Mixing 4 audio channels to 3?

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.

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

The alternative is...?

Thanks.

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.

I've yet to have a problem doing it.

NT

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

Thanks John. Good simple design.

Can you suggest a good op amp for such an audio application?

Is it possible to use single-supply amps throughout?

Thanks, Dave

No tools.

I made the L & R op amps fixed gain and added input level control pots:

Comments?

Thanks.

Should all pots be log taper?

Thanks.

Can't find them on Mouser.

Suggest another one that's good for audio?

How about LM833? ::

The SR and GBW beat the 1007... ;-)

Thanks.

Is the type of resistor important (wire wound, etc.)?

Thanks.

Only a single supply (in the amplified speakers) is available to power this circuit. I can tap this supply for my circuit:

I've added input & output caps. Are these values sound? ;-)

How do I go about getting a 1/2 Vcc ground reference? (See my non-EE attempt.) What values to use for the divider resistors?

What needs to be reference to the new "ground"? Everything between the input caps and output caps?

Thanks, Dave

The easiest way is to take two equal value resistors connected in series across your supply (as you have done R16/17). The centre point is then connected to the + input of another op-amp which has its output strapped back to its input - terminal. this forms a voltage follower and the output is your Vcc/2 reference. This provides a low impedance source and the resistors can be almost any value you like - anything between 10k and 1M say.

If you want to do it with just resistors, you could make R16 and R17 somewhere around 4k7 each, and add a few uF of bypass capacitance around one or both resistors. 1k would give a stiffer reference if you don't mind the additional power consumption.

If you can spare an op-amp section, you can get a better (stiffer) ground reference than you'll get with just resistors, with lower power dissipation (I think). Use R16 and R17, and a small cap from the junction point, to create a Vcc/2 reference, but do not "ground" this directly to your internal reference point (the rectangular-looking ground symbol). Instead, feed this to the noninverting input of an op-amp section, feed the op-amp output back to the inverting input (i.e. create a unity-gain follower), and use the op amp's output as your ground reference. In this arrangement R16 and R17 can be high-value (100k?) as their junction point will be looking into a high-impedance op amp input.

Pretty much... each of the op amps' noninverting inputs, and the "bottom ends" of the potentiometers, as you have drawn them. *NOT* the V- input to the op amp(s), of course.

You might want to add "pop preventer" resistors at the inputs and outputs... say, 100k to DC ground, from the "outside" end of each of the DC-blocking capacitors.