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Ultra Class A Superdrive Power Amp

Started by pelanj, March 09, 2009, 11:55:13 AM

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pelanj

Anyone tried that (http://www.geocities.com/tpe123/folkurban/fuzz/snippets.html)? I tried it today on a breadboard with a 230 / 12 V transformer (15 V no load), which gives twice as much load to the transistor.

The power is really minimal and the amp overdrives very easily and when played clean it is even less loud. Right now I cannot think of any reason why should it be preferred over a LM386 (or similar) based chipamp...but kind of fun to put together with minimal effort.  8)

J M Fahey

It reminds me of the *very* first amplifier I built (around 1968/69, ... don´t ask...) which was the power section of an AM car radio and not much more complex than this jewel: Class A single TO3 PNP Germanium power transistor, driven by a ¡¡¡ Plastic cased Silicon !!!! NPN transistor, using an output transformer and putting out a very loud and powerful 2 W RMS. Quiescent current: around 2 Amps. It run hot !!!
I´ll dig a little and post the schematic, just to remind us how much we advance.
By the way, those snippets are great for experimenting.
I´ll dust my ProtoBoard and start testing those weird ideas.

pelanj

I have found another one - but it seems a little strange to me - as drawn, the quiescent current would flow through the speaker winding. Is does not seem to me as a good idea... http://scitoys.com/scitoys/scitoys/computers/solderless/amplifier.html

In my quest for a low volume guitar amp, I will maybe try out something like this - http://www.google.com/patents?id=eh8yAAAAEBAJ - is that a good idea? :)

pelanj

By the way, I have recently finished a TDA7233 based chipamp (datasheet circuit with small changes). I have built in into a plastic home cinema surround speaker to get a sort of combo amp. The surround has a 3inch fullranger, so the heavy distortion sounds from a pedal sound like nordic-black-metal-hive-of-bees. But it works really well with SansAmp GT2 and Digitech's pedals with speaker emulated outputs.

I hope I can get some opamps and transistors to test the ideas from the previous post soon.

In the Teemu's book i have found a circuit, where an op-amp had a cathode follower in the feedback loop. If this was combined with the class B output stage with BD139/BD140 (corrected by the feedback) and some current feedback as well, it could make a nice small practice amp with "tube harmonics".

J M Fahey

Strange as it sounds, the "class A transformerless" amp, with DC across the speaker *was* used, specially in *very* low cost intercoms, "matchbox" (literally) radios, alarms, and similar "very Lo Fi" projects. I remember an alarm that used a 2N3055 to switch on-off a speaker across a 12V battery (chopped D.C.) between 500/2500 Hz that was unbearably loud and the speaker didn´t suffer. I don´t know how anybody could have gotten that patent: an op amp driving a couple of transistors was, even then, "prior art" or "common knowledge". He got the patent indeed, but it´s un-enforceable in Court. Otherwise the guy would be millionaire today ... and he´s not even known. Any power chip has almost exactly that inside, with the bonus that the output transistors are already part of the chip itself. They can practically be considered "big" op-amps.
Bye.