How Are AC Signals Combined in Audio Systems?

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Ok I've got an output from a mp3 player which has two channels (L & R) however the device I'm connecting it to can only output mono sound, so what I did was merely connect the left and right channel wires together but what I'm wondering is AC signals are not so clear cut to me, I'm currently studying Electrical Engineering (2nd year), but it hasn't clicked in my head previously that AC signals are not simply additive. Would I really be getting both channels combined? And would there be cancellation or some other problem that would be detrimental to the sound quality?

Thanks.
 

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You should probably use combining resistor networks.

If you are taking speaker outputs and combining them, the problem is that they will sometimes be out of phase and one will feed power into the other. The output impedance of even small amplifiers is very low and can look like a short circuit to another speaker output.

It is safer (and probably sounds better) to put a 10 ohm resistor across each speaker output, then take a 1 K resistor from each speaker output to the same 10 K resistor to ground.
You can then take this output to the input of an amplifier as a mono signal.

This way, you will get minimum interaction between the two stereo channels.
 
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