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ZEN amplifier page

Single stage, single-ended Class A amplifier, 2x15W Nelson Pass designed a very simple amplifier, called ZEN. Here you can find schematics, construction details, pictures. Available as a legacy project from PASSDIY.COM website.


Aleph 3 amplifier page

Two stage, single-ended Class A amplifier, 2x30W Nelson's popular Aleph series are out of Pass Laboratories production. The project is ongoing


Aleph 30 amplifier (updated) & Aleph 60 (new)

Improved versions of Aleph 3 and Aleph 2 amplifiers. I re-designed myself the PCB layout, to be as close to the original design, as possible.


Classé DR-3 amplifier

Classical amplifier from Canada, designed by Dave Reich.


Classé DR-9 amplifier

Classical amplifier from Canada, designed by Dave Reich.



All the things here only for DIY purposes only. Not for commercial use!


About amplifier classes...

As a quick refresher for those who may not remember the key characteristics of the various amplifier "letter" classes, the class designations for audio and RF amplifiers have been in use since the first active devices (vacuum tubes) came into use, and are standardized. Although there are classes all the way out to Class J, the most common are:
-Class A biases the stage in its linear region, where it amplifies the entire input wave cycle; the active device is consuming power all the time. Distortion can be very low, but dissipation is very high
-Class B biased the stage so it amplifies only half the cycle, and is shut off during the other half; often used with a complementary device for full coverage. (Class AB is midway between Classes A and B in operation.)
-Class C biases the stage so it amplifies less than half the input cycle, and is off most of the time
-Class D uses the active device in either fully on or fully off mode, which is very efficient but also very distortion, unless appropriate filtering is used.

If these concepts are unfamiliar to you, perhaps you are a hardcore digital or software engineer who has, um, shall we politely say, "forgotten" the concepts of a transfer function and basic circuit biasing? I suggest you do some quick studying up on this venerable classification system, since it has been in use since the 1930s, long before our modern, IC-driven world, and is still very relevant today! (And it applies to RF amplifiers, too? E.H. Armstrong's first FM transmitter used a Class C design for its final power-amplifier stage.)
Bill Schweber, Site Editor, Planet Analog


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