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I have the audio output of 2 computers connected to only one set of speakers.
Each computer uses one of these external USB sound cards: enter image description here

And I use this splitter cable to wire the signals together: enter image description here

The whole thing works, but with some minor problems:

  • There's a little bit of noise when the speakers are set at max volume
  • One audio signal sounds a bit louder than the other
  • When recording the microphone input, sometimes the audio output of the other computer becomes the mic input

So obviously the two signals are interfering each other and also each signal finds its way into the other sound card because the splitter cable is meant to be a splitter and not a mixer.

So I thought if I could solve this by adding diodes to each side of the splitter cable so that each audio signal goes to the speakers only and not to the other sound card. enter image description here

Will this idea work?
What kind of diode should I use for that?

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    \$\begingroup\$ It could sort of work but you probably won't like it. The diodes will act as rectifiers and clip off any part of the speaker drive that swings below 0V. The diode's forward drop will also affect things. You will also have some issues with grounds, especially if the PCs could potentially have different ground levels. In short, don't do it this way. \$\endgroup\$ – RBerteig Oct 27 '14 at 23:47
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    \$\begingroup\$ What I'd do is connect the output of one PC to the input of the other. You can then use the PCs mixer to balance the two signals (I assume your PCs mixer application allows you to send the input directly to the output - you might need some software for that). \$\endgroup\$ – RJR Oct 28 '14 at 0:26
  • \$\begingroup\$ @RJR, unfortunately I need the sound sources to be independent of each other. One computer could be turned off, or the sources might not be computers at all. \$\endgroup\$ – GetFree Oct 28 '14 at 0:48
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As mentioned in the comments, diodes are not the way to go. They will massively corrupt your audio signal, and it will just sound utter rubbish.

The simplest method is to use a passive mixer. That simply consists of two resistors per channel:

schematic

simulate this circuit – Schematic created using CircuitLab

You can tie all the ground wires together without resistors.

This of course is assuming you are using a powered speaker set, and not just raw speakers.

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  • \$\begingroup\$ Yes, the speakers have external power supply. I guess I will need to turn the volume up more than usual using this method. \$\endgroup\$ – GetFree Oct 28 '14 at 0:51
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    \$\begingroup\$ You shouldn't need to, no. The input impedance of the speakers should be considerably higher than the 10KΩ per input of the passive mixer. You can reduce the resistor values if you really need to though - no lower than around 1-2KΩ though. \$\endgroup\$ – Majenko Oct 28 '14 at 1:02
  • \$\begingroup\$ One question: analog audio signals are positive voltages only? or they are positive and negative voltages (in which case it's an alternating current)? \$\endgroup\$ – GetFree Oct 29 '14 at 5:14
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    \$\begingroup\$ Depends on your point of view. In general the ground point is mid-way through the signal, so yes it has both positive and negative components. There is always a chance there may be a DC offset as well, which will make it all positive when viewed from ground, but both positive and negative when viewed from the perspective of the offset. \$\endgroup\$ – Majenko Oct 29 '14 at 10:26

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