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Soft clipping power amp design found on the 'net: questions

Started by ampetrosillo, February 21, 2024, 03:26:55 PM

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I have found the following design on the 'net:


It looks interesting, although very unfamiliar to me: it's a soft clipping power amp with, by what I read, low global feedback, current feedback and (therefore) high output impedance. It looks very well suited to guitar duties, although I'm not an expert (I can find my way around preamps and I can also design them, but I'm not familiar with power amps).

I've tried deciphering the schematic, but as I said, a lot is unfamiliar to me: the input stage isn't the common differential pair and kinda looks like an output stage? Also, it employs MOSFETs while most of the things I've read up on employ BJT transistors (there shouldn't be much difference though, MOSFETs are biased similarly to bipolar transistors, with the difference that the gate is basically electrically isolated).

Could you please walk me through it and explain to me if and where it can be improved upon?


The input is simply a dual differential pair, one NPN the other PNP. Each pair has it's own active current source. The outputs of the diff-pairs feed current mirror driver transistors where most of the Voltage amplification takes place. Bias for the output stage comes from a Vgs multiplier which matches the temp-co of the output MOSFETs much better than a Vbe multiplier. The source follower type output clips softly because the output Voltage can only swing to about 5V less than the rail Voltage. In addition, the zener diodes across the feedback resistor can cause soft clipping depending on the diode's characteristics.

Hard to tell what the open-loop gain is without modeling or building one, I don't speak German or trust any translation on the net.

Designs like this can be found on the diyaudio.com forum.