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Stephen W. Moore

Pass Zen (“Penultimate” v.4) Amplifier

The Zen could hardly be called “hi-fi” with its unimpressive distortion figures. Is that 2.5% distortion at 10 Watts?


However, it sounded wonderful with the right speakers. The speakers of the day were generally inefficient, designed to be coupled with huge amplifiers with enormous damping factors. In this case, the Zen, with its feeble power output and damping factor, simply sounded awful.


With the right speakers, the Zen opened up a new sonic frontier and helped usher a new dawning of the audiophile community. Soft, luscious sounds having the emotionality of real people came through.


So, in 2001, Pass started exploring further development of the Zen, all through the DIY community. The Pass Zen version 4 “Penultimate” is a result of his continued development.

“Penultimate” Zen (v.4) Amplifier


Pass authored a series of articles building on the foundation of a 1994 minimalistic single-ended MOSFET amplifier he dubbed the Zen … “what is the sound of one transistor clapping?”. Although a single-ended amplifier design was far from novel, the Pass Zen was revolutionary in concept to the audiophile community. It represented backlash against the expensive, overindulgent, and overly complex solid-state monstrosities that were promoted by the audiophile community. It offered a new, personal, soft single-ended sound directly at odds with the era’s analytical and excessive predominant offerings.

Pass Zen Amplifier


Nelson Pass is one of the world’s preeminent audio designers. He graciously shares many of his novel ideas with the DIY audio community and is an active participant on diyaudio.com discussion forums. Several Pass amplifiers have been given the most prestigious awards in available in the audio industry.


Nelson Pass offers novel amplifier design ideas on his DIY web site, passdiy.com and publishes full schematics of his niche-market specialist amplifiers at firstwatt.com. Pass entertains discussions, dispenses advice, and generally assists the DIY community to recreate his \ amplifiers on the forums at diyaudio.com.


As his amplifier designs are patented, his sharing of amplifier designs are for the “garage enthusiast”, and not for commercial offerings unless authorized.




I won’t discuss the circuit here in detail. Follow the link to the Zen v.4 documentation.


My version used the Pass Labs circuit board, instead of drawing my own circuit board. I used mostly stock components from Digikey, although parts selection is not terribly important with this amplifier. The sonics are in the design, not necessarily the implementation. I’ve seen some awful implementations that reportedly sounded great.


I built mine with an insane Ixys TO-264 MOSFET (i.e. the IXFK 180N07) for the capacitance multiplier (Q5) and Aleph current source (Q2). I used the IRFP044 for the main active device (Q1).

Very heavy heatsinks were used. The body of the heatsink is about 15mm thick of solid aluminum, so they have huge thermal mass and stability.


Power Supply


The amplifier operates in single-ended mode and the circuit board has an integrated capacitance multiplier (references here and here). Therefore, the power supply can be expected to provide constant current. I used a big Amveco toroidal transformer with two secondaries. The left and right channels used their own separate transformer winding, diode bridge, and filter capacitors.


Big 3” computer-grade can capacitors were used, along with Solen 2.2uF film capacitors. JK Miller toroidal chokes and Panasonic ECQ-P capacitors provided RF filtering for high frequency components from the AC utility grid and the diode commutation noise. The diodes were bypassed with snubber networks consisting of 0.1uF film capacitors and 110 Ohm resistors.


Final power supply output was filtered by an air-core Jantzen Audio inductor. Metal core chokes would saturate under constant DC current.



This amplifier must agree with your speakers. I’ve tried it with numerous speakers, both commercial and DIY. The biggest surprise was the best sounding combination was with my Definitive Technology Celsius monitors. Just as surprisingly, the Zen v.4 sounded relatively poorly with my Triangle Acoustics Solaris. This was not what I expected — the consumer grade boxes designed for big solid-state tuners sounded good, and the high-sensitivity niche-market audiophile speakers designed for valve amplifier sounded bad. I figured the Triangle Acoustics would love the Zen, because they prefer amplifiers with high (poor) output impedance and damping factors. But they didn’t.


However, with the right combination (in my case, the Celsius), the Zen was singing! She had gorgeous vocals with tremendous soundstage that wrapped around the speakers and beyond. Male and female vocals were sweet and compelling. May I say sumptuous?


The amplifier got lost in big orchestral passages. Symphonies were turned into mush. This amplifier is best suited for simple passages, especially solo vocals, and chamber music. Contained within these genres, it’s a beautiful thing.


Hifi? Hardly. Audiophile approved? Maybe. Buckets of fun? Absolutely! Did it compel me listen to more music? Yes, and that’s what counts, right?


Pass Zen v.4

Tube/MOSFET Hybrid

Fostex FE207E

EL84 Push Pull