should be done very carefully). As an experiment it has the virtue of only changing the bias
and the expectations of the experimenter.
As the bias is reduced the perception of stage depth and ambiance will generally decrease.
This perception of depth is influenced by the raw quantity of bias current.
If you continue to increase the bias current far beyond the operating point, it appears that
improvements are made with bias currents which are much greater than the signal level.
Typically the levels involved in most critical listening are only a few watts, but an amplifier
biased for ten times that amount will generally sound better than one biased for the few watts.
For this reason, designs which operate in what has been referred to as "pure" Class A are
preferred because their bias currents are much larger than the signal most of the time.
As mentioned, preamp gain stages and the front ends of power amplifiers are routinely single
ended "pure" Class A, and because the signal levels are at small fractions of a watt, the
efficiency of the circuit is not important.
4) Given the assumption that every process that we perform on the signal will be heard, the
finest amplifiers must employ those processes which are most natural. There is one element
in the chain which we cannot alter or improve upon, and that is the air. Air defines sound, and
serves as a natural benchmark.
Virtually all the amplifiers on the market are based on a push-pull symmetry model. The push-
pull symmetry topology has no particular basis in nature. Is it valid to use air's characteristic
as a model for designing an amplifier? If you accept that all processing leaves its signature
on the music, the answer is yes.
One of the most interesting characteristics of air is its single ended nature. Sound traveling
through air is the result of the gas equation
PV1.4 = 1.26 X 104
where P is pressure and V is volume, and whose curve is illustrated in fig. 1. The small
nonlinearity which is the result of air's characteristic is not generally judged to be significant at
normal sound levels, and is comparable to the distortion numbers of fine amplifiers. This
distortion generally only becomes a concern in the throats of horns, where the intense
pressure levels are many times those at the mouth, and where the harmonic component can
reach several per cent.
