low ends.

The relatively poor sound production is, of course, the result of using inexpensive speakers and almost always of placing them in the console without regard for the requirements of acoustic enclosure design. The enclosure problem is always acute with speakers inside the console, because a console is designed to house electronics and be of the right shape to accommodate the organist and look nice – not to be a speaker enclosure. The acoustic problem is solved in very few organs, one of those few being the Schober Theatre Organ, which actually has (optionally) an enclosure built into it as a subassembly.

The economics dictating the lack of adequate sound production are unfortunate because an organ is not really a restricted-range instrument. When properly voiced, it has a full, round bass which goes down to 32.7 cps and gives essential foundation tone. At the other end of the spectrum, many organ voices are bland and have very little high-frequency content; but an organ which is bland as a whole, on which none of the voices can really be smoothly brilliant, becomes monotonous after a while and lacks the fascinating changes of character which are one of the main reasons for being of this versatile instrument.

The fact of the matter is that an organ needs an amplifier-speaker system with just about as much fidelity as a good high-fidelity installation – and better than some to take care of the solid 32-cycle bass. From the beginning, Schober Organs have been designed on that premise, so that any good, smooth, full-range speaker system and any lowdistortion high-quality amplifier are suitable. Many people who already have good high-fidelity (or

stereo) systems simply connect the organ output to them and don’t bother with special equipment for the organ. There is nothing wrong with this and it is one reason the organs are designed, without any compensatory audio curves, to provide the right output for a good, flat system. The amplifier of such a system should be rated at not less than 20 to 40 watts continuous sine wave power.

The most important requirement for those intending to use present audio equipment with a Schober Organ is bass performance. It is most important because so much otherwise good speaker equipment will not handle bass as low as 32 cps – which is often a surprise to the owner! The orchestra double-bass is one of the very few instruments with tones that low, and usually the volume of double-bass output is low, compared to the rest of the orchestra, so that even speakers which have response down to 32 cps sometimes will not handle much power at that frequency. The organ pedals, however, are often much more prominent by comparison, so that power handling capability must be good in the bass.

Unfortunately, the way speaker systems are specified in advertising literature by most manufacturers makes it impossible to set a useful printed criterion. One can only say that the system which gives the fullest bass and the greatest smoothness and range over the rest of the spectrum with symphony music will also sound best with the organ.

In its first few years, Schober did not make amplifiers and speaker systems, on the assumption that units available on the high-fidelity market filled the bill. This is no longer so true, especially in speaker systems – the popularity of small-size systems has practically knocked low bass out of the box. In response to an obvious and often expressed need, Schober designed an amplifier and two major speaker systems. All of them can be relied on to give the best possible organ reproduction, and in line with the philosophy mentioned above – they also make record and radio listening components of first quality.

TR-2 Power Amplifier The TR-2 is a high-quality transistor power amplifier conservatively rated at 40 watts output of continuous sine wave – not “music power” or “IHFM” power. Harmonic distortion is less than ¾ of 1% at full output, and intermodulation is 1.4%. It is very strongly protected against overloads and will not lose its output transistors even if the output is shorted. In normal operation it does not even get perceptibly warm. These points are stressed because so many transistor amplifiers either can be damaged rather easily or have protective circuits which stop the amplifier from operation when danger comes along. For an organ amplifier, neither of those possibilities can be permitted. Killing the power in time of danger can do a good job of protecting an amplifier, but it does nothing to protect the organist who is left without sound – perhaps in the middle of a church service!

The diagram of Fig. 6-1 shows the complete

diagram of the amplifier. Most of it is patterned after the RCA circuit so commonly and successfully

used today, so there is no need for a full explanation. A few unusual points are worth noting.

The output transistors are protected by the 0.3-ohm resistor in series with the output. In an amplifier of this type, an output-terminal short usually causes current through the output transistors which are limited only by the capability of the power supply.

The usual result is instant death to the transistors.

The 0.3-ohm resistor is a simple-minded and entirely effective protection, for it limits shortcircuit current to what the transistors can take, without having an appreciable effect on output impedance and damping.

The input sensitivity of the amplifier is actually 55 millivolts. While such sensitivity is unnecessary for ordinary uses, it results from the front-end design and the only extra cost is the sensitivity control which was added so that the full range of the main volume control would be available for normal purposes.

The TR-2 amplifier is only 5½ inches wide and will fit on the floor inside the console of any Schober Organ.

The LSS-10A Speaker System The LSS-10A is the primary Schober loudspeaker system. It has full, powerful bass response and is very smooth up to its maximum rated frequency of about 13,000 cps – which can be extended to 18,000

by addition of the optional small HF-1 radiator. It combines this excellent performance and a power rating of 40 watts of organ or program material with high efficiency, low price, and relatively small – though by no means shoebox – size. Fig. 6-2 shows the interior of the enclosure viewed from the rear; the back, a simple solid piece of wood, has been removed.

The 12-inch woofer is a specially built Schober unit with a 3.4-lb magnet and 8 ohms impedance.

Choice of a 12-inch speaker for a premium system, rather than a 15-incher, surprises some people, especially when they hear the sound. With its heavy magnet and low free-air resonance (below 32 cps), this speaker gives all the bass one might wish when placed in an enclosure correctly designed for it.

The enclosure is of the ducted-port type, which is a variety of the bass-reflex breed. Designing an enclosure of this kind for optimum performance is not an amateur pastime; but when it is done right, the results are hard to argue with. The enclosure must, of course, be designed for the specific driver which it will contain.

The low-frequency performance is enhanced and intermodulation very much minimized by the low crossover frequency – 250 cps. The upper part of the range is covered by a Schober 8-inch cone speaker with a 22-ounce magnet, housed in a separate sub-enclosure for complete isolation from the bass speaker. A dual half-section crossover network makes the transition, but an additional aid is the mounting of the 8-inch driver. The front-panel hole over which it is mounted is D-shaped instead of circular, and the speaker does not cover the square end of the D. The proportions of the hole are such that energy below 250 cps from the back of the cone gets through the hole to the front, and the front and back wave cancel.

The small rectangular board at the upper left corner of the front panel covers a hole which is standard in each LSS-10A cabinet. The HF-1 tweeter, a diaphragm-type very-high-frequency driver with a diffraction horn, can be added to the system at any time to extend its response to 18 kc. It has a high-frequency level control which is mounted in a hole provided in the back piece. With the HF-1 in place, there is an additional crossover at 3500 cps, where the HF-1 takes over.

The LSS-100 Speaker System The LSS-100 was originally conceived for use in churches and auditoriums where each speaker system needed to have a higher power capability than the 40 watts of the LSS-10A. Between conception and final design, however, enthusiasm took over and turned out a speaker system which not only handles high power but produces a flatter, smoother, full-range, more uncolored and effortless sound from any source than very nearly any system available today – not for its price but for any price.

It is a 4-way, 5-unit system in a ducted-port enclosure of correct (which means distinctly nonminiature) size. It is probably used more often with elaborate high-fidelity and stereo systems than with organs!

Fig. 6-3 shows the rear of the front panel of the LSS-100, on which all the components are mounted. The two bass drivers at bottom are the same 12-inch units as in the LSS-10A. The box at upper right is shown with its back removed, housing the 8-inch midrange unit, which operates from 150 to 1,000 cps. The large diaphragm driver with its diffraction horn at upper center covers 1,000 to 3,500 cps; and the HF-1 driver at upper left goes from 3,500 to 18,000 cps. The acoustic design of the enclosure is the same as that of the LSS-10A, scaled up.

The LSS-100 can handle 100 watts of program material or organ signal. If the installation requires that much sound to fill the auditorium, two TR-2 amplifiers can be used to drive the LSS-100. A special tandem-amplifier kit is available; it parallels the two amplifier inputs and places the outputs in series.

Built-In Speaker Systems Schober designers basically frown on speaker systems built into organ consoles, because usually the console makes a poor acoustic enclosure and the sound is too close to the organist, who is usually – at least in the home – his own principal audience.

Schober customers over the years have generally accepted this philosophy and have been glad of the

therefore available for all Schober Organs except the Recital.

The LSS-1 system consists of a 12-inch speaker and two 6 x 9 ovals, with a crossover network. The LSS-1 can be used with the Consolette II or Spinet.

In both cases, a special back, not normally provided, must be ordered or made when the LSS-1 is used.

The LSS-5 is a special system for the Theatre Organ. Its components are the same as those of the LSS-10A and a similar enclosure is built into the console. A special back is needed to complete the enclosure and hold the speakers. The LSS-5 is capable of

excellent quality.

Leslie Organ Speaker

results obtained with the external speaker systems most of them se. It is true that most “commercial” home organs contain their own speakers, but that is only doe to the fact that mass producers must produce that which can be sold most easily.

Sometimes, however, it proves impossible to sell the lady of the house the idea of allocating space to a separate speaker system – and once in a while there really is no room. Special systems are

For many years a very special kind of speaker system has been used by various segments of the electronic organ industry. Known as the Leslie Organ Speaker, it is a sound producer which rotates in a horizontal plane (on a vertical axis) at a vibrato rate in the neighborhood of 6 times per second, so that as it rotates the direction of sound projection travels 360 degrees. Different Leslie versions work in different ways, but the total effect is a combination of amplitude and phase modulation which, used correctly and in moderation, can add a great deal of warmth to theatre-type music and enhance the similarity of sound between an organ which is well designed in its own right and a typical theatre pipe organ. Unfortunately, many small organs incorporate a Leslie speaker but do not realize its potential by providing openings in the enclosure on front, back, and both sides –

something which would be difficult inside an organ console. The result is mainly amplitude modulation.

