DC stability issue with this audio amplifier

Question

This design has a stability issue.

The DC output point (without input signal) should be 0 V with a small offset, but it's not the case, because this point is not stable.

Initially the amplifier had an offset of about 200 mV but it wasn't stable; it changed from 100 mV to 300 mV pretty fast (so fast that you can see 2-3 changes a second on a multimeter).

I tried to add an input signal and the amplifier itself worked well but the offset was always unstable.

After a while this offset started to drift and increased to 400 mV, 500 mV, 600 mV, and more, sometime it became negative, always with fast changes.

Every time I turned off the amplifier and then on again this offset increased, even if the amplifier was cold enough. Actually there is 2 V of offset.

The trimmer on R4 should set the offset, but it has no effect on the circuit, the offset goes its own way. There is a stability DC issue but I can't find it. Maybe something is wrong with the schematic?

Answer 1

As discussed in detail in the chat (link in the comment under your question), this amplifier schematic is reasonable and will work if built properly. It should not have problems with DC offset drift, especially not as much as 2 Volts - you should expect a few tens of millivolts at most.

The amp can be improved a bit by removing C2, which might degrade its stability at high frequencies slightly.

A few troubleshooting hints:

• Is C11 okay? It must be a bipolar electrolytic type. Check if it's maybe shorted out.
• Do the current sources work? There should be about 600mV across R18 and R21.
• Does the Vbe multiplier work? The voltage across it should be around 2.75V (from Q6 collector to emitter).
• Is there enough bias current for the Vbe multiplier and driver transistors? R5 and R17 should both have a bit more than a volt across them.
• Is the input grounded properly? Check that the voltage at the input node really is zero. If in doubt, just short it to ground for testing.
• Try removing R24 and connect C1 from the collector of Q1 to ground instead. You can also increase its value to make the amp more stable (I'd recommend 220pF from Q1's collector to ground.)
• Check the voltage on the left side of R1. Is it near the input voltage? If yes, then input buffer works. Also check if perhaps R1 has a shaky connection. (Thanks tobalt!)