Technical Specifications for Audio Ampli

Technical Specifications for Audio Amplifiers (II)

In the last column we looked at some general amplifier data and we discussed the relevance of RMS Power values. Allow me to recap, that RMS is not equal to RMS measurement and this depends on the time constant that the manufacturer of the amplifier applies. If you want to compare power, then you also need to check for how long the power will be available. Besides the power, there are a few other data points that are relevant to describe a amplifier performance.

In the amplifier world there are two more data points which determine the performance of a given amplifier and they are the “Slew Rate” and the “Damping Factor”. The “Slew Rate” is the more simple value and it simply describes the rate at which an amplifier can reproduce impulses. The slew rate is given as a Voltage value per microseconds and it tells you how fast the amplifier is able to make larger voltage changes happen on the output. This means the Slew Rate is an indicator on how dynamic the amplifier is and can reproduce impulses. As a rule of thumb you should remember that the higher the slew rate, the better the reproduction of impulses. In Analogue amplifiers this is mainly limited by the power supply and its capacity (all pun intended) to supply the source power for larger impulses and then of course the ability of the amplifier channel to produce the steep impulse responses. Decent professional amplifiers start at 20 V/microseconds and they go all the way up to 100 V microseconds based on model and power supply. The second data point is the “Damping Factor” and it is used to describe the amplifiers performance in a real world load. It describes the ratio of the nominal impedance of an amplifier versus the source impedance. As you keep adding loudspeakers to your amplifier output and your impedance comes down, it is important that the amplifier can still supply a decent amount of power into this load. Again you can see that the damping factor depends on the connected load and hence is not a constant. As a rule of thumb you can remember that the higher the damping factor, the more independent the amplifier performance should be from the connected load.

I am well aware that I might have stretched your desire to read and learn more at this point already, but there are two more points I would like to make in the interest of reality check. These days the vast majority of amplifiers on the market are no longer Analogue and linear power supply units. We have mostly transitioned to CLASS D units and there are two main reasons for this. First, the efficiency of those units is much higher than the one from any analogue device. The loss through heat is much less and hence this resolves a lot of ventilation problems and many modern CLASS D units can survive with just convection cooling. The second reason is that CLASS D amplifiers typically come with much smaller and also way more lightweight power supply unit, since the design of the power supply unit is completely different and then the overall weight of the amplifiers comes down dramatically. Ultimately, no digital switchmode power supply can supply power as good and audiophile as a large and heavy linear power supply with huge capacitor banks and a very good AC filtering, but weight has a serious meaning in “live”..

The last aspect for this column for today is the question about test signal. The use of Sinewave or Pink Noise for power testing is okay. but it does not really do justice to what the amplifier can do with music signal. Both Sinewave and Pink Noise are not dynamic and create a requirement for long term power, that cannot be supplied by most modern units and their power output into Sinewave signal and Pink Noise signal would be considered rather small. However, when connected to any loudspeaker with a music signal, you will never apply Sinewave or Pink Noise to the system for any real world performance. This now requires for us to introduce one new parameter on the test signal and that is the CREST FACTOR of the test signal. The Crest Factor describes the ratio between peak and effective signal and the higher the Crest Factor of the signal you use for power testing, the higher your power output that can be claimed. Essentially the Crest Factor allows for quite a range of dynamic (as the Crest Factor increases) and this was historically called the “music signal” power testing, whereas there was no clear specification on what “music signal’ really meant. This has changed and it can be clearly described now and it is not uncommon for manufacturers to use a Crest Factor of 9 dB or even 12dB and then claim their maximum output power based on this signal and the numbers go really high then. So you have to check what measurements have been applied, so you are sure that you are comparing apples to apples. If an amplifier delivers high power into Sinewave or Pink noise, this still means more than with a Crest Factor of 9dB or 12dB and a dynamic music signal.

In general there is no good or bad data for as long as you honestly describe what you are doing and you are allowing anyone to replicate your measurements. If you see power values that seem unreasonably high, then it is wise to look at what is measured and how and based on which definition. In this sense I wish you all a “dynamic” month ahead.

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