Ground Lift (isolation) w/o transformer.

[ashcat_lt]

The title says it all. How to isolate the ground to avoid loops in an unbalanced situation (say in an AB/Y pedal) without a transformer?

Any ideas, advice, links, would be appreciated.

[GooberNumber9]

You could convert the signal to another type and convert it back. You could convert it to balanced and back, or to optical and back, or you could insert a wireless rig to convert it to RF and back.

[suppositron]

I think if you ran the signal through a high impedance buffer amplifier that might work.

[jv]

Another solution to this is to use a cable with the shield disconnected on one end. For instance, if you have your guitar cable going into the a/b switch, then 2 cables going out of the switch to 2 amps (that are properly grounded!), you will have a ground loop. If you disconnect the shield on one end of one of the cables going to the amps it breaks the loop. I think there are recommendations for which end of the cable (switch or amp end) should be disconnected, but you can try both ways and see which works best. Be sure to mark the cable that has the shield disconnected, though! Otherwise it will cause you problems if you try to use it in another situation.

[GooberNumber9]

If you disconnect the shield on one end of one of the cables going to the amps it breaks the loop.

True. In unbalanced cabling it also breaks the circuit meaning you get no signal along with your no hum.

The buffer amp is another solution for electrical isolation. A bypassed Boss, DOD or Ibanez effect pedal powered from a battery might do it.

[ashcat_lt]

Yeah, I didn't think that breaking the shield would work with unbalanced audio. With the buffer thing, I'm not sure. Aren't the sleeves of all the jacks connected via case in these kinds of pedals?

[The Scum]

In the most general sense, to be universally applicable to all systems, an unbalanced run with no ground connection shouldn't move any signal...if the two devices don't have a common ground reference, it can be hard to interface them.

At my gay gig, I work on industrial systems where we use isolated power supplies, and it occasionally hangs me up when I'm debugging them...I'll have a scope probe clipped to what I know is a live signal and see nothing, because the ground isn't connected.

However, actually isolating power systems is a lot of work, and takes careful planning to implement. Real life doesn't usually follow things stringently enough to have that sort of isolation, like your amps. Lifting one ground with your AB pedal should be fine.

I'm assuming that the amps are both plugged into the same leg of your AC power, thus sharing a common ground. Then you could potentially lift the shield on one of the cables from the AB pedal to one amp. If the ground loop formed by the pedal, the amps and the wall plug is the root of the problem, that will break the loop, and solve the noise. Since both amps already use a common ground reference, there's no problem.

Try it, and report back. I assume that the setup of either amp by itself works OK, just that the pair has problems?