Probing circuits with op-amps

Hello all. I'll admit up front i'm using you guys for some basic
electronic questions, it's not directly vector related. Hope that's
not a problem.

I'm trying to diagnose a sound circuit that has started having a hum.
 It's not a grounding problem and swapping the sound board with
another causes the problem to go away. The voltage is correct and
there is little, if any, AC ripple.

What I'd like to do is use my scope and see the sound waves that are
generated before the op-amp. My working assumption is the op-amp has
gone bad, so I'd like to see the input to the op amp is and what the
output is.

After playing around with this a bit, I realized it's not as simple as
connecting a probe (with proper ground) to the input pin and then to
the output pin. The circuit, by design, feeds the output back into
the input, striving to make the IN+ and IN- equal. Because of this,
I never see anything interesting on the scope when probing the input.

Now... is my reasoning correct?

FWIW, the circuit is a typical inverting amp with IN+ going to ground
and IN- having the source. There are the expected resistors in there
also.

What is the correct way to see the input to the op-amp? I'm thinking
I would need to pull the leg on a resistor that is inline with the
op-amp's input (to break the connection before the feedback and before
the op-amp) and measure. Is there a better way?

Or am I all wet and I've got it all wrong?

Thanks...
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Received on Tue Aug 27 10:26:16 2013