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

[Scott Gravenhorst]

Justin Owen <juzowen at googlemail.com> wrote:
>-----Original Message-----
>From: Florian Teply [usenet at teply.info]
>Received: 11.09.2008 13:57:50
>To: synth-diy at dropmix.xs4all.nl
>Subject: Re: [sdiy] Op Amp AC Coupling/DC Biasing (was Blocking audio at power	off)
>
>On Thursday 11 September 2008 15:30:52 Justin Owen wrote:
>>>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.
>
><snip>
>
>>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.
>
>I was just about to upload a schematic - then we had a blackout. Hmmm...
>
>http://www.sdiy.org/juz/SDIY_troubleshoot.pdf
>
>This is just the initial op amp inverting summer/mixer - first things first I guess.
>
>Thanks all
>
>Justin

Well, IMO, when the power is not applied, I would suspect that the circuitry in the opamp
exhibits a more or less high impedance condition between it's pins.  The resistive path from each
input, through it's cap, and through the opamp feedback resistor (both being 10K - a moderate
impedance) and finally the output capacitor is probably fairly low in comparison.  So it seems
possible that the signal makes it from input to output through those resistors and caps, but with
some (but not severe) attenuation due to some resistive path to ground inside the opamp.  I don't
think this is unusual, it just is.


-- ScottG
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-- Scott Gravenhorst
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