Fig. 6-4 is a drawing which indicates, without detail, how the particular type of Leslie Speaker offered as an option by Schober works. The speaker points down and the rotor, made of a very lightweight material, rotates directly below it. The core of the rotor is so shaped as to direct the sound to an opening in the rim, so that at any instant sound goes in only one direction. The continuous rotation, of course, sprays the sound around a full circle.

When the sound is directed right at the listener, amplitude is greatest, and as it goes to one side and then directly away from the listener, it is less, then least. Thus the amplitude modulation.

Assuming that the rotor starts facing the listener, as it begins to rotate away from him, wavelengths increase in a dynamic fashion, producing progressively lower frequencies in the same way as the classic example of the whistling locomotive speeding away from the boy beside the tracks. And as it starts back during the second half of the rotation, audio frequencies increase because of the shortened wavelengths. This Doppler Effect is continuously reversing, and its effects, combined with AM and different at every frequency, are even more complicated that can easily be produced by purely electronic means.

The complete unit consists of this speaker and rotor in a cabinet having openings in all four sides.

The rotor is belt-driven by two motors, which can be selected alternatively by a switch in a control box fastened to the organ console. One motor drives the rotor at the vibrato speed. The other drives it at a so-called “chorale” speed, which is very slow. The chorale speed does not produce a vibrato effect; it simply causes tones to “move” slightly to eliminate the unfortunate effects of the electronic precision inherent in the generators.

It is extremely important that the Leslie speaker not be used by itself as the only speaker producing organ sound. It has a fairly low rolloff so that it does not reproduce upper harmonics and tends to make strings and reeds sound like flutes. Indeed, the rotation effect applied to treble notes and harmonics would be unpleasant. The Leslie speaker is used

only in addition to a good, full-range speaker system such s the LSS-10A or LSS-100. Another control box on the console allows the organist to select main speaker system, main plus Leslie, or strictly for temporary special effects – Leslie alone.

Headphones Since electronic organ sound is produced by an electronic amplifier, it is obvious that headphones can be used (sometimes a surprise to non-technical people). They are justified when necessary to avoid eviction on account of late-evening practice, but at best they cannot give the aural satisfaction of a good speaker system. Headphone sound can, however, be surprisingly acceptable if the organ is equipped with a REVERBATAPE Unit and if the phones themselves are of the modern high-fidelity type and give a good enough ear seal to produce bass.

Headphone installation is a routine job for any technical man, and the exact method of connection to the power amplifier output is usually specified in the manufacturer’s instruction leaflet. Generally, it just a matter of inserting a dummy load and a switch across the amplifier output, and connecting the amplifier output to a 3-way phone jack through a suitable dropping resistor, part of which can be made variable for headphone volume control. An easy way to mount the jack, switch, and control is to place them on a small piece of sheet metal, and mount the metal behind a big enough hole cut in the organ kneeboard at one end, just beneath the lower manual where it will not show. Schober does not recommend any particular headsets; most are fairly good for the purpose and one good basis of a choice is to see which kind seems to cling most tightly to the ears and give the seal needed for hearing bass.

As purchased, most phones are connected for stereo – separate input to each ear. Following the usual enclosed instructions, the jack can be wired so the phones are placed in series or in parallel for singlechannel operation. The parallel connection usually gives better smoothness, and net impedance is of no importance. Schober has an Information Bulletin on headphone installation, which well be sent to any Schober Organ owner on request without charge.

Chapter 7 THE SCHOBER REVERBATAPETM UNIT There is almost no musical instrument or ensemble which does not sound better in a hall having some reverberation than in an acoustically dead room. This is especially true of an organ – pipe or electronic – since the organ is the most mechanical of all instruments and there is a great deal of “machinery” between the player and his audience (even if the organist himself is his only audience).

Reverberation is the “sound of a large auditorium” which is brought about by the bounces of the original sound against the walls of large, live halls, with consequent audible repetitions of each sound at each bounce. The total effect caused by the reverberatory bounces is an apparent slow decay of the sound after each note has ended, rather than the abrupt, cold harshness obtained in a small, padded room where everything is heard with razor-sharp precision and immediacy. Electronic organs, with their scientific precision of sound, are too precise and impersonal to begin with. The defocusing and softening effects of reverberation are really a necessity to give both player and audience the emotional satisfaction and sense of bigness.

Though not easy to achieve in practice, the requirements for an artificial reverberation system are simply stated. It is only necessary to pass the sound electronically through a device which will repeat each sound at short intervals, each repetition somewhat lower in volume that the last, until it finally dies away. This is done by the spring-type units which are available today. However, if real satisfaction is wanted, the effect must sound like a natural one; that is, the music should have the same aural and emotional effects as in real theatre or church, which means in effect that it must fool the hearer, rather than being obvious as just another sound effect. Realism in a reverberatory is the ultimate goal, and unfortunately spring devices, which offer the maximum simplicity in design, and the lowest cost, have inherent limitations that make realism impossible. They have sharp resonances, narrow bandwidth (no bass or high treble), and they sound, if anything, like very small rooms with more reverberation than a small room is likely to have,

because the period between repetitions is too short.

They also cannot be controlled to simulate realistically rooms of various sizes or with walls of varying hardness.

The best method of artificial reverberation is to use a continuous recording device, such as a tape recorder. The sound is recorded on the tape and is played back several times as a given section of tape passes several playback heads. This system has been in use for years in recording and broadcast studios. It has the advantages of complete controll-

ability, in both design and use. It can have full frequency range, low distortion, and timing and reverberation-control characteristics. A good tape reverberator can produce acoustic effects indistinguishable from the real thing. Many phonograph records have been made in small studios, the acoustics enhanced by tape reverberators, without any knowledge on the part of listeners that the job was not done in a large concert hall.

For the first time a relatively low-cost tape reverberator with all the quality and durability necessary for daily home use is available in the Schober REVERBATAPE Unit. It was designed especially for electronic organs (though it is equally usable in many other applications). Its mechanical system is simpler than that of the least expensive tape

recorder; yet because it has only one job to do – no stop, reverse, fast or slow, record, erase, or playback switching – it is as reliable as the good conventional professional records and has low wow and flutter content. Professional musicians have commented that it is the one advance in the electronic organ art that is likely to make the electronic organ acceptable to the most reactionary pip organist.

A Bird’s-Eye View There is a continuous loop of tape which continuously passes through the machine, running in turn over a recording head, three playback heads, and an erase head. Program material, such as the output of an electronic organ or a musical performance to which reverberation effect is to be added for a recording, is fed to the input of the Unit.

The output of the Unit, which is fed to the usual power amplifier and speaker (or, depending on the use, to the recording machinery or broadcast circuits) contains first the original program material, merely passed through the REVERBATAPE Unit without change of any kind. Second, and more important, it contains program material which has been recorded on the tape, then picked up by each of the three playback heads in turn. Since the tape takes time to get from the record head to each playback head (about .11 second), each playback-head signal is a delayed repetition of the original material.

The third playback head, in addition to feeding the third delayed repetition to the output, also feeds a signal back to the record head, so that the process is repeated for another tape pass across the heads.

The result is a series of equally time-spaced repetitions, each repetition decreasing by a calibrated amount from the previous one until finally the signal is too small to be heard. The effect is that of an auditorium where repetitions are obtained by bounces from the walls, the sound losing some energy at each bounce, until nothing audible is left.

The user controls the effective length of the decay, and thus the apparent characteristics of the “auditorium” with a single-knob Reverbatape Control which varies reverberation time (time for a 60-db decay) from zero to over 6 second.

The REVERBATAPE Unit consists principally of a tape-handling mechanism built on the front face of a heavy steel box, shown in Fig. 7-1. The tape itself is a very special kind. The oxide is dispersed in an unusual binder which has enormous holding power, so that continuous passing over magnetic heads causes very little deterioration. The small amount of oxide rubbed off comes away as a dry powder, which simply falls, rather than as “gunk” which remains on the head gaps and fouls them, as with the usual tape.

The tape is in the form of a single continuous 19inch loop, each of which is guaranteed for 500 hours of operation; three are furnished with each machine and replacement cost is very small. In laboratory tests single loops have been run for over 1000 hours without audible degradation. A standard splice, made with pressure-sensitive splicing tape, is

unsatisfactory. The tape is therefore formed into loops by a welding process using a specially built machine.

Tape Drive The tape is driven by a small 4-pole motor within the box, whose steel shaft emerges as shown in Fig. 7-2. The rim of a rubber drive roller on an arm is very lightly pressed against the motor shaft by a wire spring, and the tape rides over a smallerdiameter section of the roller. For positive drive, the tape is pressed against the roller by a ball bearing at the end of another arm under spring pressure.

When the machine is not running, the pressure of the rubber roller rim against the motor shaft is light enough so that no permanent indentation is made in the rubber by the shaft. However, when the machine starts, the fat that the roller is pulling the tape

against some resistance tends to snub the roller against the motor shaft. The roller, in effect pulls itself up tight against the shaft. Also, as the motor shaft drives the rubber roller, a force is created which tends to swing the arm on which the roller is mounted toward the motor shaft, further pressing the roller and shaft in firm contact. This is a “selfenergizing” system, unique in tape mechanisms to our knowledge, which does away with any need for mechanical disengagement of the two when the Unit is shut off, and considerably simplifies the system.

A tension arm near the center of the mechanism is pivoted at one end and has a highly polished cylindrical surface at the free end. The weight of the arm pressing down on the tape provides just the right tension to hold the tape in form contact with the magnetic heads without pressure pads. After the tape passes beneath the tension arm, it rides over the inertia wheel at upper right. This precisely machined brass wheel runs free on its shaft and smoothes out any variations in speed. It is a flutter filter.

The tape runs over a record head, three playback heads, and an erase head, in that order, at a speed of about 12 inches per second. The separation between heads is such that a given point on the tape takes about 110 milliseconds to go from one head to the next. After passing over the rubber drive roller, tension arm, and inertia wheel, the tape again passes over the record head.

