[sdiy] Op Amp Question

[Stewart Pye]

Hi Harry,

Hopefully the schematic I just send is readable. The other 22k resistor 
is there because when the switch is off I still want the op amp 1 output 
(at half the level of when the switch is on).

Op amp 1 is an integrator (no feedback resistor but a feedback cap). Op 
amp 2 has a 22k feedback resistor. Would the fact the OA1 is an 
integrator be a problem?

Regards,
Stewart.

Harry Bissell wrote:
> In effect, the low impedance stops crosstalk. I'm assuming each has
> a feedback resistor, the feedback would remove any residual signal.
> Just keep the resistors high enough in value to avoid loading the opamp.
>
> I didn't quite follow the ascii-matic... I think you don't need the additional
> 22K resistor, you can send the junction of the 22Ks direct to the inverting
> input of the opamp (through the analog switch)
>
> H^) harry
>
>
>
> ----- Original Message -----
> From: Stewart Pye <stewpye at optusnet.com.au>
> To: Synth DIY <synth-diy at dropmix.xs4all.nl>
> Sent: Tue, 14 Dec 2010 16:06:19 -0500 (EST)
> Subject: [sdiy] Op Amp Question
>
> I have a circuit where I want to switch two additional inputs to an op 
> amp summing circuit using an analog switch. So that I can get away with 
> using just one analog switch I had the idea of joining the summing end 
> of the resistors together and have an analog switch between there and 
> the summing node.
>
> It would look something like this.
>
>                               |---------////22k////---------|
>                               |                                          |
> OP amp 1 Output --- ////22k//// |                       |
>                                                analog switch --- Op Amp 
> inverting input
> OP amp 2 Output --- ////22k//// |
>
>
> My question is when the switch is in the off position we are left with a 
> 44k resistor between the two op amp outputs. Would this cause any bleed 
> from one output to the other, or does the low output impedance of the op 
> amps eliminate this?
> When the switch is off I want only the op amp 1 output ( with half the 
> gain of when the switch is on) but don't want any bleed from op amp 2 
> output...
>
> Cheers,
> Stew.
>
>
>
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