[sdiy] Op Amp AC Coupling/DC Biasing (was Blocking audio at power off)

[Justin Owen]

Sorry to be banging on about such a simple thing, but a) I'd like to get it right and b) I'd like to understand it and learn from it.

I'm trying to nail the fact that audio is travelling from an input through to an output when there is no power in the circuit. This is not wanted. I could use a relay as has been suggested - but I don't understand why it's happening in the first place.

For troubleshooting I've disconnected everything except the first mixing/summing op amp and the first audio input.

I have an op amp in standard single-supply summing mixer configuration. It's taking in a sine wave from an external soundcard via a mono 1/4 " jack. It's AC Coupled at input and output using 10uF electrolytic, polarised caps. Negative side facing outwards/away from the op amp.

I'm measuring 1.2 volts AC where the audio enters the circuit - and with power on - I'm getting 1.2 volts AC at the op amp output. It's going back into an input on the soundcard and looks like a sine wave when it gets back there.

With power going to the circuit, and no load on the op amp (i.e. no input or output connected) I'm measuring an even VCC/2 (about 4.5 volts) at both inputs and the output.

That all makes perfect sense to me.

What doesn't - is that when I disconnect the power - but leave the input signal on, I'm still getting 1.2 Volts AC at the input but there is now about a half a volt DC at the inverting input and the output of the op amp and I'm still getting audio through - albeit at a much quieter level.

Here's what I hope is a clue - if I disconnect the resistor between the positive side of the cap that is AC coupling the input and the inverting  input of the op amp - the cap discharges after about 15 seconds - and there is no longer a DC value readable at the negative side of the cap or any audio at the output.

I'll happily admit that I don't understand enough about the guts of AC Coupling/Biasing - and even op-amps to know how to how to diagnose the problem in order to come up with a solution...

Any help, guidance, advice on this one would be hugely appreciated.

Justin