Circuitry Being essentially a tape recorder, though a very special one, the Unit requires the usual stages – record amplifier, bias-erase oscillator, a playback amplifier for each head – plus special stages, the electronic section employs 11 identical small-signal transistors, two power transistors, and a zener diode and four silicon diodes in the power supply. All the circuitry is built on a single printed circuit which may be seen in the rear-view photo of Fig. 7-3. the board even contains the regulated power supply (except for the power transformer), which furnishes only about 50 ma at 30 volts, about 1.5 watts of power for the whole electronics! Fig 7-4 is a complete schematic diagram of the REVERBATAPE Unit. An input level control (potentiometer 7) is provided on the printed board so that the Unit

can be adjusted to place maximum undistorted recording level on the tape, whatever the available input voltage may be, to achieve maximum signalto-noise ratio. The input control is in the collector circuit of input transistor 5, and the input system as shown can be adjusted for full recording when the input source maximum is as great as 3 volts or as little as 0.3. Actually, the stage has a 26-db gain, but the input resistor 2 results in a 26-db loss, giving in effect unit gain between input terminal and the collector. Resistor 2 provides a high input impedance for the system, useful for many applications.

However, if the available signal peaks at less than 0.3 volt, resistor 2 can be reduced so that maximum record level will still be reached. The sacrifice is, of course, input impedance, which comes down to a minimum of 22,000 ohms when resistor 2 becomes zero, as it will if the input has to be adjusted for an input maximum signal of as little as .015 volt.

The Unit also has a level adjustment (119) at its output (lower left in diagram). In most applications this is set so that output is the same as input, giving zero insertion loss for the Unit. However, since each control has a 20-db range, the Unit can usually be set to give either a loss or a gain if desirable.

The input stage has most of the emitter resistance bypassed (11) sufficiently to keep the response flat to well below the audible range. The 1,000-ohm part (8) of the emitter resistance is bypassed by 9, which is small enough to result in a response at control 7 which begins to rise at 2KC and reaches +12 db at 10KC. This preemphasis (which is cancelled in a later stage by a complementary deemphasis) is instrumental in reducing to a very low level the hiss at the output of the Unit.

The signal leaving the input stage is applied simultaneously to the output stages 114-118 and to the recording system. The signal passed to the output stages eventually emerges at the output unaltered in any way. This is called the direct signal and is the original sound of the program source.

The signal going to the recording channel is first passed through a voltage divider (13 and 14) before going through control A (one section of the external three-gang REVERBATAPE control) and finally reaching the base of transistor 19, the recording preamplifier. The purpose of the voltage divider is twofold: it adjusts the signal level to that required at

the base of transistor 19 and provides the proper source impedance for control A. The recording preamplifier stage 19 is virtually identical to the input stage. Capacitor 21 provides preemphasis as in the input stage, but for a different reason, to compensate for the high-frequency loss of the playback heads.

The output of transistor 19 is direct-coupled to the base of transistor 25, the recording amplifier.

This stage operates as an emitter follower with the record head (terminals R and R1) connected from emitter to ground through suitable series resistance 28 and 29. The series resistance is sufficient to cause constant audio current at all frequencies up to about 10KC, dropping off to -3 db at 20KC.

Bias & Erase The bias-erase oscillator is a single-ended circuit using power transistor 39 to provide sufficient current with low harmonic content.

There are two unusual points about this oscillator. First, the erase had (which is connected between the E terminals) is part of the inductance which, with capacitor 37, forms the resonant circuit which controls the oscillator frequency. (Capacitor 38 merely blocks d.c.) The Q is very high, which, in addition to stabilizing the oscillator frequency, tends to keep the waveform clean.

The bias voltage to be applied to the record head is available across the oscillator coil 34, which is also part of the tank inductance. The record head is connected across the coil through resistors 32 and

33 and capacitors 30 and 31. The resistors adjust the

bias voltage and the capacitors block the d.c. The bias-erase frequency is about 45KC.

Playback Amplifiers The three playback heads are connected to points P1, P2, and P3, the inputs to the three playback amplifiers. Since the three amplifiers are identical, only one need be discussed.

Consider the amplifier for the first playback head, transistors 48 and 58. The amplifier incorporates d.c. and a.c. negative feedback. The equalization necessary for the standard correction of the head playback characteristic is incorporated in the a.c. feedback connection 54 and 55.

The advantages of distortion reduction and increased input impedance are, of course, also gained. As a result, the base impedance of transistor 48 is about 20,000 ohms, above 1 KC. With the 90mh playback heads used, there is no high-frequency loss (due to the low-pass action of the head inductance and the 20,000-ohm base impedance) below 20 KC. The coupling capacitor 46 is large enough to allow flat bass response to below 20 cps.

The d.c. negative feedback is obtained by connecting resistor 53 from the emitter of transistor 58 to the base of transistor 48, and direct-coupling the two stages. Fifty-three and 49 forms a voltage divider which adjusts the emitter voltage of transistor 58 to that required at the base of transistor 48. Bypassing 59 and 60, besides eliminating all emitter degeneration in this stage, eliminates a.c. feedback from the d.c. loop.

The d.c. negative feedback method of applying the necessary biases uses few components, and such factors affecting bias as temperature and d.c. amplification can change in one stage and be corrected in the other.

Output form the amplifier is taken from potentiometer 57. The potentiometer makes it possible to adjust the amplifier output to accommodate the normal production tolerances in magnetic head sensitivities. This adjustment is made as a factory calibration.

High-frequency rolloff must be incorporated in the playback channels in order to provide a more rapid decay of high frequencies compared to the low frequencies in the reverberated signals. In a building highs are absorbed more rapidly than lows due to selective attenuation in the air and at the

reflecting surfaces. Resistor 63 and capacitor 64 produce a very slow rolloff amounting to about 6 db at 10 KC.

Mixing and Output All three playback amplifiers are identical. The output signal of the first is passed directly to the mixer, transistor 114, through resistor 106. The outputs of the second and third playback amplifiers are passed through reverberation control sections B and C and resistors 107 and 108 to the mixer stage.

Resistors 106, 107, and 108 prevent loading and resultant upset of the control tapers, which are critical, as well as isolation. The direct signal is also mixed into this stage through 105.

The mixer stage is basically the same as the input stage with the exception that high-frequency deemphasis takes place here to offset the preemphasis of the input stage. Deemphasis is caused by 112, which shunts the collector resistor at high frequencies.

The output stage of the Unit is emitter follower 118. This stage is directly coupled from the mixer stage, its stability being controlled by that of the well stabilized mixer. The output impedance of the Unit depends on the setting of the output control 119, varying from about 50 ohms to about 1000 ohms, low enough for all normal applications.

System Noise As is true of any good tape-recorder design, saturation of the magnetic recording tape in the REVERBATAPE Unit is the controlling factor in just how hard the input can be driven. Maximum signalto-noise ratio results only when the unit receives the highest possible input signal short of tape distortion.

Therefore, any overload of the electronics that prevents the fullest possible recording on the tape can only result in a lower signal-to-noise ratio.

In spite of the fact that the REVERBATAPE Unit has the peculiar characteristic of having the noise output of three tape recorders, but the net audio signal of only one, plus rerecording feedback, the signal-to-noise ratio is about 50 db below maximum undistorted output at maximum setting of the external control, very comparable to the ratio specified for high-quality broadcast tape recorders.

At a more normal setting, the ratio is about -53 db;

with reverberation off, about -55 db. These readings include all audible hum and noise below 20 KC.

Control System The control methods usually used in artificial reverberations systems – especially the spring units – are unrealistic. Usually there is a direct channel, through which sound goes without added effect, and a parallel reverberation channel, in which reverberative repetitions are added to the original.

These are combined at their outputs. To reduce reverberation time, a control simply reduces the gain of the reverberation channel, so that relatively more direct sound is heard.

Fig. 7-5 shows the result. With full reverberation setting, the direct sound is represented by line A. The sound source ceases (the player releases a key) at time X. Then the reverberation channel causes a slow decay along line B. When the reverberation channel is turned down, the direct sound A stops at time X. There is then an abrupt drop in level (Y), after which the decay occurs at the same rate ( C) as before. No real auditorium does this. Instead, the decay always begins smoothly just when the sound source is removed. Fig. 7-6 shows this effect at various total reverberation times. Each decay curve B, C, D, E, is a smooth one beginning exactly at point X, without any abrupt drop after cessation of source. The decay slopes are, of course, different. To accomplish this necessary bit of realism in the REVERBATAPE Unit a new kind of attenuation system had to be designed.

The REVERBATAPE Unit is so calibrated that when the external control, Fig. 7-7, is at maximum, the output of the Unit due to signal from the first playback head is about 1 db less than the output due to the direct signal. Output due to the second head is 1 db less than that, and signal due to the third head is still another decibel lower. Output of the third head is also fed back to the record amplifier through resistor 104, Fig. 7-4, and re-recorded, so that this cycle continues – each succeeding repetition 1 db less than the last.

With this control setting, and with the repetitions spaced 0.11 second apart, it takes 60 repetitions or 6.6 seconds for an attenuation of 60 db, and this is the maximum reverberation time of the device. In testing, the maximum reverberation time of the unit

may not seem this long unless the original sound is loud enough so that after 6 seconds the sound, which is now down 60 decibels, can still be heard!

The ear actually senses the rate at which the sound

is decaying.

Faster music, or music requiring greater clarity, however, requires shorter reverberation times.

Those are obtained when the control is rotated counterclockwise by increasing the attenuation between played-back repetitions. For example, when the setting gives 2 db between repetitions, only 30 repetitions or 3.3 seconds, is needed to reduce level by 60 db. He control is continuous from maximum down to zero. The following explanation of hit it works requires careful attention to avoid confusion.

