TPA3118 board ground-potential/buzz

[terribleamps]

I've been going mad over the last couple of weeks with trying to get one of the TPA3118 boards to behave itself. I've coupled it to a Derailer and other drive pedals to try and figure out the source of the issue; invariably, I find that I get the buzz/cancellation that one experiences when their guitar bridge isn't grounded correctly, no matter how I try to connect groundings around the power amp itself. It seems as though it's down to the ground-potential, that my person has a better level than the circuit. The board cuts out if you try and directly ground the speaker output negative (as I'd do on a valve amplifier). I can't seem to find similar experience out in google-land... has anyone crossed this issue before and solved it?

 

[Robert]

The speaker output can't be grounded. You need an isolated jack for the speaker.

 

[terribleamps]

The speaker output can't be grounded. You need an isolated jack for the speaker.

Yes, I'm using nylon jacks as a matter-of-course. More that I'd tested the theory of whether there was any need to ground the output (with obvious failure).

The power source - a mains-connected toroid - is earthed and connected to the chassis properly, and the negative of the bridge rectifier is also coupled to the chassis ground.