Zen Variations 4 — Nelson Pass / 2002

The Penultimate Zen is the sum of several incremental improvements to the original Zen amplifier of 1994. Eight years just flies by, doesn’t it? These improvements are contained in parts 2 through 4 of the Zen Variations, and is likely the last version of this amp, although by no means the end of the variations on the theme of single stage amplification. In part 2 we developed an improved active current source load for the single gain device which is at the heart of the amplifier. Originally designed for the Aleph amplifier series, this current source doubles the output current… More...

Construct a Class - A Amplifier — Nelson Pass / 1977

Most audio power amplifiers use class A circuitry except in the drive and output stages, where they use class B or AB operating modes to achieve high efficiency. In class B and AB modes, the output stage operates in a pushpull configuration, where one set of output devices delivers positive voltage and current and another set delivers negative voltage and current. When one set is working, the other set is turned off. This scheme operates efficiently, but has two serious flaws, the extremely nonlinear characteristic of the transistors at the collector cutoff region and the turn-on/turn-off times of the devices.… More...