Maximum reverberation time is obtained with the sliders of controls A, B, and C all the way up.

(These controls are ganged on one shaft and available to the organist.) Control A then sends full signal to the recording amplifier 25. Signals from the three playback amplifiers join at the base of transistor 114. With the Control all the way up, the calibration potentiometers 57, 76, and 95 have been so set at the factory that the signal reaching 114 due to head 2 is 1 db less than that due to head 1; and

the signal at 114 due to head 3 is 1 db less than that due to head 2. In other words, the outputs due to the three playback heads are 1 db apart.

Let us specify certain terminology. Let us always refer arbitrarily to the level of signal appearing at the Unit’s output terminals due to passage through the direct channel and not from the recording system as 0 db. With the Control at maximum the three calibration potentiometers 57, 76, and 95 have been adjusted so that signal appearing at the output terminals due to playback head 1 is at a level of -1 db, that due to head 2 at -2 db, and that due to head 3 at -3 db.

Signal from playback head 3 not only goes through resistor 108 to join the other signals at the base of 114; it also goes through resistor 104 to control A for re-recording or feedback. This signal appears at the control along with the signal from the Unit’s input. However, the signal appearing at the control from head 3 is 3 db lower in level than the signal due to the device input. This 3-db-less relationship, which is determined by choice of the components, is always present. Since the signal obtained at the base of 5, recorded and played back through head 1, resulted in a system output of -1 db, the fed-back signal produces a system output due to

head 1 which is 3 db less than that, or -4 db. This is 1 db less than the output due to the original head 3 signal, and it continues the scheme of continual 1db apart repetitions.

We may also consider that, due to the setting of calibration potentiometer 76, the output of head 2 has a permanent 2-db attenuation, and that of head 3, due to the setting of 95, has a permanent 3-db attenuation. Adding 3 db to each of these for the second round of signals produced as the result of the feedback gives 3 db less than the original -3 for head 2, or -5 and 3 db less than the original -3 for

head 3, or -6. Thus action continues repetitions at the output always 1 db apart.

So far the three ganged control potentiometers have always been at maximum. Before moving them, we must explain that they have special tapers, so that in any position the attenuation given by control C is always twice that (in decibels) of controls A or B, which are identical. Thus, if the control is set so that A attenuates its signal 3 db, then control B has 3 db of attenuation and control C attenuates by 6 db.

Let us now reduce the control setting so that A and B have 1 db of attenuation each – and C has 2 db.

The input signal passes through control A and suffers 1 db of attenuation. The output of head 1 is thus 1 db less than with the control wide open.

Instead of appearing at the system output at -1, it appears at -2.

The output of head 2 is also 1 db less than before due to control A. It also suffers another 1 db of attenuation in control B, making a total of 2 db.

Therefore, instead of being at -2 as originally, it is now at -4.

The output of head 3 is also 1 db less that before due to control A. It also is attenuated 2 db more by control C, making a total of 3. So instead of appearing at the output at -3, it appears as -6.

You will note, therefore, that at this control setting, the system outputs due to the three heads are respectively -2, -4, and -6 with respect to our standard 0-db direct channel. The reverberation time is reduced, but the decay curve, composed of equal decrements in repetitions, is still linear.

Now let us consider the fed-back signal with this control setting. Since the signal to the recording head is attenuated 1 db by control A and control C attenuates the head-3 signal by 2 db more, the fedback signal arrives at control A 3 db lower than it was before. In going to the recording amplifier it passes through control A, for 1 more decibel of extra attenuation. It therefore comes to the record amplifier attenuated by a total of 4 db more than it did when the control was wide open. When the control was wide open, the fed-back signal produced a system output of -4 db due to head 1.

Subtracting 4 db from that, the fed-back signal with the new control setting due to head 1 is -8 db, which continues the cycle of 2-db decrements as advertised. For a little mental exercise, you can

postulate other control settings and additional feedback cycles; you will find that decrements between repetitions are always equal!

Control rotation does not produce linear increments of reverberation time; there is a special over-all time-rotation taper. The numbers on the control dial do not indicate seconds of reverberation time; they are merely reference numbers. As the control is advanced from 0, reverberation time increases rapidly up to a setting of about 1½ or 2 on the dial. As the control is advanced beyond about 2, reverberation time increases much more slowly.

The number-2 setting on the dial provides about the minimum reverberation necessary for any effect at all. Thus, virtually unusable reverberation times are relegated to a small portion of the total control rotation while those which are the most useful are spread out over the greatest part of the range.

Reverbatape Installation The REVERBATAPE Unit is essentially a voltage amplifier stage, usually of unity gain, with high-impedance input and low-impedance output. It can therefore be inserted in any audio system where these conditions are appropriate, and where the available signal voltage maximum is something between 0.3 and 3 volts. (as indicated earlier, it can accommodate input signal maximums as low as .

015 volt, but for this input the output voltage maximum is still 0.3 volt, and external attenuation may be needed.)

Schober Organs end in a preamplifier or MixerCompressor output of low impedance, which is normally connected to the input of a power amplifier. The REVERBATAPE Unit is usually connected between this organ output and the amplifier. Other organs and other kinds of audio systems are made up differently, and sometimes the best place for Unit insertion is between a pair of stages. The Unit is used extensively for other organs and a simple installation system, usually involving connectors and tube-socket adapters has been worked out to fit each individual make and model.

It is also used for recording and some home audio systems. One version called the RV-3C, sold through dealers for multi-channel organs, includes a power amplifier and speaker system, with everything mounted inside the speaker enclosure. It has six inputs so that as many channels can be reverberated, and the direct channel is removed from the Unit so that direct sound continues to come from the organ’s individual speakers.

REVERBATAPE kit builders can obtain on request full information about an installation of any kind.

After any installation is made, the Unit’s input and output controls, available through holes in the front panel, must be adjusted. The input control is turned up so that the loudest possible signal just fails to cause distortion; and the output control is then adjusted for the desired volume level. These adjustments are made by ear and require no special training or knowledge. Assembly of the kit, too, in common with all other Schober kits, is open to anyone and involves no previous skills.

Chapter 8 THE SCHOBER PERCUSSION GROUP It has long been the Schober design creed to concentrate design efforts and customers’ money principally on organ voicing in terms of regular organ stops. Voicing makes or breaks an organ and determines whether or not it will give long-term satisfaction. Special effects and other gadgetry, by comparison, are at best unnecessary and at worst can – and often unfortunately do – use up so much production cost that not enough is left for the wide variety of distinctive and useful voices which are the backbone of the instrument. Things like sustain, percussion, glide, and cow moos often impress the prospective customer who is unfamiliar with organs, but they do not an organ make!

Some special effects, however, can be genuinely useful and, in contrast with others, easy enough to use so that the usual amateur player gets his money’s worth from them. If they are available on an organ which already has good voicing, so that their presence subtracts nothing from the organ basics, they are well justified. The Schober Percussion Group falls into this category by satisfying two primary requirements; (1) the percussion effects sound genuine and are musically pleasing and useful – and they do not require any special playing techniques; and (2) THE Percussion Group is optional equipment which can be added at any time, so that 100 percent of the money paid for the organ proper buys real, basic organ benefits.

A rather simple Percussion Kit has for many ears been available for each Schober Organ at a relatively low price. With the advent of the Schober Theatre Organ, however, the new Percussion Group was introduced. Recognizing the growing interest in theatre-type instruments, and taking advantage of the versatility of transistor design (in which Schober has been one of the leaders), Schober engineers spared no effort in making the new persuasions completely authentic and as versatile and satisfactory in actual use as were those found on the theatre pipe organ. The result is one of the two or three finest Percussion Groups in the entrie organ industry. The new Percussion Group can be added to any Schober Theatre, Recital, or Consolette II

Organ, and owners of older Schober transistor organs can use it to replace the earlier percussion kit, which has been discontinued.

What it Does The Percussion Group includes eight special and separate percussion voices, each in one of two pitch registers (8′ or 4′Piano, Chrysoglott (struck metal bars with resonator pipes in the theatre pipe organ), Celesta (higher-pitched and brighter metal-bar sounds), Orchestra Bells, Reiterating Orchestra Bells, Harpsichord, Mandolin, and Xylophone. The Percussion Group covers four octaves, all but the lowest octave of the upper manual.

For the “normal” percussion voices, pressing a key causes the “instrument” to make its percussive strike, and the sound dies away at whatever decay rate is appropriate for the particular voice. The decay begins immediately, regardless of whether the key stays down or is released, exactly like the real instrument. (The effect is not like that of the common “sustain,” where the decay begins only when the key is released.) Thus a percussion voice can be combined with an ensemble of regular organ stops, striking when the keys are first pressed, then dying away and leaving the sound of organ stops as the keys are held. Each key has its own keyer circuit, so that striking or restriking one key has no effect on any others being held – again just as in the real instrument.

Three of the voices reiterate – repeating constantly instead of decaying and staying silent.

These are the Mandolin (simulating the usual strumming), the Xylophone (the effect of two hammers striking the bars alternately at a rapid rate), and one of the Orchestra Bells stops (alternate-hammer striking to obtain a ringing effect).

Two of the voices have different decay rates which depend on whether keys are released or held down – Piano and Harpsichord. On these instruments there is a felt damper for each note. If a key is held down, the sound decays quite slowly; when the key is released, the damper falls on the string

and silences it almost at once. A Piano or Harpsichord could not be played normally without the "“damper effect,” but with it the realism of the sound, especially that of the Piano, is truly amazing.

