The Penultimate Zen (v4)

The Penultimate Zen (v4)

image Brendon clearly has had a long and persistent love affair with the Nelson Pass Zen Amplifiers.  This article describes the build of his Version 4 Zen amplifier.  The casework for this amplifier is hefty and by his own admission over engineered cooling wise.  I cant help be impressed though with the sheer size and finish that he has achieved on his mono block pair.

Nelson Pass’s Zen amplifiers are Class A devices which expend a lot of their power as heat.  Within the Hobby community they are renowned for their quality.

Thanks to Brendon for his kind permission to re-publish this article.  Originally hosted on his Geocities site, I am proud to preserve it beyond the closure of Geocities web hosting

DOB: Born March 5th, 2004                     Place: Christchurch, New Zealand
Dimensions L x W x H: 675mm (26.57") x 370mm (14.57") x 220mm (8.66")

image Simple power switch in the front. I don't desire the on/off switch in the back (who does?)

image Bottom with simple square rubber feet. Only raises the chassis by 6mm above the ground

image Heat-sink rails treated with silicone compound

image Mosfets ON!


Stuffed PCB using generic parts. All capacitors are Panasonic 105c temp rating because they are simply better. Vishay/Dale 1/4w resistors.

image Power supply is a potted 500VA toroidal made in Czech Rep. Pri: 0V-230V with dual Sec 0-35V.

Very well made and no mechanical hum!!! Wire leads are double insulated to boot!

Capacitors are 2nd hand Sprague CG type. "Powerlytics" 34,000uF 50VDC

image Like the handle bar? I often wonder why so many large amps have the rack mount handles mounted vertically on each side? Especially when it's clear that the amp is too large to fit in a flight/rack-mount cage.

The amp can be operated either 2 channel RCA or single channel balanced. Both amps have the female IEC power socket making it handy to daisy chain power. NOTE! not to exceed the rating of the 1st power cable.


Hook-up wire I've used regular solid core house wire.

Only special wire is the silver Teflon (Tefzel) 28awg wire for the input signal.


Structurally, it has many of the design features as in my SoZ project. I will admit that the chassis is definitely over heat-sinked and increasing the bias current would be of no problem.

The sound is superb. I'll have to agree that in balanced mode, the sound is much more authoritative than single ended RCA. It's just more of everything in sound, more bass control, more high freq. more of everything in clarity. In other words, why run it in single ended?

Biasing issues:

As in the schematic, R1 is 1 ohm to give 2amp bias. I wonder how much better would the sound be if R1 is 0.47ohm (2.8amp bias)? I've also heard about upping bias on the Zetek 3310 buffer to like 20ma is a major improvement? My question is if such tweaks offered a great improvement, Nelson Pass himself would of illustrated it or simply, just tell us to DO IT!