Re: Probing circuits with op-amps

Your understanding is correct. When an opamp is connected in a negative
feedback configuration (the most common case) the difference between the
two input pins is 0 volts, and as one is tied to ground, they will both
measure 0 volts with respect to ground. If you measure voltage across the
input resistor (or equivalently the voltage at the input of the input resistor
with respect to ground) you will see the signal you are looking for.

However, my guess would be the problem is with electrolytic caps on the
sound board. They dry out with age and heat, and start to become less
effective. Try replacing all the electrolytic caps on the sound board,
and see if the problem goes away.

___
Ken

On 08/27/2013 07:26 AM, Altan Pinball wrote:
> 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 Fri Aug 30 15:06:54 2013