On the Schober Theatre Organ, eight stop tablets are provided for the Percussion Group; on the Recital and Consolette II Organs, percussion voices are selected by a 9-position rotary selector switch, which includes an off position. The circuitry is the same for all three organs, but some values and voltages are different. In this chapter, we shall refer specifically to the kit for the Theatre Organ.

Component values for the other two instruments can be obtained from the instructions which accompany the Percussion Group Kit for each organ.

Key Switches One normally open switch is required per key to transmit d.c. keying voltage to the corresponding keyer. The switches are shaped silver wire fingers mounted on four small printed circuit boards, 12 switches to a board. Fig. 8-1 shows how these switch boards are attached to the front apron of the steel upper keyboard chassis. When a key is pressed, the key, which is a steel channel to the top of which a plastic shell is attached as a playing surface, comes down on the slanted upper part of the contact spring, pushing it aside and making a

positive wiping contact. The keying voltage source

is connected to the keyboard chassis, so that pressing the key places the voltage on the silver contact.

Each contact must be wired to one of the keyer circuits. The 48 wires from the contacts are collected into cables and led back under the treble end of the keyboard chassis to the inside of the organ.

A unique method of attaching these key-switch boards to the chassis involves small aluminum plates and special foam rubber strips with pressuresensitive adhesive on both faces. Once the switches have been attached, they cannot come away accidentally, but any of the four boards can be taken off with removal of a couple of machine screws in case of necessity. Putting the boards in place does not require any organ disassembly except temporary removal of the decorative wood trim strip under the manual, a matter of less than a minute.

Keyer Circuits The heart of the system is the series of 48 keyer circuits, one for each note. Each key circuit produces tone output from the main organ generators when a key is pressed, and the amplitude envelope is whatever is required or the particular voice. Two separate outputs are produced for each keyer, 8′ and 4′.

Fig. 8-2 is the schematic diagram of a single keyer. Tones of the correct pitches are permanently connected to terminals 4 and 8 (T is simply to facilitate interconnections), and thus through resistors 43 and 83 to the bases of the transistors.

KC is the source of B-, applied from a special percussion voltage regulator. Outputs from the transistors are taken from the lower ends of emitter resistors 41 and 81.

Each transistor is an emitter follower which is biased to cutoff by a negative voltage applied at point KB. This negative voltage passes through a diode 42 or 82, so that in addition to bringing the transistor to cutoff, it also places a low resistance – the forward-biased diode – between emitter and the bias supply, which is bypassed heavily to ground.

The low diode resistance shunts the emitter to ground when the transistor is at cutoff, so that any leakage from base to emitter is effectively shorted out, and the outputs are quite silent. This diode method of killing leakage is also used in the Theatre

Organ coupler circuits (page 34). Leakage output from any keyer with this system is on the order of 100 db below normal output in the on condition, an unusually high figure for a solid-state keyer. It makes unnecessary any high-frequency rolloff it kill leakage, which would also affect the voicing.

To turn the stage on and transmit tone to each output, it is only necessary to apply negative voltage to the bases. As soon as current flows in the transistor and the emitter resistors, which lead through an external network to ground, the voltage drops from emitters to ground place negative voltage on the diode anodes. This quickly becomes greater than the KB bias voltage and biases the diodes off, so that they no longer short-circuit the output signals.

This negative base voltage is supplied by the key switch through the remaining components in Fig. 82. the voltage actually keyed in the Theatre Organ is -14, and it is applied by the key switch to terminal K. Basically, the keying voltage must produce a pulse which charges capacitor 1. The pulse then disappears. The charge on capacitor 1 is applied to both bases, turning on the transistors and producing outputs at maximum amplitude. The pulse having disappeared, capacitor 1 then discharges gradually.

The resulting gradual decrease of negative voltage at the bases causes transistor conduction to decrease, until finally nothing is left and the transistors are again off.

The next trick, then, is to transform the steady voltage applied by the key switch to point K into a pulse, so that even if the key is held down, capacitor 1 will begin to discharge immediately after having charged.

The applied negative voltage passes through resistor 5, which is too small to affect it, and is applied to capacitors 11 and the diode-7 circuit in series to ground. This creates a negative pulse across resistor 7, which is applied through 6 to charge capacitor 1.

The capacitor charging takes place very quickly.

Once 11 has charged, it remains charged and can transmit no more negative voltage to 1; in effect it is an open circuit as far as 1 is concerned, and its left plate is positive, so that diode 6 also isolates capacitor 1 from 11. Diode 3 has a relatively high resistance in series (9) and is also biased nearly off; so that neither of these diodes passes enough current to hinder the charging of 1.

As soon as capacitor 1 has charged and further charge from terminal K is impossible, 1 begins to discharge through resistors 45 and 85 and the transistor bases, and also through the diode-3 circuit. When it finishes discharging, the outputs are silent.

Whenever the key is released and negative voltage no longer applied to K, capacitor 11 discharges quickly through diode 2 and resistor 10.

And whenever the key is pressed again, the action takes place anew, recharging 1. The discharge of 11 is fast enough so that quick repeated strikes of the key will recharge 1 each time, allowing the player to repeat notes as fast as he wishes.

The main controlling factor on the discharge rate of capacitor 1, and thus on the duration of the audio decay, is the diode-3 circuit, known as the shortener. (There can be no discharge through anything to the right of diode 6 because the negative discharge cannot pass through 6.) For the voice requiring the longest decay, the Chrysoglott, diode 3 is biased nearly off by a negative voltage applied to its anode, point SH. Capacitor 1 can therefore discharge through diode 3 at the start of its discharge, when its negative charge is high enough to overcome the external off bias of 3, but soon that negative discharge voltage is insufficient and diode

3 opens, requiring 1 to discharge through 45, 85, and remaining incidental resistances.

The Orchestra Bells require a somewhat shorter decay time. To achieve this, the external circuitry places a lower off bias on diode 3. Thus more of the discharge of 1 can occur before diode 3 opens. Two still shorter decay times are required for other voices, and for each the off bias in 3 becomes less, so that even more of the discharge of 1 can take place before 3 opens.

In addition to controlling total decay time, this system adds realism to the percussion. Real percussion tones do not have the simple exponential decay of a capacitor. They decay relatively quickly right after the initial strike, and then the decay rate slows down. If this were not true, it would not be possible to play fast repeats on one note; so little decay would have taken place by the time of the next repeat that there would be no detectable sound of individual notes.

The action as thus far described is what takes place for all voices except Piano and Harpsichord – that is, all voices for which the decay must be the same whether or not the key is held down. As can be understood, holding the key has no effect on the tone so far. Once capacitor 1 has been charged, all further influence of everything to the right of diode 6 is removed, and holding or releasing the key merely determines whether and when 11 is allowed to discharge.

For Piano and Harpsichord, the action must instead depend on whether or not the key is held down. To bring this about, using the Piano or Harpsichord stop tablet places a cutoff bias on point PB so that diode 7 no longer conducts. It also removes nearly all of bias from point SH so that diode 3 conducts whenever there is a negative voltage at its cathode.

Capacitor 11 cannot now charge through resistor 12. It must instead charge through resistor 9, which is of much higher value, and as a result, it charges much more slowly. Assume that the key is held down. Negative voltage is applied to K, and 11 begins to charge. Initially, as in any capacitor charge, there is a maximum rush of electrons from the positive plate of 11 to the negative plate through resistor 9. This causes a maximum voltage drop across 9 and of course capacitor 1 assumes a like charge, since it is too small with respect to 11 to

have any appreciable effect on 11. It does, in other words, what it is told to do by the drop across 9.

As the key continues to be held, the charge on 11 continues to accumulate, but of course as time goes on the electron transfer between plates assumes a slower and slower rate in an exponential fashion.

The drop across 9 thus becomes less and less and so does the similar charge on 1. Since the charge on 1 determines the base voltage on the transistors, the sound follows the action, being maximum at key closure and slowly dropping off as 11 charges.

Eventually, 11 becomes fully charged and no further drop occurs across 9, so the audio stops.

In actual fact, off bias is not fully removed from diode 3, only reduced to a relatively low value.

Therefore, while the charging action of 11 is as described, a point is reached when 11 cannot complete its charge through 9. The charge can then be completed only principally through capacitor 1, which is at a considerably slower rate. Thus the circuit fills the requirement of a fairly rapid tone decay immediately after the strike, but a very long decay from there to zero – the sound of a typical piano tone.

Resistor 9 is graded in value from bass to treble.

The 150,000-ohm value is used at the bass end. It results in the longest charging time for 11 and the longest tonal decay, typical of a piano in the low registers. The 56,000-ohm value is used at the treble end and hastens the charge of 11, shortening the note decay because piano notes do decay more quickly near the top. This grading has very definite effects on the piano because of the way resistor 9 affects the charging of 11 for Piano and Harpsichord. The Function of 9 for other stops is to control the discharge of 1, and in that role the value differences have little audible effect.

What happens with Piano and Harpsichord when the key is released? Capacitor 11 discharges quickly through 2 and 10, just as with other voices and 6 prevents this from having any effect on 1. When the key is lifted, therefore, all special “piano” action ceases and the circuit acts as with other voices.

Whatever charge happens to be on 1 is dissipated through 9; and since 3 is almost fully conducting, this discharge is very quick. So, lifting the key silences the piano as though the damper had fallen on a string. The “damping” can, of course, take place at any time; either just after the piano tone has begun and is still loud, or after the key has been

held for some time and the tone has decayed considerably. If the key is held until the sound has decayed entirely, lifting it does nothing except ready the circuit for a new strike.

In each of the four keyer boards, several busses run the length of each board. The shortener bus is connected to all the SH points and carries the voltage which controls the conduction of diodes 3.

The piano bus connects to all points PB and carries the voltage which turns on and off all diodes 7. The keyer collector bus brings B- to all transistor collectors, points KC. The keyer bias bus carries transistor off-bias voltage to all diodes 42 and 82, points KB. And there is a ground bus. All of these busses are simply connected together from board to board so that 48 keyers are connected to each bus.

