I'm looking for an existing design for a good yet simple class D audio amplifier using discrete semiconductors, no ICs. The purpose is educational, with a final product that actually could be used.
- not bad quality (but we don't need top quality, or to compete w audiophile gear)
- enough power to drive modest size speakers (it's hard to be quantitative about this)
- no exotic parts.
- within reach of serious hobbyists, EE students
- not too simple; not for beginners or kids to make.
- need not be commercially viable or survive outside an electronics hacker's home.
- small transistor count prefered, but no need shave every penny or cut corners.
- illustrates nicely the operation of class D amplifiers
Best Answer
I've done this before (though only in simulations) with a quad high-speed high slew rate op-amp. So there is one IC in it, but if you're really anal about that, you could implement it at the transistor level. Anyway, two op-amps are used as a triangle/ramp wave generator, which is then fed into a third op-amp which compares the audio signal with this ramp - this generates a PWM drive, which is applied to a MOSFET gate driver (a simple transistor driver I found on the 'net) which drives a MOSFET which passes current on request and filtered as required using inductors and capacitors. I don't have schematics, but this is how I would implement it.
The PWM frequency decides the quality of the audio. Higher frequency PWM will increase MOSFET heating and decrease efficiency due to increased switch rate but it will improve audio quality.