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Mixing 4 audio channels to 3?

Discussion in 'Electrical Engineering' started by DaveC, Oct 26, 2011.

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  1. DaveC

    DaveC Guest

    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.
     
  2. DaveC

    DaveC Guest

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

    Thahks.
     
  3. Don Pearce

    Don Pearce Guest

    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
     
  4. DaveC

    DaveC Guest

    Is there something wrong with my browser? I can't see any of these
    Sorry. They didn't make onto the jpg file. Added here:

    OK, can do that.

    Suggested values for resistors?

    Thanks.
     
  5. DaveC

    DaveC Guest

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

    Don Pearce Guest

    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
     
  7. DaveC

    DaveC Guest

    For that particular circuit with that chip, the caps are necessary
    And the value for the potentiometer?

    Thanks.
     
  8. Don Pearce

    Don Pearce Guest

    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
     
  9. DaveC

    DaveC Guest

    That depends entirely on the input impedance of the subwoofer. You
    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.
     
  10. Don Pearce

    Don Pearce Guest

    No, that would be about right.

    d
     
  11. DaveC

    DaveC Guest

    Yes the caps are necessary, but for the amp IC, not for the mixer.
    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.
     
  12. DaveC

    DaveC Guest

  13. 10 kohms for all fixed resistor should work...
     
  14. DaveC

    DaveC Guest

    Or, as is not uncommonly done these days in portable digital players,
    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.
     
  15. Les Cargill

    Les Cargill Guest

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

    Don Pearce Guest

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

    d
     
  17. Guest

    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.
     
  18. DaveC

    DaveC Guest

    is it OK to connect 2 computers' sound cards' grounds together?
    This kinda kills the project, doesn't it? (At least, the straightforward
    solution.)

    The alternative is...?

    Thanks.
     
  19. Don Pearce

    Don Pearce Guest

    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
     
  20. DaveC

    DaveC Guest

    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
     
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