The test setup of my new install has a ground loop; the same ground loop as my first install because they both use the same components. The old install was done at a shop, so they put "filters" on the RCAs to cure it (and hurt my sound quality?). But the new install is my job, so I'd like to fix it properly.

The setup is: empeg and Alpine deck, switched with MPSS-4 relay, all connected to the car's wiring harness to use its internal amp and speakers. That's three ground wires and three+ power wires connected to each lead from the car's harness. Problem? And for now, they're only twisted together with caps. Another problem?

The loop occurs when the empeg is docked, powered on or in standby. The noise can be heard when the empeg or Alpine is playing. The noise adds static to the radio, crushing some frequencies (88.3).

The problem seems to be from empeg RCA to relay RCA. Even after reading the FAQ and its links, I can't imagine why this is or how to fix it correctly. FAQ asks for heavy-gauge cable, but the empeg harness (and Alpine harness) uses wimpy cables and there's not much I can do about it. I don't understand the other FAQ terminology (potential, "filter power feeds", etc).

To diagnose, I connected the empeg "directly" to the car's wiring harness, bypassing the relay. All I did was move its four RCAs to the harness and directly connect emepg's amp sense line to the car (instead of through the relay). I left the Alpine and relay power and ground connected. The ground loop was eliminated in this test, even with all the other components connected and receiving power (but with no audio connection).

I hope this doesn't require a ground line run off the battery since each component works while individually connected to the car's harness. It's just that pesky relay which causes the loop. Maybe Crutchfield can help. Any ideas? Thanks.