Mixing 4 audio channels to 3?

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.

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.

the

ting

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

On Sat, 29 Oct 2011 00:49:32 -0700, DaveC wrote:

--- Why not go active and get rid of the caps altogether?

.LA>-+---[10k]-+-[10k]-+-------------> -(LA+LB) . | | | . | | +V | . | | | | .LB>-|-+-[10k]-+--|-\ | . | | | >-+ 10K . | | +--|+/ +-[POT]-+ . | | | | | | . | | GND -V | +5 | . | | | | | . | +------[10K]---+---+-|-\ | . | | | >--+--> -(LA+LB+RA+RB) . +--------[10K]---+ +-|+/ . | | . +--------[10K]---+ GND . | | . | +------[10K]---+ . | | . | | . | | .RA>-+-+-[10k]-+-[10k]-+ . | | | . | | +V | . | | | | .RB>-+---[10k]-+--|-\ | . | >-+-------------> -(RA+RB) . +--|+/ . | | . GND -V

Seems to work pretty well...

Version 4 SHEET 1 880 968 WIRE 112 -544 48 -544 WIRE 224 -544 192 -544 WIRE -96 -448 -528 -448 WIRE 32 -448 -16 -448 WIRE 48 -448 48 -544 WIRE 48 -448 32 -448 WIRE 112 -448 48 -448 WIRE 224 -432 224 -544 WIRE 224 -432 176 -432 WIRE 256 -432 224 -432 WIRE 112 -416 80 -416 WIRE -96 -352 -432 -352 WIRE 32 -352 32 -448 WIRE 32 -352 -16 -352 WIRE 80 -352 80 -416 WIRE -528 -240 -528 -448 WIRE -416 -240 -528 -240 WIRE 64 -240 -336 -240 WIRE 112 -240 64 -240 WIRE 240 -240 192 -240 WIRE -432 -192 -432 -352 WIRE -352 -192 -432 -192 WIRE 64 -192 64 -240 WIRE 64 -192 -272 -192 WIRE -272 -144 -336 -144 WIRE 64 -144 64 -192 WIRE 64 -144 -192 -144 WIRE -176 -96 -240 -96 WIRE 64 -96 64 -144 WIRE 64 -96 -96 -96 WIRE 128 -96 64 -96 WIRE 240 -80 240 -240 WIRE 240 -80 192 -80 WIRE 272 -80 240 -80 WIRE 128 -64 96 -64 WIRE 96 0 96 -64 WIRE 112 112 48 112 WIRE 224 112 192 112 WIRE -336 208 -336 -144 WIRE -96 208 -336 208 WIRE 32 208 -16 208 WIRE 48 208 48 112 WIRE 48 208 32 208 WIRE 112 208 48 208 WIRE 224 224 224 112 WIRE 224 224 176 224 WIRE 256 224 224 224 WIRE 112 240 80 240 WIRE -240 304 -240 -96 WIRE -96 304 -240 304 WIRE 32 304 32 208 WIRE 32 304 -16 304 WIRE 80 304 80 240 WIRE -528 688 -528 -240 WIRE -432 688 -432 -192 WIRE -336 688 -336 208 WIRE -240 688 -240 304 WIRE -144 688 -144 656 WIRE -48 688 -48 656 WIRE -528 832 -528 768 WIRE -432 832 -432 768 WIRE -432 832 -528 832 WIRE -336 832 -336 768 WIRE -336 832 -432 832 WIRE -240 832 -240 768 WIRE -240 832 -336 832 WIRE -144 832 -144 768 WIRE -144 832 -240 832 WIRE -48 832 -48 768 WIRE -48 832 -144 832 WIRE -528 928 -528 832 FLAG 144 192 +5 FLAG 144 256 -5 FLAG 80 304 0 FLAG 144 -464 +5 FLAG 144 -400 -5 FLAG 80 -352 0 FLAG -528 928 0 FLAG -144 656 +5 FLAG -48 656 -5 FLAG 160 -112 +5 FLAG 160 -48 -5 FLAG 96 0 0 SYMBOL Opamps\\LT1007 144 160 R0 SYMATTR InstName U1 SYMBOL voltage -240 672 R0 WINDOW 3 24 96 Invisible 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V3 SYMATTR Value SINE(0 1 1000) SYMBOL res 0 192 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 10k SYMBOL res 0 288 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 10k SYMBOL res 208 96 R90 WINDOW 0 -43 58 VBottom 2 WINDOW 3 -37 60 VTop 2 SYMATTR InstName R3 SYMATTR Value 10k SYMBOL Opamps\\LT1007 144 -496 R0 SYMATTR InstName U2 SYMBOL res 0 -464 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R4 SYMATTR Value 10k SYMBOL res 0 -368 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R5 SYMATTR Value 10k SYMBOL res 208 -560 R90 WINDOW 0 -43 58 VBottom 2 WINDOW 3 -37 60 VTop 2 SYMATTR InstName R6 SYMATTR Value 10k SYMBOL voltage -48 784 R180 WINDOW 0 -50 102 Left 2 WINDOW 3 24 16 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 5 SYMBOL voltage -144 672 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value 5 SYMBOL res -80 -112 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R7 SYMATTR Value 10k SYMBOL res -176 -160 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R8 SYMATTR Value 10k SYMBOL res -256 -208 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R9 SYMATTR Value 10k SYMBOL res -320 -256 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R10 SYMATTR Value 10k SYMBOL Opamps\\LT1007 160 -144 R0 SYMATTR InstName U3 SYMBOL res 208 -256 R90 WINDOW 0 -43 58 VBottom 2 WINDOW 3 -37 60 VTop 2 SYMATTR InstName R12 SYMATTR Value 5k SYMBOL voltage -336 672 R0 WINDOW 3 24 96 Invisible 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V4 SYMATTR Value SINE(0 1 1700) SYMBOL voltage -432 672 R0 WINDOW 3 24 96 Invisible 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V5 SYMATTR Value SINE(0 1 2300) SYMBOL voltage -528 672 R0 WINDOW 3 24 96 Invisible 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V6 SYMATTR Value SINE(0 1 4260) TEXT -562 952 Left 2 !.tran .005

-- JF

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.

--- The input pots, OK. The feedback pot, probably not.

Plot the change in output voltage as a function of wiper position to get a better handle on it.

-- JF

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.