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 :)
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.)