[sdiy] Op Amp AC Coupling/DC Biasing (was Blocking audio at =?iso-8859-1?q?power=09off?=)

[Florian Teply]

On Thursday 11 September 2008 17:41:43 you wrote:
> >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.
>
hmm, looks somewhat like i expected ;-)

Let's imagine having some sort of source connected between IN1 and GND and a 
load connected between OUT and GND.

For the sake of argument, let's imagine the source being some ideal voltage 
source and the load simply being a 10k resistor, both caps being some sort of 
AC-only short circuit.

Now let's switch on the source while keeping the OpAmp unpowered. As the OpAmp 
has no power, it simply does nothing, so - it being an ideal OpAmp - it acts 
just as if it wasn't there in  the first place.
So we have something like this:
Source - C13 - R18 - R13 - C19 - Load, leaving the Ground connections out for 
clarity. As said, those caps act as short circuits at audio frequencies, so 
they're virtually not there as well, which leaves a simple voltage divider 
consisting of both R18 and R13 as one part and the load resistance as the 
second part. As we imagined the load being another 10k resistor, this makes 
for:
Source - 20k (R18+R13) - 10k (Load) - GND,
so, the voltage measured on the load would be one third the voltage of the 
source.

Do we agree so far? ;-)

Greetings,
Florian
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