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I'm trying to get an understand of the effects of stereo and mono cables and jacks. This is complex because my mixer has both only mono jacks, and my devices -- most of them -- only have stereo outputs. Further, my microphone in on my computer is a mono and all of stereo-outs our balanced mono.

Which one of these do I need an adapter for? Can I plug a stereo cable into a mono jack? Obviously -- it doesn't make sense to plug a mono cable into a stereo jack, but what happens if I do?

S = Stereo; M = Mono;

Could someone file in the ?

  CABLES:       M  | S
J               
A           S|  ?  | X
C            ----------
K           M|  X  | ?
S
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3 Answers 3

The simplest way to see these for yourself is to look at the metal connections on your jacks. You'll find a stereo one has one more metal ring than a mono one.

The mono jack has a tip and a ring, and where that ring is overlaps two of the stereo jack's rings, so plugging a stereo jack into a mono plug effectively shorts your right channel to ground at the source, giving you a left channel only. Similarly plugging a mono jack into a stereo socket will allow you to hear your left channel only as your right channel in the amp will be grounded.

Either of these combinations probably won't cause problems if your circuitry isn't designed to cope with that shorting, but it could, especially with old electronics, or badly designed circuits.

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The best plan is to use an adapter - a stereo socket to two mono plugs avoids you shorting right channel to ground.

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Plugging a stereo jack into a mono plug input shouldn't actually short the R channel to ground; only plugging a mono jack into a stereo plug output will. Which should, in a properly designed device, not cause problems, but it certainly can in a badly designed one. –  leftaroundabout Oct 25 '12 at 10:01
    
Sorry - Reworded - it will short R to ground in the source device. –  Rory Alsop Oct 25 '12 at 10:04
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You can without any concerns plug a stereo cable into an unbalanced mono jack; as long as you only send mono signals (i.e. L and R channels equal) it will work just fine, except for the usually -3 dB pan law and possible short-cicuiting of the ring connector; the latter is however unlikely and not harmful for line connections. (It can be harmful for e.g. headphones connections, which you shouldn't be using.)

You can also plug a stereo cable into a balanced mono jack, but there's a caveat here: if you send mono signals, the L and R channels will exactly be cancelled by the common mode rejection, so you should pan anything you send hard-left. (If that's not a problem, you should also pan hard-left for stereo->unbal.mono, that also solves the two small problems I mentioned there).

Both balanced to unbalanced mono vice versa usually works without problems, except obviously you won't get noise surpression in either case.

Plugging a mono cable into a stereo jack will not work well. If it's unbalanced mono, the result will come out only on the left channel; if it's balanced it will come out L/R in opposite phase. Neither is usually acceptable, that's a major reason why hardly any professional device has stereo inputs.

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So if you have a stereo jack on a computer, a mono jack on the mixer, does an adapter merge the two waves? or does it just ignore the right channel and connect the left? –  Evan Carroll Oct 27 '12 at 19:13
    
And, in the case I want to connect a stereo-headphone jack on my computer -- to a mono-jack on my mixer, how do I do it? –  Evan Carroll Oct 27 '12 at 19:15
    
It will ignore the right channel. If you want to mix the two channels you need to use two channels on your mixer or a stereo channel; in either case you need a stereo -> 2xmono. –  leftaroundabout Oct 27 '12 at 22:47
    
If I don't want to mix the two channels but, I want ot merge them so things played on the left channel can still be heard in mono what do I need? –  Evan Carroll Oct 27 '12 at 23:55
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That is mixing both channels (there is not such thing as a special "merging" of audio sources, what you probably mean is mixing with both channels at the same level), so you need what I said: stereo to dual mono. –  leftaroundabout Oct 28 '12 at 20:49
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If you only have one mono input channel to work with, you would need a PCDI box to merge the Right and the Left without cancellation. Example: http://whirlwindusa.com/media/uploads/pcdi.pdf

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