All these busses derive their voltages from circuits on the main percussion board, described later.

Keyer Outputs Each keyer board also has four output busses, two each for 4′ and 8′ tones. The 8′ outputs of the lowest six keyers are connected to one bus and their 4′ outputs to another. Similarly, the 4′ and 8′ outputs of the upper six keyers on the board connect to two additional busses. For the whole four keyer boards, therefore, there are 16 output busses, and for each of the two pitch registers the eight output busses are combined through a special network, one of which (the 8′) is shown in Fig. 8-3.

The four keyer boards are indicated by the four dashed-line boxes, and each board is identified by a letter – W for the board controlling the lowest octave and Z for that handling the treble octave. The eight up-pointing arrows indicate connections to the eight 8′ output busses. The resistors amount to a special kind of ladder-type mixing network, used

for scaling.

Just as in the keying system of the organ itself, tone-coloring filters with a high-frequency rolloff

must be fed tones with the higher-frequency tones emphasized to offset the filter rolloff, in order to maintain a reasonably good final balance between bass and treble. In Fig. 8-3 it is obvious that if total output were taken from the arrowhead to the right of the number 11237, the F#-B tones from board Z would be the loudest, while those from the C-F of board AW would be the softest. This output is known as the 8′ treble output because its treble tones are the loudest. An output taken from the arrow to the left of the word TO, on the other hand, is exactly the reverse, with bass tones loudest. It is called the 8′ bass output. Another complete network like that of Fig. 8-3 is present to handle 4′ tones.

Piano and Shortener Control We pass now to the main percussion board, which contains all the other elements of the system except the stop tablets. Each tablet operates a single-pole, single-throw, normally open switch.

When the switch is closed, -30 volts (in the Theatre Organ) is applied to one of eight terminals on the board. While there is only one terminal for each stop, the applied voltage does several things. In breaking down the total diagram for easier explanation, we have repeated these tablet terminals, each of which is labeled with two code letters indicating a percussion stop. Here is the key: CA: Celesta CT: Chrysoglott MN: Mandolin OB: Orchestra Bells OR: Reiterating Orchestra Bells PF: Piano HD: Harpsichord XP: Xylophone

Throughout the board a system of diodes is used, which, once explained, need not be referred to again. The voltage obtained by turning on a stop is used for several purposes – connected to various points. To many of these points come voltage from several stop tablets. To prevent voltage applied by turning on one stop tablet from getting back through

a common point into circuits meant only to be energized by other tablets, diodes are inserted in series wherever they are needed. Each diode has its cathode in the direction of the stop-tablet terminal which is meant to cause voltage to be applied through the diode. Thus, Voltage from the proper tablet will pass through the diodes to the intended points; but voltage from other tablets cannot back up through the diodes and get to verboten places.

Fig. 8-4 shows the circuits which control the voltages on the piano and shortener busses of the keyers, as well as the method of creating the repeat or reiteration effect.

Transistor 264 is the piano bus control. The transistor is normally cut off because no negative bias is applied to its base. The PB connection to the collector, therefore, carries approximately the same B- voltage as is applied to collector resistor 266, since there is almost no drop across 266. This is the

condition of the stage when the Piano or Harpsichord is in use, at which time the relatively high negative voltage at PB biases diode 7 in Fig. 8-2 to non-conduction.

When any of the other six stops is on, tablet voltage applied to one of the terminals shown is applied through voltage divider 263-265 to transistor base.

This causes the transistor to conduct heavily. The resulting voltage drop across 266 lowers the pianobus voltage drastically, so that diode 7, Fig. 8-2, conducts most of the time and capacitor 11 can charge through it. (While terminals of only five stops are shown, the one for OR is connected directly to the OB terminal through a diode elsewhere on the board, so that when OR tablet goes down all the OB circuits are also energized.)

Control of the shortener bus, terminal SH, is obtained in the same way, except that it has a number of degrees. Note that SH is connected to the collector of transistor 279, which derives B- through 280. The collectors of 269 and 274 are also connected to the 279 collector through their own collector resistors.

If all three transistors are cut off, as they normally are, there is no voltage drop across 280, so that almost full B- is placed on SH and all diodes 3, Fig. 8-2, are cut off soon after capacitor 1 begins to discharge. This gives the longest decay time. When the Chrysoglott tablet is used, the situation stays this way; there is no shortening, since the Chrysoglott requires the longest decay time.

When the Xylophone tablet (XP) is turned on, however, -30 volts is fed to the base of 279 through 277, and 279 conducts heavily. The drop across 280 then reduces SH voltage to its minimum, so that keyer diodes 3 conduct until capacitor 1 is well along in its discharge, giving the shortest decay time. This short time is also needed for Piano and Harpsichord, so the HD and PF tablets also feed voltage to 279.

The next longest decay time is required for Mandolin and Reiterating Orchestra Bells.

Terminals OR and MN therefore cause transistor 274 to conduct. The conduction of 274, however, is limited by resistor 275 and not so much current is drawn through 280, so that the SH voltage is not reduced so much as when 279 was energized.

Diodes 3 (Fig. 8-2) become open circuits sooner during the discharge of capacitor 1, and the audio decay is lengthened.

In the same way, the Orchestra Bells stop requires a still longer decay, so it energizes transistor 269, which has a larger collector resistor and causes still less voltage drop across 280.

Keying and Repeating Two different kinds of keying voltage must be applied to all the terminals K in Fig. 8-2—that is, must be supplied to the keyboard chassis to be picked up by the key switches of Fig. 8-1. The first is, of course, a simple, steady d.c., which causes the keyer to strike and decay as we have described.

However, certain of the stops (Xylophone, etc.) reiterate as long as a key is held down. For these stops, the keying voltage must be intermittent – on and off at a selected, rather rapid rate, just as if the playing key were being truck repeatedly. A method for supplying steady d.c. is, of course, obvious. And the intermittent keying voltage can easily be supplied by a multivibrator – but there is one important complication.

Suppose that the intermittent keying voltage is being supplied by a multivibrator. If the organist plays very slowly and connectedly (a musical legato) there would be little trouble, even if at the instant he struck a key there did not happen to be a simultaneous keying pulse from the multivibrator.

However, suppose he is using the Xylophone and is playing a rapid passage, some of the notes only long enough for one strike of the Xylophone. If the keying pulse does not happen to be present at the instant he strikes each key, some notes are lost, and the effect is intolerable. There must, therefore, be a means to “tell” the multivibrator each time the organist has touched a key, and to force the multivibrator to produce a keying pulse at just that moment – and then go on repeating the pulses. Or if some notes are being held and a new note is added, the multivibrator must, if necessary, stop itself in mid-stride and instantly supply an “on” pulse so the first strike of the new note will be heard exactly when the organist intends it to be. This calls for a little finagling, and involves transistor 241 and all those to its right in Fig. 8-4.

In the first place, there must be no keying voltage available until a stop tablet goes down. If there is, the 48 keyer circuits will always be

operating, so that if keys are pressed and released and a percussion tablet then turned on, the tail end of the decay of the last few notes played will be heard – which is highly disturbing to the organist.

To control this factor, B- is normally not connected to 223, a transistor which is essential to either steady or reiterative keying. When a tablet goes on, voltage is applied through diodes to divider 284-285 and thence to the 223 collector circuit.

Only three special diodes, 281, 282, and 283, are needed for all eight stops, since they make their pickup from three points already supplied by all stops.

The multivibrator for reiterative keying is transistors 240 and 233. The emitter of 240 is connected to ground through 241, which acts as a switch. With none of the reiterative stops on (OR, MN, or XP), 241 is off and the multivibrator is dead. When one of these stops goes on, negative voltage is applied to the 241 base, the emitter of 240 is connected to ground through the heavy conduction of 241, and the multivibrator multivibrates.

When the multivibrator is off, B- (B1 in the diagram) goes to both the base and collector of 233, producing a fixed collector voltage, which is applied through 224 to the base of 223. Voltage divider 221-222 in the 223 collector circuit applies a fixed voltage to 219, a d.c. emitter follower, and its emitter voltage, passed through the red-blue winding of the transformer, goes to the keyboard chassis and becomes the steady keying voltage. The Transformer winding has such low resistance and inductance that it is simply a dead short for this purpose. None of the remaining transistors do anything.

When a reiterating stop is switched on, the multivibrator starts and keeps going at its regular rate, determined by the setting of the speed control 236 – until a key is pressed.

When a key is pressed the multivibrator is in one of its two states. If the collector of 233 is at maximum positive, the emitter of 219 is negative, and since a negative voltage is needed to work the keyer, all is well.

However, if the 233 collector happens to be at maximum negative, things are not so good, since a positive pulse at 219’s emitter will not operate a keyer. Nonetheless, some current is still drawn through the key switch, and thus through the red-

blue transformer winding, by resistors 5 and 10 in the keyer. The resulting transformer current induces a voltage of opposite phase in the yellow-green winding. The negative pulse thus placed on the base of 223 is amplified by 223, 227, and 229 and fed back to the junction of 231-232, where it appears as a positive pulse. That is enough to make the multivibrator flip and start again with the correct polarities to key the new note. The action is like that of a marcher who finds himself out of step and with one practiced hop change phase and comes in step again. Because of the direct coupling and lack of capacitors, the corrective action is effectively instant.

The benefits are very great. Typical Xylophone playing, for example, may well alternate between held notes, with the continuous repeat, and little staccato ones just long enough to hear a single strike. The player makes no adjustment, either of the control settings or of his playing technique. A staccato touch produces single hammer strikes; a legato touch produces repetitions.

