Tony,

I respectfully disagree. Why would it be called a ground loop when the shields (at ground potential in a single ended installation) aren't involved? That's not really the point here anyway though.

Basically as I understand it generally (as an electrical engineer) for a normal unbalanced situation:

- The signals from the head unit to the amp are signal ended and referenced to it's ground at the head unit end.
- The amp simply amplifies the voltage it sees between the centre pin of the RCA and the shield. It's merely amplifies the "difference" in potential but right at the input to the amplifier.
- If the ground of the amp is not very good OR the ground of the head unit isn't very good, the ground reference point of either the amp or head unit (or both) end will move, particularly with voltage fluctuations (stuff like alternator, switching loads such brake lamps, indicators etc).
- If the ground reference point moves at either end, that's when the noise gets in, since the ground has moved with respect to the centre signal wire.
- If the grounds are bad, the amplifier's and/or head unit's supply return current will travel via the grounds of the audio cable. This current will change according voltage fluctuations. Any resistance will then cause potential differences when current flows. And again the potential difference is what is amplified.
- The supply currents for the amp and head unit are unlikely to be flowing on the audio level signal lines of the RCA cables.

Grounding the shields should effectively achieve nothing since the should already connected to ground. What it does do though is provides another current path for the supply currents to take which can help with the problem.

Well, that's my take on it. Feel free to debunk it. I don't mind being corrected.