amplifiers and D/A and A/D converters. They are not monotonic: the distortion increases as
the level decreases.
The usual electrical picture of an audio signal is as an AC waveform, without a DC
component. Audio is represented as alternating voltage and current, where positive voltage
and current alternates with negative in a reciprocal and symmetric fashion. This fiction is
convenient because it lends itself to the use of an energy efficient design for amplifier power
stages known as push-pull, where a "plus" side of an amplifier alternates operation with a
"minus" side. Each side of a push-pull amplifier handles the audio signal alternately; the
"plus" side supplying positive voltage and current to the loudspeaker, and the "minus" side
supplying negative voltage and current.
Problems with push-pull amplifier designs associated with crossover distortion have been
discussed elsewhere at length, and one of the primary results is non-monotonicity. Class B
and many AB designs have distortion products that dramatically increase with decreasing
signal. This is reduced greatly by Class A mode, but crossover distortion remains as a lower
order discontinuity in the transfer curve.
For reproducing music as naturally as possible, push-pull symmetric operation is not the best
approach. Air is not symmetric and does not have a push-pull characteristic. Sound in air is a
perturbation around a positive pressure point. There is only positive pressure, more positive
pressure, and less positive pressure.
Push-pull circuits give rise to odd ordered harmonics, where the phase alignment reflects
compression at both positive and negative peaks and crossover nonlinearity near the zero
point.
Push-pull operation in amplifiers is commonly portrayed by the analogy of a two-man saw
cutting down a tree. Certainly if we are cutting down trees by hand, we would opt for this
method, as it would be much more efficient.
As we are not cutting down trees, I much prefer the image of a violinist holding the bow at one
end with one hand. Only in this manner does the musician gain the degree of control and
precision required to produce the range and subtlety required by music. And so it is with
single-ended amplifiers.
Only one linear circuit topology delivers the appropriate characteristic, and that is the single
ended amplifier. Single ended amplification only comes in pure Class A, and is the least
efficient form of power stage you can reasonably create, typically idling at more than twice the
rated output power
under the best of conditions.
Single ended operation is not new. It is routinely found in the low level circuitry of the finest
preamplifying stages and in the front end circuits of the finest power amplifiers. The first tube
power amplifiers were single ended circuits using a single tube driving the primary of a
transformer.
In 1977 I designed and published in Audio Magazine a single ended Class A amplifier using
bipolar followers biased by a constant current source. A considerable number of amateurs
have built the device, rated at 20 watts output, and many have commented on its unique sonic
signature.
