Audio Engineering

Introduction to Audio Amplifi cation 165

waveforms within the audio frequency spectrum, should not exceed some agreed

upon level. [In practice, this is very diffi cult to defi ne because the tolerable

magnitudes of such waveform distortion components depend on their frequency

and also, in the case of harmonic distortion, on the order (i.e., whether they are

second, third, fourth, or fi fth as the case may be). Contemporary thinking is

that all such distortion components should not exceed 0.02%, although, in the

particular case of the second harmonic, it is probably undetectable below 0.05%.]

● The phase linearity and electrical stability of the circuit, with any likely reactive

load, should be adequate to ensure that there is no signifi cant alteration of the

form of a transient or discontinuous waveform such as a fast square or rectangular

wave, provided that this would not constitute an output or input overload. There

should be no ringing (superimposed spurious oscillation) and, ideally, there

should also be no waveform overshoot under square-wave testing in which the

signal should recover to the undistorted voltage level, 0.5%, within a settling

time of 20 μ s.

● The output power delivered by the circuit into a typical load—bearing in mind

that this may be either higher or lower than the nominal impedance at certain

parts of the audio spectrum—must be adequate for the purpose required.

● If the circuit is driven into overload conditions, it must remain stable. The clipped

waveform should be clean and free from instability, and should recover to the

normal signal waveform level with the least possible delay—certainly less

than 20 μ s.

In addition to these purely electrical specifi cations, which would probably be diffi cult to
meet, even in a very high-quality solid-state design—and most unlikely to be satisfi ed in
any transformer-coupled system—there are a number of purely practical considerations,
such as that the equipment should be effi cient in its use of electrical power; that its heat
dissipation should not present problems in housing the equipment; and that the design
should be cost-effective, compact, and reliable.