Also decreased R13 to 68ohm so as when there is only voice audio being played it wouldn't pick up the signal well, so having a lower ohm resistor will help discharge the cap further down with the short detection periods of voice.
What you have said indicated that values of R13 and R15 are very critical to the present operation of the circuit. In that case, I would propose to disable the audio signal in another way which does not affect the function of R13 and R15.
That's very smart. Thank you.
Now it works, picks up the audio signal fine when there is audio, and after it switches to standby it bypasses opamp 2 signal to ground and does not pick up any further signal.
One problem though, When you play audio, and stop it there is a sound that comes out of the speakers like a static (or a discharging of something?) that lasts for 2 seconds and then disappears. This sound does not happen with the unmodified design (maybe happens once when you switch the unit on but does not repeat after that). I just removed the jumper connecting pin 5 to the collector of TR2 and the sound disappeared. Should R18 be increased to maybe 100K? What exactly is the purpose of R18?
Edit: I changed R18 to 20K and that static sound seems to disappear. But at the same time I also changed C1 to a nichicon FG audio cap. Don't know which eliminated that static sound but it's gone.
Thanks again.
Edit2: That static sound is there when playing low volume sounds even after removing TR2 and the modification. If I remove the shielded audio in cable it's gone. And with that cable removed if I touch the probes of the multi meter to ground and the negative of C1 the static sound comes back and this time continuous and not with low volume audio. I think something is leaking from this board to the pre amp board of the sub. But where? And how do I prevent it?