I have a Yamaha R-N500 amplifier which has a slight vibration on the face plate when I touch it. It feels like it could be 50Hz. The amp has a fixed 2 pin cord, so isn't grounded.
I happen to have a USB grounding cord which connects the USB shield to the ground pin of a mains plug. I connected it to the USB-A port on the back of the amp and the vibration disappears but also the sound improves quite a bit, it seems to tighten up and have a blacker sound.
Why would grounding the amp reduce the vibration on the faceplate?
Also if I connect my MacBook to a headphone DAC and then to the amp via RCA cables, the surface of the laptop vibrates as well (note that the MacBook is not connected to mains, just running off battery, the DAC is not connected to mains either but running off the MacBook's USB power). Connecting the USB grounding cord to the back of the amp also causes the vibration in the MacBook to disappear.
I'm intrigued to know why the grounding removes the vibration, and improves the sound, and why most of the mass-produced hi-fi doesn't have it?
Edit: I should mention that the amp faceplate vibrates even if nothing is connected to it, and also if I plug the USB grounding cord into one of the MacBook's USB ports (or through a hub) the vibration disappears in the amp and MacBook.
Edit2: The MacBook when connected to mains with its own power supply but not to the amp, vibrates as well, and that disappears when the USB grounding cord is connected.