Cloning a classic…

I have been looking at class AB amp designs for a while, trying to find a “compromise” between my low-power (but very large) class A and high-power (and compact) class D builds. For some reason very few among the class AB designs managed to “stand out” to me with the right combination of simplicity, compactness and reputation. It’s not that I really had anything specific in mind, I just kept looking at stuff and thinking “naah, that’s not what I want” 🙂

However, one design that did keep cropping up was Rod Elliot’s “Project 3A” (or just “P3A” for short). This is a discrete amplifier with a reasonable power level for normal use and a very simple design. There’s plenty of evidence out there to support that the performance is good and even a way threads to evolve the design (search the “solid state” forum on diyaudio.com)

Rod sells PCBs for the P3A and that would of course have been the easy route, but because I had a specific form factor in mind I decided to “roll my own” 🙂 The end result is app. 70mm square (flat mounting on a 75mm heatsink were one of the key design criteria) and quite compact. My only concession over the original is that I removed the input capacitor. Well, I didn’t actually remove it on this version since there is space for a bipolar electrolytic from Muse or Blackgate, but the main version is intended for having the input cap off-board.

As I wanted a matching power supply I “recycled” the last Gainclone PSU I did but added a second capacitor bank (which just fit on a board that is still constrained by me using the free version of Eagle). Since the P3A runs on 35V rails it is possible to use 50V capacitors and then a reasonable capacitance is still feasible with this PSU footprint – especially in a dual-mono setup. Of course the board has space for normal 35mm snap-in caps as well, but that’s so boring 😀

I have only done this test version of the amp so far and confirmed that it works and that it plays music. (This is also the reason for the transistor pins not being trimmed properly – bias adjustment). That said, I do have a couple of case ideas in mind for this one where the lower heat of a class AB amp will be welcome (or should I say “required”) 😀

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One Response to Cloning a classic…

  1. Pingback: Project files: Little helpers – Capacitor boards | theslowdiyer

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