I see what you're getting at.... You're talking about amps where the - floats. Well, then the signal never does get inverted inside the amp, they are amplified in two seperate stages and instead of summing it is now subtracting. Same difference, now it's happening on the output stage, when it subtracts the inverse from the original it still produces the original in phase signal, when you subtract commom mode voltage(noise) your net is still 0.

The amp I was describing had - tied to grnd on the output side.

My point still stands, if the noise is induced out of phase in the signal lines, than it gets reproduce with the music, in both scenarios the noise has to be in phase on both lines and the music be out of phase.

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