Im not sure how to phrase my question or what to google, but does anyone know how this feature is implemented (eg a simple example circuit):

On some audio mixers, if only the left channel is plugged in, the audio from this input will be split across the left and right channels. However if both left and right inputs are connected, then the audio from the left and right inputs will remain separated on the left and right channels.

For example, a behringer q502 mixer has this functionality, it is marked on the mixer as "mono L"

enter image description here

I know this could be implemented by having a separate switch to toggle between stereo and mono inputs, but im curious how this is done by simply plugging in a TRS cable.

  • \$\begingroup\$ Might have already found an answer: electronicspecifier.com/products/cables-and-connecting/… \$\endgroup\$ Jun 19, 2020 at 7:14
  • \$\begingroup\$ Jacks have switches built in. Plug into the right and its switch disconnects from the left. \$\endgroup\$ Jun 19, 2020 at 10:12
  • \$\begingroup\$ Remember, you can vote up each helpful answer to thank the writers. Then you can follow up with comments if your question is not fully answered, or mark one answer as your preferred answer. \$\endgroup\$
    – P2000
    Jun 21, 2020 at 5:42
  • 1
    \$\begingroup\$ Ah, thanks P2000 I didnt realize that. Ive accepted Andys answer :) \$\endgroup\$ Jun 21, 2020 at 21:54

2 Answers 2


It will likely use a mono jack socket with a transfer switch like this: -

enter image description here

Then inter-wire NC and NO contacts like this (blue and purple): -

enter image description here

So, if the LEFT jack is inserted it will naturally connect its tip to the amplifier and also connect the tip to its NO contact. That NO contact connects to the right NC contact (blue wire) which connects to the right tip circuit when the right jack plug is not inserted: -

enter image description here

The purple wire is for when the right jack is inserted.

This should produce a left and right balanced mono signal if either left or right jacks are plugged in. When both are plugged in, you get normal left and right stereo and no cross-connection between the channels.

  • \$\begingroup\$ That looks unlikely, Andy, as there is no easy way to ground and mute the inputs when nothing is plugged in. The left priority solves that issue by allowing grounding through the right jack socket. \$\endgroup\$
    – Transistor
    Jun 19, 2020 at 18:35
  • \$\begingroup\$ We’re talking about line input impedances and hence maybe 10 kohm to 50 kohm and not very likely to be significant in terms of noise generation. I’m proposing the above as an idea not a definitive method used by the equipment in the question. \$\endgroup\$
    – Andy aka
    Jun 19, 2020 at 19:31

One of the jacks has a switch that connects the L and R inputs together.

If you use only the L jack that has no switch, the switch connects the single input to both L and R input channels.

If you use the R jack with the switch, the switch then disconnects the L and R inputs and the R jack goes only to R input and L jack goes only to L input.


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