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

Tim Daugard daugard at sprintmail.com
Tue Mar 18 23:26:54 CET 2008

From: "Justin Owen" <juzowen at googlemail.com>

> 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.

Huh . . . I think this description and the next are the same.

> 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.

My preference. If the non-signal leg of the pot is connected to the negative supply rail -
especially on a single supply circuit then there is a good chance that the capacitor is
biased in the proper direction.

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

Opamp doesn't matter much - I pick the opamp for low noise and don't worry about single v
dual supply. All my circuits run dual supply.

> a) Is there a right and wrong way?

Yes, the right way is to use non-poalrized caps in this application. I use 100 nF caps.
But, I've been know to use poloarized also.

> b) Why?

Polarized caps have a limmited life span with an AC signal. Though the only one I've had
fail so far was in a tube amp - but that failure was impressive. My son still talks about
it when he discusses being a mechanic instead of an electrician for car repairs.

> c) Since I'm not using dual voltage or AC - do I still need the caps anyway?

Probabily, but depends on the source and what you connect the non-signal side of the pot

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

I don't think so, I like caps after the volume pots, it kind of implies that you pull no
DC voltage (okay there is some DC involved in charging the cap) through the wiper on the
pot - hopefully extending the pot life.

Anyway, here is mine, not necessarily 100% correct but works for me (until one fails.)


Tim Daugard
AG4GZ 30.4078N 86.6227W Alt: 12 feet above MSL
http://home.earthlink.net/~synthfred/h_toctop.htm (Fred's online edition)

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