How to make a stereo switching input

[cloutier]

I'm building a passive stereo splitter/mixer. I also want to have LEDs indicating whether it's in stereo or mono mode, and have it powered by battery. I understand the use of a stereo jack for the input to break the ground path when nothing is plugged in, but how would I do it for a stereo input? I need the ring connection for one of signals. And a switching jack goes open when a connector is inserted.

Any idea would be greatly appreciated.

[suncrush]

4-pole jacks are available at 1/8".  Does it need to be 1/4"?

[cloutier]

Yes it does. Could I just use one of the outputs for the switching?

Thanks.

[Phoenix]

Well, let's try to solve your battery power switching problem first. If it were easy to get 1/4" sockets with MAKE connections on the sleeve rather than break, that would make things very easy - but they're custom items I've always found.
You can find jacks with isolated switches that would also work to make the battery connection when a jack is inserted, but they tend to be pretty janky, so I'd avoid them.
What I'd suggest is to use a jack that has a normally closed (open when jack is inserted) connection on the sleeve. Cliff, Neutrik and Rean (budget Neutrik) all make this type of jack, here's a link to a Neutrik example.

Then you could add a circuit like this.

I just threw it together, but I believe it should work, but keep in mind it is untested! It includes reverse polarity protection (the first mosfet) as inspired by RG's Advanced Power Switching and Polarity Protection for Effects article. It can be replaced with your favourite polarity protection scheme, or ommitted (not recommended).
The "SLEEVE_NC" connection would go to the normally closed sleeve terminal on the jack, when there is no jack inserted this connection is grounded, which pulls the base of the pnp transistor low, turning it on, which then pulls the gate of the second mosfet high, turning the circuit off.
When a jack is connected, this connection to ground is removed, the base of the PNP is pulled high turning it off, and the gate of the second mosfet is pulled to ground instead, turning the circuit on.
The circuit does not completely disconnect the battery - some current still flows (through the three 10MEG resistors and the PNP); but it is exceptionally little, on the same order as the self-discharge rate of the battery itself, so no worries about the battery being drained when a jack is not inserted.
The mosfets should be selected for the lowest Rds(on) that you can find in a suitable package at a reasonable price to avoid excessive voltage loss - but less than 100mV drop is quite easy with easily available and cheap parts.

As for the mono/stereo detection, you could modify RG's Millenium Bypass (edition 1, edition 2, edition 3) to detect if the ring is being shorted to sleeve or not. This is how mono/stereo detection is normally accomplished in all the commercial examples I've seen/taken note of. I think there are some Boss schematics floating around that show implementations of such.

Anyway, hope this helps.

[anotherjim]

There are jack sockets with isolated changeover switches. They have been used in guitar amplifiers so look for amp spares - Fender etc. They are 9pin used in some Fender amps. The ring contact isn't wired in the amps, but they are TRS with 2 changeover switches. Presumably, they couldn't get a mono jack with the 2 switch pairs?
I'm not certain that the switches are electrically isolated from the plug contacts, although the schematics suggest they are.
This one?
http://www.tubeampdoctor.com/en/shop_Cables_Jacks_amp_Plugs_Connectors/Fender_Jack_stereo_w_switch_4350

You could DIY from a normal jack IF you can mount a microswitch reliably so it operates with a plug inserted. A pcb mounted Cliff type with the plastic bulge over the plug tip cut off from the back. The jack tip may be through just enough to operated the button of the switch.