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Combining two audio outputs

  1. Apr 30, 2014 #1
    Essentially what I'm trying to do is combine two headphone level outputs to one set of headphones. The earbuds are generally 23 ohms, so got four 24 ohm resisters and wired it like so:

    Left output #1 -> 24 ohm -> left earbud
    Left output #2 -> 24 ohm -> left earbud
    Right output #1 -> 24 ohm -> right earbud
    Right output #2 -> 24 ohm -> right earbud
    All commons combined

    I tried it out and it essentially works but there is some spillage between the left and right channels. I'll try a left/right channel test without the circuit and it sounds perfect, but with it, there is still some sound from the left/right channel spilling over into the right/left channel.

    How would I correct this?
     
  2. jcsd
  3. Apr 30, 2014 #2

    berkeman

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    Staff: Mentor

    Can you just use a dual-mono to stereo adapter cable?

    http://thumbs3.ebaystatic.com/d/l225/m/mEuaUl9TfAZ7mrEDUudPQxA.jpg
    mEuaUl9TfAZ7mrEDUudPQxA.jpg
     
  4. Apr 30, 2014 #3
    I think the OP wants to go the other way. Combine 2 source signals to one headphone. He has taken a passive approach in using resistors. Not sure based on the way it is worded as to whether the resistors are set as load or in series with the source to drive to the phones. Ultimate solution would be quiet OP-AMP in summing configuration with the 24 ohm as load for each output channel in input side. Or headphone amp, with the input channels set with summing resistor network.

    As for the crossover bleed, you might use a DMM and verify that the headphone ground is chassis ground on both outputs. Make certain that your two outputs are only tied at the headphone ground and not at the chassis.
     
  5. Apr 30, 2014 #4

    berkeman

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    Staff: Mentor

    Ah, well if so, there are certainly different sex versions of that Y-connector/adapter... :smile:
     
  6. Apr 30, 2014 #5
    There are certainly adapters that would allow two male stereo inputs to one female output. However I don't consider it good practice to tie two power outputs together without some isolation circuit. The OP used resistors, and says it it has some crossover issues. A small headphone summing amp or mixer would be best. But might cost a little and require power even if from batteries.
     
  7. Apr 30, 2014 #6

    berkeman

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    Staff: Mentor

    More good points -- I guess we need to hear from the OP about what these two mono headphone sources are.

    @Jd0g33 -- can you give us more info about what you are trying to do and why?
     
  8. Apr 30, 2014 #7
    Hey sorry for late reply and thanks so far. Essentially what I want is a passive 'mixer' of sorts. I have the stereo audio signal coming from the tv headphone output, and I have the stereo audio chat signal coming from my gaming system. So I just want the two different sources of audio signal combined to listen to in one pair of stereo headphones. I don't mind a slight volume decrease.

    This is exactly how I have wired it:

    Left output #1 -> 24 ohm -> left earbud (straight to left terminal on 3.5 mm female)
    Left output #2 -> 24 ohm -> left earbud (straight to left terminal on 3.5 mm female)
    Right output #1 -> 24 ohm -> right earbud (straight to right terminal on 3.5 mm female)
    Right output #2 -> 24 ohm -> right earbud (straight to right terminal on 3.5 mm female)
    Common #1 -> Common #2 -> earbud common (ground terminal on 3.5 mm female)

    This works except for the previously stated bleeding of signal into the opposite earbud. What would cause such bleeding?
     
  9. May 1, 2014 #8

    analogdesign

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    Science Advisor

    If it absolutely positively has to be passive I would try increasing the resistance values. Sounds like your headphone amps are having difficult driving the resistors you put in but this depends entirely on how your headphone amp works.
     
  10. May 1, 2014 #9

    meBigGuy

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    Gold Member

    You might get bleeding (crosstalk) from two sources with this approach.

    1. Common ground has some resistance
    2. The output amp has output resistance (all amps have some, some have a lot) so the 2 24 ohm resistors in series between the amps produces crosstalk. This might be especially evident at low frequencies since most headphone amps have a series capacitor.

    There is not much you can do about #2.

    You could try an audio diode in series with each channel (joke).
     
  11. May 1, 2014 #10

    sophiecentaur

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    Gold Member

    This is not a trivial problem - which is why it's usually done Actively! (Sorry if the potential cost is upsetting you.)
    It could also be the power supply / decoupling in one or both of the power amplifiers. Are you confident that the crosstalk is low for the stereo output from each device independently? It is possible that the power supply for one of the amplifiers is not good and there is some (cross) feedback included in the actual R and L amplifiers, to reduce crosstalk. That would mean that an external signal, applied to one to the outputs, would appear on the other output - and, hence into the other ear.. If you use different standoff resistors, does your strange effect vary? Could you solve the problem (to your satisfaction) by just doubling the resistor values?

    Another (passive) way to tackle the problem could be to use a transformer Hybrid. This is a four port device and provides you with two inputs that are isolated from each other and the input signals appear at the other ports - one of which gives you A+B and the other gives you A-B. (Very much 'telephone' quality audio)See this link. I was searching for a source on Google but, apart from something on eBay, I could only find adverts for those pesky Transformer Toys. But that link does describe what they're all about. Good versions of those transformers are expensive to buy ('wound components' all tend to be) and You would not be able to wind one without the right winding machine, I think.

    If you really want a good solution, then you should consider a small, battery operated mixer box. Is there nothing available in the Electric Guitar trade, amongst all the fuzz boxes and things?

    Farnell has hybrids
     
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