amps audio jack 1/4 stereo switch

[morphingstar]

this is about music amplifiers for small stage. The have several input channels, for keyboard, microphone etc. The signals are mixed together and fed to the output jacks.
The situation I need explanation for is where there are 2 jacks, 1 left, one right (stereo).
My observation shows the jack marked Left is actually a stereo jack (1/4 inch, 6.3mm), the one marked Right is only picking up a mono signal, Right channel.
When I plug in 2 mono jacks, 1 for each channel, the signal is processed as stereo and turns up at the output just alike.
If I plug in 1 Stereo jack into the socket marked Left the 2 signals are processed as stereo, the socket marked Right is then not used.
In a similar way the stereo output with 2 sockets handles the signal.

What happens within these sockets?
Stereo-in on LEFT disconnects RIGHT.
Stereo-out on LEFT disconnects RIGHT.
Mono-in LEFT must make sure the RIGHT contact ring within the same socket is not shortened to ground by the mono plug.

I guess the circuit was developed decades ago. I have just never come across it. I would like to know how this signal separation for 2 sockets (mono) and signal combination for 1 socket (stereo) works.
Do not forget, I stated the LEFT will accept mono and stereo jacks. It operates as mono if the RIGHT is plugged in, and as stereo when the RIGHT is not plugged in, in the later case the jack must be stereo. The tip of the jack always carries the LEFT channel of a stereo signal. A mono connector has only tip and ground, it shortens the RIGHT socket contact to ground.
Please provide a link to the circuit diagram. As said I just can't find it. Thanks.

logged in 3 times to finally post this thread - just for my record - once was not enough.

 

[shrtrnd]

I don't KNOW, but this sure sounds like it's designed to be used for whatever input
signal is needed. Particularly, because it's an older system, my guess is it was
designed when both mono and stereo were frequently used (1960's & 70's).
If the user had a mono input, you'd use the right jack.
If the user had a stereo input, you'd use the left jack.
I don't think it was intended to use both the left and right jacks at the same time.
That's my guess with the info I read.

 

[KrisBlueNZ]

I would guess something similar to shrtrnd.

I think the right jack socket probably has a switched contact so that when nothing is plugged into it, the right input to the amplifier will connect to the ring connection on the left/stereo jack.

In that case, here are the scenarios. "Tip" is the tip or end of the plug, which is present for both mono and stereo plugs; for stereo plugs, this is the left channel. "Ring" is the extra ring-shaped contact that's only present on stereo plugs, where it carries the right channel.

1. Mono plug in Left/Stereo socket; mono plug in Right socket:
- Right socket's switched contact is disconnected.
- Right amplifier input comes from tip contact of Right socket.
- Left amplifier input comes from tip contact of Left/Stereo socket.

2. Stereo plug in Left/Stereo socket; nothing in Right socket:
- No plug in Right socket, so Right socket's switched contact connects the Right tip contact to the Left/Stereo ring contact.
- Left amplifier input comes from tip contact of Left/Stereo socket.
- Right amplifier input comes from ring contact of Left/Stereo socket via the switched contact on the Right socket.

3. Mono plug in Left/Stereo socket; nothing in Right socket:
- Same as 2 but the plug in the Left/Stereo socket doesn't have a ring, so the socket's ring contact contacts the plug barrel which is ground, so the right amplifier input sees no signal.

This doesn't provide for the case where you have a mono signal that you want connected to both amplifier inputs. To support that case, the designers might have been even cleverer, and added a switched contact on the Left/Stereo socket so that when nothing is plugged into the Left/Stereo socket, the left amplifier input is connected across to the tip contact from the Right socket. In that case, as shrtrnd suggests, you could plug a mono source into the Right socket and it would come through both channels.

In that case, the Right socket should be labelled "Right/Mono".

 

[morphingstar]

Thank you. The matter is about OUTPUT jacks. Modern keyboards tend to have the described jacks: LEFT / Left+right, and RIGHT.
Hence he qualifications _old_ and _input_ do not apply.
Thanks for trying to get one step further. I did the guesses before I came here.
KrisBlueNZ you have been very elaborate and may be correct in the analysis. I was hoping for a source of standardization and diagram from there, with the connector-switch-logic.
There also keyboards which have no LEFT / Left+right, only 2 1/4 outputs, one per stereo side, and a stereo mono switch on the control panel. These always need 2 cables for stereo. This situation drops out of my question.

 

[shrtrnd]

You're very welcome.