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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.
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