this post was submitted on 18 Nov 2023
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1 Watt Practice Amp (alien.top)
submitted 11 months ago by coffeecalmsmedown@alien.top to c/diy@hi-fi.community
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Hey folks! I'm trying to design a 1 watt guitar amp with quasi-complementary output, using a 12v supply. The quick sketch below is partly inspired by the Leak 30, as well as a book by Teemu Kyttälä on solid state amps, and some other reading. Not shown is the gm-compensated mirror on the preamp side.

My questions are as follows:

  • do you think 12v is adequate for this circuit?
  • is there anything that's just plain wrong here? Yes, it looks crude/simple, though I'm mostly asking whether I'm going to fry something.
  • would it be worth my while adding a Zobel network at the output? I don't mind some dampening if the ends are justified.

This is a small guitar amp I want to use in my apartment; No more than a single watt is required. And I'm looking for an American-style sound with good distortion at the higher end.

Thanks in advance :)

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[–] bitcrushedCyborg@alien.top 1 points 11 months ago (1 children)

With an efficient and well designed output stage, 12V should probably be able to get you close to a watt of peak power through an 8 ohm speaker. Some suggestions/criticisms:

  1. Add another coupling capacitor between the volume pot and the first transistor to prevent popping when you change the volume.

  2. Something doesn't look right with the right half of the schematic. You have wires jumping over stuff, just to end up being connected to the points they jump over. The output transistors' bases are shorted together, as are the two outputs from the preamp. R3&R5 are in parallel, as are R4&R6. I'd suggest checking out examples of class B push-pull amplifier and class AB amplifier circuits to see what to do.

  3. You'll probably want some way to minimize crossover distortion from the output stage. Look at class AB amplifiers for examples of how.

  4. Simulate it so you can thoroughly test everything. CircuitJS for quick and easy, LTSpice for detail and accuracy.

[–] coffeecalmsmedown@alien.top 1 points 11 months ago

Hey thanks, I'm going to work on it a bit more today and post the whole circuit, input stage and edits included.

I'm kind of stuck on quasi-complementary output for now, though maybe an AB amp would be a good design for afterwards. The Leak 30 Stereo, Mullard 5-10(tube), El Cheapo, Armstrong 600, and the Sinclair Z50 all used quasi-complementary output, and I'd really like to try to get a less-modern sound, if that makes sense. More to come!

[–] fomoco94@alien.top 1 points 11 months ago

With a properly designed amplifier, you'll have no trouble getting 1 watt of output.

These are the problems I see:

  • You probably don't have enough gain in the output stage. There's a reason why darlington outputs are used in almost every commercial design.

  • The voltage divider bias you have won't be stable. As the outputs heat up the Vbe drops. This means the bias current will increase. Which makes them even hotter. Which means bias increases more. Which means... Thermal runaway is possible. The usual cure is the use diodes (or in a typical design a transistor as a Vbe multiplier) mounted to the heatsink so that bias voltage tracks with temperature.

  • Your input stage can't swing close enough to the rails and that will limit the ability of the output stage to achieve maximum possible voltage swing.

  • There's a reason why all modern designs use a diff pair on the input and complementary pairs on the output. It's the best way to do it. The quasi output stage was used when PNP power transistors performed much more poorly when compared to NPNs.

  • In my opinion, you never omit the zobel on the output. It's there to help prevent high frequency oscillation in the power stage.

Personally I'd just use a op amp buffer followed by a diode limiter and then an IC power amp. (Or a discrete amp using the normal modern topology.)