Zen Revisited Power amplifier



SE power amplifier with just one active device (MOSFET) per channel

Used in the Attic


Associated equipment:                                                                                      Last updated: November 30, 2005


Homemade multimedia speakers (Monacor)

Homemade line-level preamp (Pass BOZ)

Homemade FM tuner (based on Ambit modules)

Homemade phono preamp (with pencil tubes)

Pioneer PL-12D turntable with Audio Technica AT66 cartridge



The Zen (single-ended MOSFET) amplifier was published by Nelson Pass in The Audio Amateur  1. In a subsequent issue of this magazine, Nelson slightly improved the original design 2. I decided to build the "Revisited" version with the following circuit modifications:


1. More extensive power supply filtering to eliminate hum (C-L-C filter , L1 in combination with C1,C2,C7,C8).

2. Even higher quiescent current (3 Ampere / channel, by lowering R1 from 0.33 to 0.22 Ohms).

3. Slight amount of negative feedback (via resistor R11) to improve damping factor and loudspeaker control.


(see the schematic shown below)




Construction details can be downloaded from the Passlabs site 3. I used a printed-circuit board which can be purchased from Old Colony Sound Laboratory (Peterborough, NH, USA) and accomodates two channels (i.e., a stereo version of the amplifier).


My copy of the amp worked immediately, but not "without any flaw". Although I had used the largest heatsinks which I could obtain (0.3 K/W, one per channel) and had mounted these vertically on an open chassis to ensure proper ventilation, the amp ran EXTREMELY hot. After ten minutes of operation, one could not touch the heatsinks. Even the PCB became very hot due to thermal conduction. I began to feel the burden of TRUE class A operation....


Something had to be done about this, or the lifespan of the MOSfets and electrolytic capacitors would be very short. Since I did not want the amp to become even larger and heavier than it already was, I decided to use forced cooling and mounted a powerful 12 V fan on each heatsink. The fans are series-fed from a separate 24V DC supply based on a LM317 i.c. With the fans in action, the amp does not get more than lukewarm even after several hours of operation. But it should stand in the open to ensure the circulation of air. The movement of air by the fans generates a lot of noise. Thus, the amp must be far from the listening site (preferably in another room). But how does a Zen sound? Or, as Nelson Pass said: “What is the sound of one transistor clapping”?


When you have appropriate speakers: very good indeed. By "appropriate speakers" I mean: speakers with high efficiency and simple crossovers (or no crossover at all). Low-efficiency speakers with complex crossovers will not mate well with the Zen. Such speakers require a powerful push-pull amplifier (100 W or more) to sound decent. With the multimedia speakers in my attic, the Jericho horns in my living room and with Stax electret headphones, the Zen sounds great.


One could call it an "American sound" in the style of old recordings from the Philadelphia Orchestra: Warm, lush, powerful bass, sweet-sounding strings. Without artificial sibilants during reproduction of the human voice. Perhaps not the ultimate in refinement, but a VERY good result overall. Baroque violins which can sound harsh on solid-state equipment sound excellent on the Zen. Even ardent tube lovers (like myself) must admit that Nelson's Zen is something special.


Warning: For proper alignment of a Zen, a sinewave generator and an oscilloscope are required. P1 should be adjusted for symmetrical clipping of the gain stage.


Questions, comments: aren.van.waarde@hetnet.nl


(Tip: Use only my initials if you want to reach me)


Finally, some pictures of this monster:




Zen seen from the front. The power transformer, bridge rectifier and fuse are below the chassis. All other parts are above the chassis where they are well-ventilated.




Zen seen from the side. Notice the large fan on the heatsink. Although built on a single chassis, my Zen is a dual-mono construction. The power transformer has a separate secondary winding for each stereo channel.





1  Nelson Pass, “The Pass Zen Amplifier: 10 Watts of Single-Stage Single-Ended Class A”, The Audio Amateur 2/94.


2  Nelson Pass, “Zen Revisited: Upgrade to the Single-Stage, Single-Ended, Class A MOSFET Amplifier”, The Audio Amateur 3/94.


3  Website of Nelson Pass: www.passdiy.com



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