Do i need a ground lift switch?

[Sensei Tim]

A buddy asked me to build him a simple passive AB box so he can switch between his wireless and wired set up on his pedalboard.

question is: do i need to add a ground lift switch? gut says no since it's not an ABY, but not 100% sure since all of the grounds are connected together.

[Neil mcNasty]

As far as I know: It has to be transformer coupled if you are going to get a proper Ground Lift function...
If you "lift" or remove the ground on a passive AB box, you will get either; loud noise/hum or oscillation issues.

I've put a transformer with ground lift switch in a 1590A box.
So that I can bring it out/add it to the noisiest of the two amps, if I get extreme ground issues.
It's a very handy device to have in your toolbox, but notice that it has never been used since I built it 2 years ago, so these kind of issues does not happen often.

[Sensei Tim]

That's the thing - I'm not ab'ing between 2 amps. I'm ab'ing two different input paths into the same amp - in this case it's a preamp pedal that has its own ground lift.

[Neil mcNasty]

Oh, sorry.
I see what you meant now...
In my mind there should be no need for a ground lift in this case, since it will behave like if you run one instrument directly to the amp/preamp...