Lawrie's description of the boat issue brought to mind a recent issue I had with trying to use a small portable speaker system designed for use with PCs, connected to my laptop (I've had this arrangement in use for many different incidents) but this time, I used them powered through a GFCI outlet in our home. Hum/buzz city.

I still strongly suspect that there might be some evil in your power signal chain that is not ground related but from some crummy switching power supply or pulse-width-modulated signal to give certain RMS power values. I've had way too many experiences with this to ignore it without peril. Again, I refer to T-series ThinkPad laptops - built to withstand a near nuclear attack, put a terrible PWM-related buzz/hum on the supposed 5V DC in the lappy's USB.

-Scott

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