[sdiy] Blocking/Decoupling caps on a summing op-amp mixer

Justin Owen juzowen at googlemail.com
Tue Mar 18 21:15:30 CET 2008

Hey all,

Apologies for asking such a basic op-amp question here, but I'm having a hard time finding a definitive answer/explanation.

I've just breadboarded a standard op-amp-based summing mode/inverting amp mixer - all three schematics I had all showed polarised capacitors at each audio input (channel) with the positive side facing towards the op-amp - so: audio input to negative side of cap to (feedback?) resistor to (inverting?) input of the op-amp.

Works fine - no probs. But then I found 3 new schematics - exactly the same layout - but the caps have the negative side facing the audio input.

Seems to work either way, I'm only using a single-supply op-amp (TS27L2) - but I'd like to know:

a) Is there a right and wrong way?
b) Why?
c) Since I'm not using dual voltage or AC - do I still need the caps anyway?

Oh - I'm guessing having the caps before any Volume pots would also make more sense?

Many thanks,


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