Hum free A/B/C?

[m-theory]

I've seen layouts for hum free A/B/Y switches, but not for A/B/C.  I've built a vanilla A/B/C to switch between two inputs on one amp and another separate amp.  As might be expected, I've got hum issues that are going to be unacceptable at stage level volumes. 

Does anyone have, or know where I can find a layout for a a cure?  Does it absolutely require building additional circuitry in front of the outputs, or is there another solution, besides using a cheater plug on one of the amps?  I'm not terribly fond of the idea of running an ungrounded amp in any situation. 

[R.G.]

No biggie.

The same hum isolation cures for two amps work for three or more.

The big issues here are how are you going to do the switching? Rotary switch? Some kind of footswitch?

There is no existing layout that I know of, but the circuitry is a pretty simple adaptation of existing stuff.

Yes, it absolutely requires building additional stuff in front of the outputs. The hum comes from the connected grounds on the amps. When you switch between them, you need to isolate the grounds. And congratulations on not wanting to risk your life on a ground-lifted amp.

[m-theory]

Thanks for the reply.  I've wired the outputs to 2-3pdt switches, using the Fulltone A/B switch example.  Switch 1 sends signal either to output 1 or switch 2.  Switch 2 sends signal either to output 2 or output 3.  Unused output is sent to ground.

So, could I use this layout http://aronnelson.com/gallery/humfreeaby/Improved_Hum_Free_ABY_Perf, and just leave out the "both" part?  Would I need a third transformer?