Bus Amplifiers Let us return now to the audio signals. When last

seen, they were available at the two ends of the mixing networks of Fig. 8-3 – a treble and bass signal for each of 8′ and 4′ registers. Fig 8-5 shows where they go next. These are the bus amplifiers on

the main percussion board. They are very much like the bus amplifiers in the Theatre Organ itself, and

they even include woodwind circuits – which we will not bother to explain, since you can find them explained on pages 13 and 34. The four inputs, 4T, 8T, 4B and 8B (8′ and 4′ treble and bass), appear at the top terminals. Six outputs appear at bottom 8′ and 4′ treble and bass signals, plus 8′ treble and bass woodwind tones with the hollow sound characteristic of missing even harmonics.

Gates and Filters As in the organs themselves, the tone colors of the percussion stops are determined by formant filters, the theory of which (page 10) is not different from that governing ordinary organ sounds.

There are six voicing filters (Fig. 8-6) for the eight stops. The Orchestra Bells are produced by the same filter whether reiterating or not, and differ only in that the reiterating version has a faster decay so that the repeats can be heard cleanly. The Harpsichord operates like the Piano; these differences are taken care of in the control circuits of Fig. 8-4.

Each filter is preceded by a transistor-diode gate, which admits tones to the filters or excludes them, just as the organ stop tablets turn the filters on and off. The electronic gates are more appropriate here, since the stop switch has so many things to do in addition to controlling the filters.

All gates operate the same way, and transistor 71 is typical. The transistor, an emitter follower, does not conduct because no forward bias is applied to its base. In addition, an off bias (C-) is applied through a diode to its emitter. This diode functions exactly like those in the keyer circuits to eliminate leakage.

When the Chrysoglott tablet is pressed down, negative voltage is applied to the transistor base through a time-constant filter 72-32-74. The filter eliminates the click or pop that would accompany instant turn-on.

Tone from two of the bus amplifiers is applied to the transistor base. The Chrysoglott is a bland sound and so has a low-pass filter; its principal tone is taken from the 8′ treble woodwind but amplifier.

However, it is not necessary in each case to choose either treble or bass tones. They can be mixed in whatever exact proportions give the best scaling – the most even volume from top to bottom – for each stop. Thus, some bass 8′ tone is also mixed in, though through a much larger resistor 70.

The emitter of 71 feeds tone to a 4-section lowpass filter, the output of which is connected in common with all the other filters and sent to a 2stage final amplifier, transistors 211 and 215. The output is emitter follower with a volume control 216, which the organ owner sets to this taste to balance the percussion stops with the rest of the organ.

The shunt load on the stop filters, in addition to the base of 211, is a pair of resistors selectable by a 3position Percussion Balance switch on the stop

board (on the Theatre Organ only). This gives the player additional flexibility in balancing percussive sounds with regular stops. This control is not standard for the Consolette II and Recital Organs, but the owner can easily add it with a couple of resistors and a locally purchased 3-position switch.

Shielded cable should be used between the BS-SS terminals on the board and the switch.

Output Connection The output terminals in Fig. 8-6 are PO and OS (the latter for the cable shield). The percussion can, of course, be connected to the organ preamplifier, all of which in Schober Organs have suitable terminals. However a standard component of the Theatre Organ is the Mixer-Compressor (page 75), and it should be used with the Consolette II and Recital as well.

Forgetting for the moment the compressor section of this device, its mixing action is almost indispensable. If the percussion tones are mixed into the preamplifier and the latter is connected to a REVERBATAPE Unit and/or the Leslie/Organ Speaker LSL-150, as is so common for Schober Organs, the Leslie will cause vibrato in the percussion, destroying the authentic effect, and the REVERBATAPE Unit will lengthen the decays and make them indistinct and unnatural. The MixerCompressor provides a way of connecting percussion to the power amplifier after the REVERBATAPE Unit. It also provides an output from which percussions can be connected to a separate amplifier-speaker system so that percussions do not go through the Leslie speaker. At the same time, the 2-gan potentiometer supplied gives control of percussion volume right along with that of the organ stops.

Power Supply The Percussion Group does not have its own power supply, but a section of the main board is devoted to a simple power control system, diagrammed in Fig. 8-7. B- from the organ power supply is connected to the B- terminal. The KC terminal and point B1 carry a resulting voltage which is regulated against changes in load by the power transistor 288 acting as an emitter follower.

KC supplies collector voltage to all the keyers. It requires regulation because the load varies

according to the number of keys being pressed. The regulator has no voltage reference source and does not therefore supply a particular voltage of its own

choosing. However, the organ power supplies are closely regulated, so the voltage produced by the divider action of 286 and 287 is very nearly the same from one organ to the next. B1 is the Bsupply for the keying control circuits of Fig. 8-4.

This, too, should not move as the load is varied by keying and by the multivibrator.

The remaining voltages are obtained by simple divider action and need no regulation because they supply reasonably fixed or small loads. B2 is for the output amplifier, bus amplifiers, and filter gates. Cis the off bias applied to the gate diodes (Fig. 8-6).

B3 is B- for the shortener and piano control transistors (Fig. 8-4), and KB is cutoff voltage for the emitter diodes on the four keyer boards (Fig. 82). The latter is adjustable, as the keyers must just exactly be cut off for minimum leakage, but the cutoff must not be overdone as that would shorten all the decays. Control 299 is adjusted by ear – turned slowly clockwise until all percussion tones just disappear when no keys are pressed.

Chapter 9 THE MIXER-COMPRESSOR One of the distinguishing quality features of the Schober Organs is that stops add as they do in a pipe organ. That is, if one or more stops are already on, turning on one or more additional stops contributes to the total loudness and adds the voices of the new stops to the existing ensemble (unless of course, the new stops are so low in inherent volume that they are drowned out). This is in contradistinction to what happens in some electronic organs, where interaction exists between stops – turning on more stops loads down some circuits and does not cause a proportionate rise in total tone, and the total tone color becomes a single net “voice,” rather than simply a combination of all those on, with each one separately audible.

The fact that added voices do not change the characters of themselves or others already on is definitely desirable. However, the progressive rise in volume as stops are added to on ensemble is not always an unmixed blessing, particularly in rooms smaller than auditorium size, and especially in homes, where the limit on maximum volume is

often not the organ or the amplifiers, but the family and the neighbors. When the internal controls are set so that maximum volume, with loud stop combinations and the swell shoe full on, is as much as the amplifier, speaker system, and nearby humans can take, the soft stops are sometimes less than one would like. In common audio terms, the dynamic range of the organ is too great.

The problem of dynamic range cannot be solved by any fixed circuitry, because the requirements are different in every installation. If things were arranged so the inherent dynamic range of the organ were reduced to suit the average home installation, organs placed in churches, auditoriums, and homes of larger than usual size and with more amplifier power than most would not be at their best. One quality element, in other words, would be scaled down to suit the minimum.

Real quality is rare enough these days to be worth preserving at any cost. Fortunately, the Mixer-Compressor, which makes it possible to adjust dynamic range for every installation, costs

very little. It is a standard element of the Theatre Organ, and is optional for the Consolette II and Recital Organs – even being easily addable to those assembled before the Mixer-Compressor was designed.

The unit does several other things as well. The realistic sound of the Percussion Group is largely dependent on the amplitude envelope of each voice, which can be affected for the worse by reverberation. Ideally, one would play a piano, harpsichord, bells, and the like in a room having much less reverberation than an organ needs. In a real auditorium, there is no choice. But in an organ using the Schober REVERBATAPE Unit, it is perfectly simple to feed organ tones through the reverberation system, while the Percussion voices bypass reverberation and reach the amplifier directly. The Mixer-Compressor does exactly that.

It also provides an optional separate output for Percussion, so that percussion sounds can be channeled through a separate amplifier and speaker system.

One of the principal values of doing that is keeping percussion out of the Leslie speaker to avoid giving them the Leslie vibrato-tremolo effect, which would sound wrong with percussions.

As a final fillip, the Mixer-Compressor provides an input for a crystal microphone, whose signal goes through the REVERBATAPE Unit along with organ tones. Thus, someone can sing along with the organ, sounding as though he were singing in a large hall. The effect is highly flattering to the voice – and it is also lots of fun!

The Mixer-Compressor is a single printed circuit plus a small plate with three jacks for connecting the amplifier, the optional separate percussion amplifier, and the microphone. All the controls are presetting potentiometers mounted on the board.

Once the unit is installed and adjusted, the organist can forget it.

Fig. 9-1 is a complete schematic diagram of the Mixer-Compressor. Certain component values differ for the different Schober Organs; the values shown are those for the Theatre Organ.

Audio Channel Organ output from the Preamplifier-Vibrato Unit is connected to the main input, terminal PR. (A shielded cable is used, but the shield is connected only at the preamplifier end to avoid ground loops, as the Mixer-Compressor is grounded to the organ circuits by the GND terminal.) The audio channel is

the connection to the base of transistor 36, through the level-adjusting and frequency-compensation RC network. Emitter follower 36 feeds a cable attached to terminals RI (IS is the shield connection), which ends in a phono plug and goes to the input jack of the REVERBATAPE Unit. A similar cable connected to terminal RO (again no shield connection at the Mixer-Compressor to avoid a ground loop) brings REVERBATAPE output to the emitter of 55, which acts as a grounded-base amplifier for the main organ signal. Emitter follower 57 furnishes total output (with preset level control 58) for the power amplifier. Phono jack RJ is furnished to eliminate the REVERBATAPE Unit during adjustment, or in case no REVERBATAPE Unit is used at all. Plugging the cable attached to

RO into the jack transmits signal directly from transistor 40 to 55.

The main organ signal of which we have spoken thus far undergoes automatic volume compression by the action of the photosensitive resistor, the P block of component 23. That will be explained in a moment; meanwhile, other than the photoresistor, no compressor components are in the circuits we are discussing.

