Mixing 4 audio channels to 3?

Latest rev:

You do mean each of the op amps' *inverting* inputs, yes?

Is this what you mean (see link)?

Thanks.

Thanks guys. Fixed:

All else looks good?

Are cap values reasonable? I added C8 & C9 out of habit of seeing in other designs. Values for these?

Thanks.

You still have the signal go to the non-inverting input. The way the schematic is, U1-U3 will throw their output hard to a rail or oscillate with positive feedback from R13-R15.

I worry that the RC time constant would have the reference be not at the

The current version of the drawing has signal going to the inverting input

Suggestions?

That means you're not looking at the right version of the drawing. Copy & paste this into a browser:

Suggestions?

Thanks.

What does U4 do?

Agreed.

I'd eliminate C9. Some op amps aren't able to drive capacitive loads without exhibiting instability.

If you do want some noise reduction on your reference, I'd add a small decoupling resistor (say, 47R) between U4 and C9, and perhaps use another .1 uF for C9. If you're using a good low-noise op amp, you can probably just omit the filtering here and feed U4's output directly to your "common".

I'd also recommend decoupling your 16-volt power supply, with a .1 uF located as close as practical to the V+/V- pins of each op amp.

Remember to get the polarities of C1-C7 correct when you install them (+ to the op-amp side, - to the outside world).

OK, done.

Sound like basic good advice. :-)

I presumed that such coupling caps should be non-polar. No?

Thanks.

Provides a Vcc/2 local "ground" so I can use these op amps with a single supply voltage.

First you can replace C1-5 with 1uF each. Replace C8 with 220uF, and omit U4 & C9 entirely. You dont want to use a 50k pot followed by a 10k load (R5-12). I'd go with 10k pots and 100k for R5-12, adjusting the nfb Rs accordingly.

NT

Since the V- pin is already PS ground, I need decouple caps only on the V+ pins, yes?

Leave C5-7 as is?

Teach this man to fish: why don't I want to use 50K pot & 10K load combination?

"adjusting" means replace those with 100K's also?

Thanks.

More useful to let it oscillate as a square wave generator at 100kHz or so, and rectify the output into a negative 15V rail. That way you can run the op amps the way they are meant to be run.

KISS

It's largely redundant anyway.

Suggest a circuit...?

Thanks.

No need for that. You're going to have an 8-volt bias sitting on each cap (half of your supply voltage), and the audio signals that they see will only be a volt or two, peak-to-peak, so the caps will always be polarized in the direction I indicated.

It's entirely usual and standard practice to use polar electrolytics in this sort of situation. If you want to get fancy I'm sure you could find an exotic 'lytic (like one of the new solid-electrolyte types), but I see no need for that in this application.

You *could* use nonpolar 'litics if you have them around, but as they're usually more expensive I don't see the point.

Good practics is to put the bypass caps as close to the IC leads as is practical, and run short traces (or wires) to the IC pins.

I wasn't suggesting one bypass cap from V+ to ground and another from V- to ground... since you're using a single-sided supply and V- is DC ground, that would be redundant (as you have noted).

that would work

NT