The Performance Requirements for Amplifiers

Monday, April 2nd, 2018 - Electrical Note

The performance requirements for amplifiers is not a recapitulation of international standards, which are intended to provide a minimum level of quality rather than extend the art of audio music. The performance requirements for amplifiers is rather what you should be worrying about at the start of the design process.

The Performance Requirements For Amplifiers

The Performance Requirements for Amplifiers

1. Safety

In the drive to produce the fi nest amplifi er ever made, do not forget that the Prime Directive of audio design is – Thou Shalt Not Kill. Every other consideration comes a poor second, not only for ethical reasons, but also because one serious lawsuit will close down most audio companies forever.

2. Reliability

If you are in the business of manufacturing, you had better make sure that your equipment keeps working, so that you too can keep working. It has to be admitted that power amplifi ers – especially the more powerful ones – have a reputation for reliability that is poor compared with most branches of electronics. The ‘ high end’ in particular has gathered to itself a bad reputation for dependability.

3. Power Output

In commercial practice, this is decided for you by the marketing department. Even if you can please yourself, the power output capability needs careful thought as it has a powerful and nonlinear effect on the cost.

The last statement requires explanation. As the output power increases, a point is reached when single output devices are incapable of sustaining the thermal dissipation; parallel pairs are required, and the price jumps up. Similarly, transformer laminations come in standard sizes, so the transformer size and cost will also increase in discrete steps.

This does not mean that fractions of a watt are never of interest. They can matter either in pursuit of maximum efficiency for its own sake, or because a design is only just capable of meeting its output specification.

Some hi-fi reviewers set great value on very high peak current capability for short periods. While it is possible to think up special test waveforms that demand unusually large peak currents, any evidence that this effect is important in use is so far lacking.

4. Frequency Response

Frequency ResponseThis can be dealt with crisply; the minimum is 20 Hz – 20 kHz,  0.5 dB, though there should never be any plus about it when solid-state amplifi ers are concerned. Any hint of a peak before the rolloff should be looked at with extreme suspicion, as it probably means doubtful HF stability. This is less true of valve amplifi ers, where the bandwidth limits of the output transformer mean that even modest NFB factors tend to cause peaking at both high and low ends of the spectrum.

Frequency Response

Having dealt with the issue crisply, there is no hope that everyone will agree that this is adequate. CDs do not have the built-in LF limitations of vinyl and could presumably encode the barometric pressure in the recording studio if this was felt to be desirable, and so an extension to  0.5 dB at 5 or 10 Hz is perfectly feasible. However, if infrabass information does exist down at these frequencies, no domestic loudspeaker will reproduce them.

5. Noise

There should be as little as possible without compromising other parameters. The noise performance of a power amplifier is not an irrelevance, especially in a domestic setting.

6. Distortion

Once more, a sensible target might be: as little as possible without messing up something else. This ignores the views of those who feel a power amplifi er is an appropriate device for adding distortion to a musical performance. Such views are not considered in the body of this book; it is, after all, not a treatise on fuzz-boxes or other guitar effects.

Distortion

Digital audio now routinely delivers the signal with less than 0.002% THD, and I can earnestly vouch for the fact that analog console designers work furiously to keep the distortion in long complex signal paths down to similar levels. I think it an insult to allow the very last piece of electronics in the chain to make nonsense of these efforts.

To me low distortion has its own aesthetic and philosophical appeal; it is satisfying to know that the amplifi er you have just designed and built is so linear that there simply is no realistic possibility of it distorting your favorite material. Most of the linearity-enhancing strategies examined in this book are of minimal cost (the notable exception being resort to Class-A) compared with the essential heat-sinks, transformer, etc.

7. Absolute Phase

Concern for absolute phase has for a long time hovered ambiguously between real audio concerns like noise and distortion, and the subjective realm where solid copper is allegedly audible. Absolute phase means the preservation of signal phase all the way from microphone to loudspeaker, so that a drum impact that sends an initial wave of positive pressure towards the live audience is reproduced as a similar positive pressure wave from the loudspeaker. Since it is known that the neural impulses from the ear retain the periodicity of the waveform at low frequencies, and distinguish between compression and rarefaction, there is a prima facie case for the audibility of absolute phase.

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