Mixing takes place in transistors 36 and 55, as well as simple amplification. The crystal microphone signal has its own volume control 24, which is preset, after which volume is easily controlled by the distance the microphone is held from the mouth. The signal goes through frequencycorrection network 25-26 and blocking capacitor 31 to the base of 36, to join the organ signal which passes through the REVERBATAPE Unit.

Transistor 55 is a grounded-base amplifier for the organ signal coming from the REVERBATAPE Unit, as we have said. It is also a standard grounded emitter stage for signals inserted at terminals PN and RH. Output from the Percussion Group is connected to PN, and the RH input is provided for a possible future use. Either or both of these signals are passed through input isolating resistors 45 and 46 to a swell-shoe potentiometer, which has the customary bass compensation components 47-48.

This potentiometer is the second gang of the 2-gang control which is a standard part of the Theatre Organ (the first potentiometer is the regular organ control) and comes with the Mixer-Compressor kit for use in the other organs to replace the usual single one.

The swell shoe potentiometer arm goes through level-control resistor and blocking capacitor 49 and 50 to the base of 55. The collector of 55 therefore includes regular organ signal which has been passed through the REVERBATAPE Unit and percussion signals which have not been reverberated.

Two sets of connection holes are provided for each of the input isolation resistors 45 and 46. If the percussion, for example, is used and connected to terminal PN, resistor 45 is as shown. If not, the resistor is placed at position 45X. The same is done with resistor 46. Each of the two input devices acts as part of the load impedance of the other. If one is not used, its loading effect is provided by the alternate resistor connection.

There is one more possibility. The connection between the two J terminals can be removed, and the output of the swell shoe, terminals J and JS, can be connected to a second power amplifier rather than being mixed into transistor 55.

Compression Volume compressors are extremely well known in the audio art, and expander-compressors were at one time quite popular in home audio equipment.

As designed in former days, the scheme for a compressor was to rectify the audio voltage and use the d.c. to control bias on a tube in the audio chain.

The polarities were such that as the audio signal grew larger, the tube bias became more negative, reducing the gain and thus keeping the output signal from getting as large as it would without compression.

Such a system has some basic faults, which can be minimized but not removed entirely by astute design. The d.c. signal proportional to the audio amplitude envelope, derived by rectification, had to be quite pure d.c.; at least all audio components had to be removed by filtering. In addition, it was not possible to allow the d.c. to change too suddenly, as this would cause a plate-current transient which would be heard in the output as a thump. For both reasons, compression action was always delayed, so that a large, sudden peak could often get by without compression.

The problem of sudden peaks is very serious with organ music. If the organist sets up a large combination of loud stops and then strikes and holds a chord, the old type of compressor would let the chord sound immediately at almost full volume, after which it would audibly come down in volume.

Th effect would be unnatural and annoying. A compressor is necessary, therefore, which will act nearly instantly, so that when the chord is struck, the compressor adjusts its volume to the compressed value before the ear has time to evaluate its volume. And this fast compression must not be accompanied by any extraneous noises. This is exactly what the control section of Fig. 9-1, transistors 5, 12, 18, and 22, does, in conjunction with the light sensitive resistor 23.

Audio signal, in addition to going from input terminal PR to the base of 36, also enters the control system through compression control 3 (which does

not affect the signal to 36). With the arm of 3 at the top, compression is at maximum, and we shall assume this condition.

The object of the control system is to drive the lamp section (L) of light-dependent resistor assembly 23. The photocell of 23 (P) is the shunt leg of a voltage divider in the audio circuit feeding the base of 36. As the lamp grows brighter (the result of increased input signal to the lamp from the input audio through the control transistors, as we shall see), the cell resistance decreases, so that less of the audio can get to 36. This action effectively lowers the transmission of the audio section of the unit whenever the input increases.

Control-section input transistor 5 is an emitter follower, isolating the audio line from any distortion due to the control components and presenting high impedance to potentiometer 3 to avoid loading it.

Transistor 12 is a voltage amplifier, which feeds emitter follower 18, from which signal is applied to 22 to control the brightness of the lamp.

The circuits of 12 and 18 have special characteristics not obvious at first glance. Transistor 12 is a linear a.c. amplifier over most of its range, but when signal at the arm of 3 reaches about 2 volts, 12 begins clipping symmetrically. This prevents overdrive to 22 and the lamp. It also limits compression so that output signals due to a very loud input do not eventually become softer than those due to a moderate input.

Transistor 18 is biased only very slightly on. In the absence of signal, its emitter connection to 22 causes 22 to conduct only slightly. This keeps the lamp just barely illuminated rather than completely extinguished, so that when a signal does come along, the lamp will light up much more quickly, and compression begins much faster, than if the lamp were out.

The low “on” bias of 18 also makes 18 a rectifier for all but the very smallest input signals, since only negative alternations of audio applied to its base can cause significant emitter current. The result is that audio applied to the control system by the arm of 3 appears at the emitter of 18 as varying negative d.c. which charges capacitor 20.

When a big signal peak comes along, 20 charges very quickly, since the charge is supplied by a lowimpedance source – the emitter of 18. Compression is thus very fast. When the signal (perhaps a sudden loud chord) is removed (the organist lets the chord

go), the only “on” voltage remaining at the base of 18 is the slight fixed bias. However, 20 is still charged negative, effectively biasing 18 completely off. Since 18 is off, it is no longer a “live” emitter follower and 20 must discharge through resistor 19 and the base of 22, both fairly high-impedance paths. Thus 20 discharges very much more slowly than it charges. The result is that compression takes hold very quickly, as it must to take care of sudden loud sounds; but the audio gain of the unit does not return to its no-signal value for some time. This fast-attack, slow-decay action is very desirable in a compressor, to avoid repeated great changes of gain when a succession of loud sounds, separated by short silences, is produced by the organ.

Adjustment The compression and volume levels must be adjusted when the organ is installed, but the procedure is very simple and requires no instruments.

First the REVERBATAPE Unit is removed from the circuit by plugging the cable attached to terminal RO into jack RJ. Then the organ output is set at some nominal specified point, such as maximum or ¾ maximum.

With the compression control, potentiometer 3, at zero (arm grounded), the organist plays some small combination of stops with the swell shoe wide open. A typical ensemble might be an oboe solo with a soft flute accompaniment and a soft pedal stop. As he plays, the output control 58 and the power amplifier volume control are adjusted to give the minimum volume which is still judged to be enough for pleasant listening. The soft stop combination should not be any louder than necessary, but it should not be so soft that it lacks interest and effectiveness.

Now the organist selects a large ensemble of loud stops, something which he feels should be about as loud as anything he will ever play, again with the swell shoe open. As he plays, the compression control 3 is turned up (arm away from ground) so that compression begins. The control is advanced until loudness is reduced to the maximum the organist desires or to just below the level where amplifier or speaker distortion is evident. The dynamic range can now be checked by playing various combinations of stops and kinds of music,

to see that, while there is considerable variation in loudness, nothing overloads the amplifiers or tends to knock down the walls, but nothing is too soft to be effective. Obviously, the desirable dynamic range depends on the room size, amplifier-speaker capabilities, and the taste of the organist.

Next percussion volume can be adjusted so that it sounds right with relation to the organ stops (another matter of personal taste). The control is on the percussion board, not the Mixer-Compressor.

And finally, the REVERBATAPE Unit can be set after the cable is plugged back into its output

jack. The input control is set as high as possible without distorting on loud sounds; then the output control is set so that volume is the same as it was with the RO cable plugged into the RJ jack. From here on, the Mixer-Compressor is on its own – no further adjustments are needed and the organist usually hears no evidence of its existence. If a microphone is to be used, the control 25 should be set so that gain is not enough to cause microphoneto-speaker feedback. Any crystal microphone is suitable. The one sold by Schober is inexpensive and gives good results.

THE SCHOBER ORGAN CORPORATION The unusual idea of the assemble-it-yourself organ, one of the most elaborate kit products ever marketed, has prompted many friendly questions about the company which dominates this field.

The Schober Organ Corporation was formed in 1954 by C. G. McProud, then Editor of the magazine Audio, Henry Schober, Audio business manager, and Richard H. Dorf, engineering consultant and writer, known throughout the English-speaking world as a foremost authority on electronic musical instruments. Never guessing that any substantial number of people would want to build so elaborate a series of kits, the three conceived (and Mr. Dorf designed) the Schober Organ as a spare-time venture. (Mr. Schober’s name seemed to give the authentic Teutonic touch to a classic organ!) The avalanche of orders gave the lie to the part-time assumption and shortly Messrs. Schober and McProud severed connections with the company. Mr Dorf took over as president and floor sweeper in a New York loft.

After three expansions in the original loft building, even 5 times the original space was insufficient. In 1960 Schober moved to its present location in a modern building on the edge

of Lincoln Center for the Performing Arts, where the 10,000 feet of specially constructed showroom, laboratory and working space is again beginning to feel tight!

Mr. Dorf and Louis H. Exstein, who joined in 1958 as vice-president and general manger, plus a substantial staff of capable, interested employees, run the company on a highly personal basis, deriving their greatest satisfaction from giving Schober customers a little more than their money’s worth in the form of organs of genuine musical satisfaction at low cost, and in fast, helpful service. Their second great satisfaction is the continually increasing financial success of the company, which they believe proves that a business can be run for the simultaneous benefit of its customers and its owners!

Schober organs now dot the landscape in every cranny of the U.S., and are to be found in most other parts of the free world as well. Laboratory activities, which have yielded the modern transistor organs described in the booklet and other Schober products, continue on a high level, constantly working on improvements for existing organs (most improvements are made available as modifications to present owners) and new products.