Mixing 4 audio channels to 3?

If I'm on the right track, what are the suggested values for resistors?

Thahks.

s to

SB

Lose Rs 1-4, keep 5-8, lose R11, and lose C1,2,3, then itll be good.

NT

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.

Sorry. They didn't make onto the jpg file. Added here:

OK, can do that.

Suggested values for resistors?

Thanks.

Callouts added:

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:

Why would you do away with them?

What values for the resistors do you suggest?

Thanks.

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.

And the value for the potentiometer?

Thanks.

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.

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.

No, that would be about right.

I don't see component references, either. However, assuming R1-R4 are the resistors from the inputs to ground, they may be necessary to provide a proper impedance to the source driving them.

To reduce crosstalk, I'd replace the two resistors from the 220nF cap junctions to the pot with four resistors from each of the four inputs to the pot. Presumably, the output impedance of the amps is low compared to the resistors reducing crosstalk.

For mixing signals there is a method using amps and feedback where the connection between multiple sources is at a virtual ground, eliminating crosstalk.

ards to

a USB

s to

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

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?

Thanks.

Latest rev:

All resistors 5K-10K. Pot 100K.

Comments welcome.

Thanks.

IC

ted

Nothing like that, no. The amp IC requires the caps, so theyre needed, no question about it. But theyre needed for the amp, not for mixing purposes, so if the 2 circuits can be disconnected by the end user, then the caps need to be with the IC amp part, not the mixer part.

NT

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.

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).

Thanks.