Mixing 4 audio channels to 3?

I want to connect the audio out (headphone jack) of 2 computer sound cards to a desktop & woofer speaker-amp combination. (The original input was via USB
only and I'm modifying this for analog audio input.)
This is my guess at the necessary resistors to mix down these 4 outputs to the 3 inputs in the amplifier (L, R, sub):
<
http://i41.tinypic.com/97mpud.jpg
The data sheet for the Philips TDA8510J amp IC shows an application for 2 channel input:
<
http://i40.tinypic.com/5ets9w.jpg
Suggestions welcome.
Thanks.
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Lose Rs 1-4, keep 5-8, lose R11, and lose C1,2,3, then itll be good.
NT
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Callouts added:
<
http://i40.tinypic.com/5ets9w.jpg
R11 provides some adjustment for sub. Otherwise no way to have any difference between L & R channel volume and sub volume.
The caps are suggested on the data sheet example:
<
http://i40.tinypic.com/5ets9w.jpg
Why would you do away with them?
What values for the resistors do you suggest?
Thanks.
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For that particular circuit with that chip, the caps are necessary because those inputs are not at ground potential. If you are feeding normal hi fi unit inputs the caps aren't needed. Resistor values around 5 to 10 k would be what you need.
d
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And the value for the potentiometer?
Thanks.
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That depends entirely on the input impedance of the subwoofer. You could try a 100k pot and connect it as a normal volume control - one end to the incoming signal, the other end to ground and the slider to the subwoofer.
d
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The data sheet says that the 2 single-ended inputs (R & L) are 50K impedance. The bridge-tied load ("BTL") input -- used for the sub -- is 25K impedance.
Does this suggest any change to your recommendation of a 100K pot tied to ground?
Thanks.
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No, that would be about right.
d
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Latest rev:
<
http://i40.tinypic.com/300feaq.jpg
All resistors 5K-10K. Pot 100K.
Comments welcome.
Thanks.
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wrote:

Is there something wrong with my browser? I can't see any of these component references. Actually I see what you mean. One problem here is that R9 and R10 are going to cause crosstalk. Without some active electronics there is no way around that apart from taking the sub from one channel only. This is actually quite common.
d
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On Oct 26, 5:09pm, snipped-for-privacy@spam.com (Don Pearce) wrote:

I was hinting that the OP might decide to add numbers to the components, otherwise its sufficiently tedious to talk about them that people often just wont bother.
Yes the caps are necessary, but for the amp IC, not for the mixer. House them together and its the same difference, house them apart and the distiction matters.
NT
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This is unclear to me.
Do you mean that if the mixer resistor matrix is housed close to the amp IC that the capacitors won't be needed? And if they are separated and connected by shielded audio cables that the caps will be needed?
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--
Since the chip is being powered by single positive DC supply, The
only way to get signals which swing below ground into and out of the
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wrote:

Or, as is not uncommonly done these days in portable digital players, establish a floating ground for the purpose of providing and signal for the (floating) load, so that the difference between the output and the floating ground can become negative.
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Which raises the question I was pondering recently: is it OK to connect 2 computers' sound cards' grounds together?
The PS for the powered speakers is via "wall wart" (not grounded).
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DaveC wrote:

It should be. If you are concerned, check with a meter first.

-- Les Cargill
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No. You will certainly create a ground loop which will result in mains hum.
d
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On Thu, 27 Oct 2011 17:59:59 GMT, snipped-for-privacy@spam.com (Don Pearce) wrote:

This is certainly likely but not a foregone conclusion. In any case, it's "OK" (i.e. not dangerous). As long as the computers are plugged into the same circuit, it's quite possible that he can get away with it, assuming line levels. If hum is a problem, coupling capacitors or cheap audio transformers will usually solve it.
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This kinda kills the project, doesn't it? (At least, the straightforward solution.)
The alternative is...?
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Transformers. You can buy decent ones in Maplin for use in car audio. Capacitors have been suggested, but they don't help. If they are big enough to pass bass, the ground loop is still there. It doesn't require a DC connection to function.
d
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