24-04-2012, 05:10 PM
This amplifier uses dominant pole stabilisation that includes the output stage (C9). This means that output stage distortion at HF is reduced, but including the output stage is normally risky and can risk oscillation. If replacing C9 fails to help, adding 100pF or so between collector and base of T2 might help.
But it would help to know the oscillation frequency. If it's a couple of MHz or less, then you have Nyquist oscillation - i.e. the whole damn lot is oscillating. But if it's much higher, then it could be local oscillation in one of the output transistors - very common with emitter followers. In which case, putting 100 ohms in series with each of the bases should stop it.
I suspect it's the latter...
Remember the old expression: Amplifiers oscillate; oscillators don't
Good luck,
Mark
But it would help to know the oscillation frequency. If it's a couple of MHz or less, then you have Nyquist oscillation - i.e. the whole damn lot is oscillating. But if it's much higher, then it could be local oscillation in one of the output transistors - very common with emitter followers. In which case, putting 100 ohms in series with each of the bases should stop it.
I suspect it's the latter...
Remember the old expression: Amplifiers oscillate; oscillators don't
Good luck,
Mark
