> How can I check if the keyboard/power supply is grounded? The power > supply > itself is a three pronged one.... Then in most countries it is required by law to be effectively grounded. Next question is if you power socket is... It should be if it accepts the plug. Since you also have this interference with an amp, but not with the headphones it really seems to be a grounding problem. To only other way to explain it would be to assume it's some high-frequency stuff which your headphones would just ignore, but any active input such the amp or soundcard could demodulate into the audio band. If you're using a laptop, does running it on battery make a difference ? Anyway I'd try a DI box, it's good practice anyway with all instruments. Ciao,