I had a vicious ground loop in a system that used a residential cable modem with an incoming coaxial connection. The cable modem was connected to the same circuit as my home theater preamp and self powered speakers. I tried disconnecting one signal cable at a time in the system until I isolated it to the coaxial cable entering the apartment. So I drove to Fry’s, bought a coaxial grounding block and ran a ground wire from the block to the ground on a wall outlet. Boom, ground loop was silenced. It turns out that cable tv/internet companies love to run coax signals extremely hot to prevent loss, especially in apartment and condo complexes.
Later, a different system with a headphone amplifier connected to a DAC with a pair of XLR cables in a balanced differential setup was getting horrendous ground hum. It only occurred when either of the two (or both) was connected to power AND to each other via the balanced connection, right channel, left channel or both. Neither hummed on its own. On the advice of an old school audio repairman who was factory authorized by brands like Luxman, McIntosh, Mark Levinson, etc, I snapped both ground pins off the (removable) power cables. Hum gone instantly. (I don’t recommend this unless you know what you’re doing and what the source of your hum is. Another option would be to put a very nice isolation transformer inline in the signal path; breaking the XLR ground.)
As you can see, hum can come from many different kinds of ground faults in a complex audio setup!
Later, a different system with a headphone amplifier connected to a DAC with a pair of XLR cables in a balanced differential setup was getting horrendous ground hum. It only occurred when either of the two (or both) was connected to power AND to each other via the balanced connection, right channel, left channel or both. Neither hummed on its own. On the advice of an old school audio repairman who was factory authorized by brands like Luxman, McIntosh, Mark Levinson, etc, I snapped both ground pins off the (removable) power cables. Hum gone instantly. (I don’t recommend this unless you know what you’re doing and what the source of your hum is. Another option would be to put a very nice isolation transformer inline in the signal path; breaking the XLR ground.)
As you can see, hum can come from many different kinds of ground faults in a complex audio setup!