Mixing 4 audio channels to 3?

Latest rev:
<
http://i40.tinypic.com/35m026h.jpg


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

Is this what you mean (see link)?
Thanks.
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In article

Distortion performance might be better if you run the amps in inverting mode. If you run the audio into the non-inverting inputs, the inverting input follows and the amp has to operate throughout it's common-mode range. If you use the amps in inverting mode, both the + and - inputs will stay very close to ground.
Isaac
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No.

Your drawing is wrong; signal goes to the inverting (-) inputs and the
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Thanks guys. Fixed:
<
http://i44.tinypic.com/r1k8qa.jpg
All else looks good?
Are cap values reasonable? I added C8 & C9 out of habit of seeing in other designs. Values for these?
Thanks.
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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
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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?
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OK, here's opamps 101:

You're using the inverting - or "summing" - amplifier configuration
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Thanks for the great explanation JF.

The link doesn't work, and my service doesn't carry binaries.
How about putting it here? ::
<http://www.tinypic.com
Thanks!
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--
They don't accept .pdf's.

Send me your email addy and I'll email you a copy.
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On 11/12/2011 7:21 PM, John Fields wrote:

Our hearing is more or less logarithmic, so it might be about right.
Further more most volume pots in audio are logarithmic.
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1uF
that would work
NT
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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 1/2 way point while C8 charges on powerup. I don't see a C9. Might do something not so good to the subwoofer.
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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:
<
http://i44.tinypic.com/r1k8qa.jpg

Suggestions?
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What does U4 do?
d
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Provides a Vcc/2 local "ground" so I can use these op amps with a single supply voltage.
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Hint: Don't use the chassis ground symbol as a "Vref" symbol. It's confusing and someone along the line might get hurt. Grounds should be.
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confusing
Yeah, I wasn't sure about that. How should I have indicated 2 separate grounds?
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It's not ground. It's Vcc/2. I generally call it Vref, or some such thing.
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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.
d
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snipped-for-privacy@spam.com (Don Pearce) wrote:

??
What is wrong with using an op-amp to "amplify" DC? Surely they are "meant" to do that.
Isaac
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