Audio Amplifi er Performance 317frequency associated with the input stage collector will no longer work so well. (I have
used the term Nyquist stability or Nyquist oscillation throughout this book to denote
oscillation due to the accumulation of phase shift in a global NFB loop, as opposed to
local parasitics, etc.)
The LF feedback factor is likely to be about 6 dB less with a 4- Ω load due to lower gain
in the output stage. However, this variation is much reduced above the dominant pole
frequency, as there is then increasing local NFB acting in the output stage.
Two-stage amplifi ers are not popular; I can quote only two examples, Randi^3 and
Harris.^4 The two-stage amplifi er offers little or no reduction in parts cost, is harder to
design, and, in my experience, invariably gives a poor distortion performance.
10.4 Power Amplifi er Classes .......................................................................................
For a long time the only amplifi er classes relevant to high-quality audio were Class-A
and Class-AB. This is because valves were the only active devices, and Class-B valve
amplifi ers generated so much distortion that they were barely acceptable, even for public
address purposes. All amplifi ers with pretensions to high fi delity operated in push–pull
Class-A.
Solid state gives much more freedom of design; all of the following amplifi er classes
have been exploited commercially. Unfortunately, there will only be space to deal in
detail in this book with A, AB, and B, although this certainly covers the vast majority
of solid-state amplifi ers. Plentiful references are given so that the intrigued can pursue
matters further.
10.4.1 Class-A
In a Class-A amplifi er, current fl ows continuously in all the output devices, which enables
the nonlinearities of turning them on and off to be avoided. They come in two rather
different kinds, although this is rarely explicitly stated, which work in very different
ways. The fi rst kind is simply a Class-B stage (i.e., two emitter–followers working back
to back) with the bias voltage increased so that suffi cient current fl ows for neither device
to cut off under normal loading. The great advantage of this approach is that it cannot
abruptly run out of output current; if the load impedance becomes lower than specifi ed,
then the amplifi er simply takes brief excursions into Class-AB, hopefully with a modest
increase in distortion and no seriously audible distress.