Mixing 4 audio channels to 3?

[DaveC]

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.

 

[DaveC]

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

Thahks.

 

[Don Pearce]

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.

d

 

[DaveC]

Is there something wrong with my browser? I can't see any of these

component references.

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

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

OK, can do that.

Suggested values for resistors?

Thanks.

 

[DaveC]

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

NT

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.

 

[Don Pearce]

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.

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

 

[DaveC]

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.

 

[Don Pearce]

And the value for the potentiometer?

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

 

[DaveC]

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

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.

 

[Don Pearce]

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.

d

 

[DaveC]

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.

 

[DaveC]

Latest rev:

<http://i40.tinypic.com/300feaq.jpg>

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

Comments welcome.

Thanks.

 

[Tomi Holger Engdahl]

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

10 kohms for all fixed resistor should work...

 

[DaveC]

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.

 

[Les Cargill]

Which raises the question I was pondering recently: is it OK to connect 2
computers' sound cards' grounds together?

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

 

[Don Pearce]

Which raises the question I was pondering recently: is it OK to connect 2
computers' sound cards' grounds together?

No. You will certainly create a ground loop which will result in mains
hum.

d

 

[[email protected]]

No. You will certainly create a ground loop which will result in mains
hum.

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.

 

[DaveC]

is it OK to connect 2 computers' sound cards' grounds together?

No. You will certainly create a ground loop which will result in mains
hum.
d

This kinda kills the project, doesn't it? (At least, the straightforward
solution.)

The alternative is...?

Thanks.

 

[Don Pearce]

This kinda kills the project, doesn't it? (At least, the straightforward
solution.)

The alternative is...?

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

 

[DaveC]

10 kohms for all fixed resistor should work...

Hi Tomi. Thank you for your advice. I have enjoyed your creative designs for
many years.

Do you have any comments on my revisions of the circuit (ie, are caps
recommended? should I be concerned about cross talk if I take resistors from
all 4 channels to drive the sub woofer? etc...)

Cheers,
Dave