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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 are 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 fill in the ?

  CABLES:       M  | S
J               
A           S|  ?  | X
C            ----------
K           M|  X  | ?
S
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4 Answers 4

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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 sleeve, and where that sleeve is overlaps the stereo jack's ring and sleeve, 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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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
    Commented Oct 27, 2012 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
    Commented Oct 27, 2012 at 19:15
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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
    Commented Oct 28, 2012 at 20:49
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    @TonyS if you short tip and ring of a stereo jack together, and route that to the tip of a mono connector, you get a “poor man's mixdown” to unbalanced-mono. Not recommended, but should usually work, though impedance differences may cause the mix to be skewed (unlike in a balanced connection, a stereo connection is technically not required to have matched impedance) And if the impedance is too low (headphone connections), the stereo difference is dissipated as a lot of current which may damage the output.
    – leftaroundabout
    Commented Aug 7, 2020 at 14:53
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    If you short tip and ring of a balanced connection together, well, you basically just short both signal phases against each other and thus get no signal at all or a very feeble signal.
    – leftaroundabout
    Commented Aug 7, 2020 at 14:53
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Please note, iPhone has a handy feature to convert stereo to mono output - no need to buy more connectors if this solution will work for your application :-) - thanks everyone.

http://www.technipages.com/iphone-ipad-enable-mono-sound/comment-page-1#comment-808000

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I have also had this problem. No adaptor plugs seem to exist for 3.5 mm stereo plug to 6.3mm mono socket. So, I used a 3.5mm stereo plug to 2x RCA plugs with a short cable.

Then a plug that is 2x RCA sockets to 1x RCA socket - true stereo to mono "joiner". Then I used an RCA male to 6.3mm female plug . It's a hassle but that then allows a standard instrument jack to jack lead to head off to the line in input on the amp and not lose either of the left or right channels coming from your laptop or phone's 3.5 mm stereo output.

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  • I also did this to get around the problem. I don't know why this was downvoted (ohwell, I probably do know, it's a very ugly hack). But I upvoted you again because this actually works and is easy to do without making your own cables.
    – odinho - Velmont
    Commented Oct 10, 2020 at 20:20

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