Not too sure what your goal is. Messing with the supply voltages of an
opamp will make them unstable. They will cause a dc "pop" when switched
on or off due to settling. Are you trying to switch audio on and off or
linearly fade the audio? There are many other ways of obtaining your
goal if you can be a little more explicit in your application. Muting
audio can be done with just one bipolar or fet driven from a pic by
shunting the audio to ground. Also, as long as you have a limiting
resistor on the input of an opamp you shouldn't hurt the device with
power removed.

Rick

Joshua Hertz wrote:

> Hello all,
>
> I have searched through the archives and haven't seen
> anything like this, so I thought I'd ask everyone.  I
> would like to build an audio pre-amplifier with a PIC
> to control the user interface so that it is remote
> controllable and so that I don't have to use a plain
> knob-type input selector.  I would like to keep the
> signal path away from mechanical switches which can
> dirty over time or be noisy.  I was thinking of doing
> something like this to accomplish the switching of
> components, but I don't know if it will work:
>
>                                ^
>                                |
>                               RC
>                   +12__   _____|
>                        \_/     |
>           |---|    |----|      |
> Buttons---|   |----|       in--O/A>----out
>           |PIC|    |----|      |
>           |   |     ___/-\_____|
>           |---|  -12           |
>                               RC
>                                |
>                                V
>
> That's an awful drawing...The idea is that the PIC
> reads the buttons and sends one line high.  Each line
> (I've only drawn one) corresponds to one input.
> Sending that line high will turn on a couple of
> transistors that provides one opamp (O/A) power.  The
> RC's to ground are to provide some amount of soft
> on/off for each line.  Each line in (CD/DVD/tuner...)
> has its own op-amp unity gain input buffer stage.  The
> input buffers are all joined at the output.  So, I am
> thinking that without power applied to the op-amp very
> little signal will pass and so only the signal desired
> makes it past the buffer stage and onto the rest of
> the circuitry.  Will this work the way I'm thinking it
> will?  Ideally, the line coming into the "off" opamps
> will be muted to below the noise floor of the line
> turned on (-100dB?).  Can I damage an opamp by
> providing input signal w/o power?  If this doesn't
> work, would a solid state relay work?  Any suggestions
> as to components to use?
>
> Thanks for your help!  I've gotten lots of good
> suggestions form reading the archives!
>
> Josh
>
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