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

[Florian Teply]

On Thursday 11 September 2008 15:30:52 Justin Owen wrote:
> 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.
>
Umm, maybe i missed it in another post, but do you possibly have a schematic 
at hand or on a website we can refer to?

I seem to have some sort of idea of what could possibly happen there (the 
circuit acting as some sort of voltage divider when not powered), but that 
would be a lot easier to verify and explain with at least a very basic  
schematic to point to.

Greetings, 
Florian
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