Legacy projects:

These projects are available as downloads only, due their age no PCBs or special parts will be offered. See part substitutions or information on possible component crosses and or substitutes. Adobe Acrobat 4.0 is required to view these files and can be downloaded for free from Adobe.

Bride of Zen: by Nelson Pass
"This is the second installment of a trilogy of construction projects centered around the performance obtainable from absolutely minimalist circuitry. Part one described the Zen amplifier, a 10 watt single-ended class A power amplifier using a single MOSFET gain stage. In this piece we will examine its preamplifying mate, also a single gain stage MOSFET circuit. " (The Audio Amateur, 4/94 by Nelson Pass)... download (301k).

Balanced Zen Line Stage: by Nelson Pass
"So, is this the anticipated Bride of the Son of Zen? I suppose it is. It has an identical topology and is perfectly suited for driving the Son of Zen, but there is much utility to this circuit. It also serves as a nice balanced-in to unbalanced-out or unbalanced-in to balanced-out converter". ( Audio Electronics, 5/1997 by Nelson Pass )... download (350k). gerber files (327k) Please note that gerber files can only be read by PCB software and machines.

Zen: by Nelson Pass
10 Watts of Single-Stage Single-Ended Class A: "There are two most essential principles to audio amplifier design. The first is simplicity. The second is linearity.... download (228k).

Son of Zen: by Nelson Pass
"Letters suggesting design improvements to the ZEN provide the direction and the excuse to proceed with the Son of Zen. Son of Zen has been designed scalable, and can be built with an output from one to fifty watts without any change in the schematic of the amplifier itself."...download (380k).

A75: by Norman Thagard and Nelson Pass
"While the A75 delivers more power, particularly into low-impedance loads, that is not the primary goal of this project. We wish to introduce more advanced concepts in amplifier design, including balanced inputs, dual differential inputs, true DC response, cascode and folded cascode operation, tiered and regulated supplies, operation with and without feedback, and details of designing with the new generation of power MOSFETs."........part1 (331k) & part 2 (1135k).

D.I.Y. Op Amps: by Nelson Pass
The How and Why of building your own op amps. Simple high-performance op amps are easy to make out of as few as six discrete components whose total cost is less than a dollar. It is the purpose of this article to show how easy it is. (Audio Electronics, 6/1998 & 1/1999 by Nelson Pass)......download (320k)

ZEN Revisited: by Nelson Pass
Upgrades to the Single-Stage, Single-Ended, Class A Mosfet Amplifier. ( The Audio Amateur, 3/94 by Nelson Pass) download (50k)

Build a MOSFET Citation 12: by Nelson Pass
In its time the Citation 12 was a truly excellent amplifier, employing a number of concepts which have only recently become popular: dual split power supplies, DC output coupling, and no active current limiting. These characteristics allowed it to sonically outperform the Dynaco 120 which was its popular competitor. It could drive anything with high reliability, and in the time I spent as a repair technician I fixed a few Dynaco, Phase Linear, McIntosh, Pioneer, and Sansui amplifiers, but I've never seen a broken Citation 12. (The Audio Amateur, 2/81 by Nelson Pass) download (619k)

The Pass A-40 Power Amplifier: by Nelson Pass 
Given then that the circuit should be simple, we must find a way to achieve the exceptional performance as advertised. While we want simple distortion types, we also want a lot less of them, which brings us to the question: what techniques will extract maximum performance from a few parts? In this case I have chosen two very effective approaches: constant current sourcing and class A operation which are combined in a deceptively simple 40 Watt per channel amplifier. (The Audio Amateur, 4/1978 by Nelson Pass)   download (815k)

Construct A Class-A Amplifier: by Nelson Pass
In spite of their high cost and low efficiency, class A power amplifiers have recently been receiving more attention from audiophiles who demand uncompromising accuracy. Both the price and quality of these amplifiers result from the operation of their output stages in class A mode, where the amplifying devices are constantly operated in their linear region, above cutoff and below saturation. Whether made from tubes or semiconductors, circuits operating in class A mode yield the smoothest transfer functions and widest bandwidths, hence their near universal application in preamplifiers and other low power circuitry. (Audio, February 1977 by Nelson Pass) download (277k)