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.
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 four times the rated
output power.
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.
Single ended Class A operation is generally less efficient than push-pull Class A. Single
ended Class A amplifiers tend to be even bigger and more expensive than their push-pull
cousins, but they have a more natural transfer curve.
The "purity" of Class A designs has been at issue in the last few years, with "pure" Class A
loosely defined as an idling heat dissipation of more than twice the maximum amplifier output.
For a 100 watt amplifier, this would be 200 watts out of the wall at idle.
Designs that vary the bias against the musical signal will generally have bias currents at or
below the signal level. This is certainly an improvement from the viewpoint of energy
efficiency, but the sound reflects the lesser bias point.
I authored the first patent on the dynamically biased Class A amplifier in 1974, however I have
not used the technique for the last 15 years. The reason is that I found the quality of sound
associated with an efficient Class A operating mode inferior in depth and less liquid at high
frequencies, simply because it operates at reduced bias at low levels. Given the plethora of
cool running “Class A” amplifiers on the market, you might say I opened a Pandora’s box.
Until the output current reaches the single ended bias point of the Aleph 1, it is considered a
single ended Class A amplifier as the bias is provided by a constant current source attached
to the negative power supply. Beyond the single ended bias point at 150 watts rms. (8 ohms)
it is designed to operate as a push-pull amplifier.
A very important consideration in attempting to create an amplifier with a natural characteristic
is the selection of the gain devices. A single ended Class A topology is appropriate, and we
want a characteristic where the positive amplitude is very, very slightly greater than the
negative. For a current gain device, that would mean gain that smoothly increases with
