I’ve built a scaled-down version of Nelson Pass’ Zen power amplifier to feed my Grado headphones. For this use, the Zen topology is perfect: excellent sound quality, simplicity, linearity and no multi-stage feedback. It is a single stage class A MOSFET design with the right gain and a low output impedance. Here we don’t have the limitations of the Zen amps (at least in the single-stage implementations) regarding speaker compatibility. A single stage topology with correct interfacing values misses very few things in the original music message.
The gain device in the original Zen amplifier is biased by fixed current source. For this amp, I employed an active current source described in Pass’ patent no. 5,710,522 (see Zen Variations Part 2). The benefits of an active source include higher output current, lower distortion and 50% theoretical operating efficiency (compared to the 25% efficiency from a fixed source). This type of current source is featured in the Aleph power amplifiers from Pass Labs.
I designed this circuit to obtain the best sound from my Grado SR-325 headphones. Portability here was not imperative, so the high power consumption of a real class A amp was not a big drawback. I developed this project at the beginning of 2002 and it was first presented at DIYaudio.com in February 2002. Since the intellectual property belongs to Pass Labs, this circuit is intended only for notforprofit use, unless the current source topology is altered. I wish to thank Nelson Pass for his open-mindedness and involvement in the DIY audio world; the DIYer community will be always grateful to him!