Q:

Mono to stereo circuit only if one cable is plugged in

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"

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.

A:

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

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

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: -

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.

