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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.

Circuit

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.

Impressions

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?

π

Pass
Zen v.4

Tube/MOSFET
Hybrid

Fostex
FE207E

EL84
Push Pull