OpAmp based Summing circuit -- Is greater than Unity Gain here a technical problem? - diyAudio
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Old 7th July 2012, 05:49 AM   #1
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Default OpAmp based Summing circuit -- Is greater than Unity Gain here a technical problem?


I'm designing an active circuit that follows a passive summing network. The resistance is 47K on each input channel that is being summed. So, if I use the standard "Virtual Ground Summing Amplifier" design, I'd put a 47K resistor in the feedback loop and after the opamp, I'd end up with a signal of inverted phase and unity gain.

Originally my plan was to use a second opamp after the summing stage to provide some additional gain to the signal. I'm wondering, though, if I couldn't get away with adding my gain here by lowering the feedback resistor value.

Is there any particular reason that I can't use a single opamp for both summing AND adding gain at the same time? Seems like I could kill two birds with one stone...

Always looking to learn,
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Old 7th July 2012, 06:37 AM   #2
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Yes, you can add gain at the summing stage. There is no particular reason you can't, but the later stages need to be taken into consideration; you don't want to overdrive other inputs.
It is error only, and not truth, that shrinks from enquiry. - Thomas Paine
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Old 7th July 2012, 01:57 PM   #3
DF96 is online now DF96  England
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For gain you would raise the feedback resistor value.
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Old 7th July 2012, 05:42 PM   #4
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Thanks for the advice -- and yes, thanks for the correction.
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Old 9th July 2012, 12:14 AM   #5
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What you've described is not a passive summing.

For best gainstaging active summing stages conventionally have a gain *loss* from any single input. Remember that all inputs may sometimes be added, best S/N ratio requires as-large-as-possible signal into the summer, all inputs' series resistors contribute to output noise, and that the summing amp cannot be allowed to clip. The summing amp is actually one of the hardest thing to get right in a mixer.

All good fortune,
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Old 9th July 2012, 12:22 AM   #6
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Hi Chris, thanks for the reply.

I'm repairing an existing mixer and the passive summing stage is already present, intact, and working great. I'm only having to design and build the makeup gain portion of the mixer.

In the original design there were a lot more active stages (an effects loop for instance) so there were a lot more opamps and circuitry. My thought was just to add the makeup gain at the point just after the signal has been passively summed and be done with it, since I don't need any of the extra stuff that was there, and I'm certain I never will.

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