I get a background buzzing which varies according to the Empeg's activity e.g. disk accesses, button presses etc.
It has been present since I originally installed the amp/Empeg and now we have better weather over here I've spent some time recently trying to cure it.

For the record, I also get slight alternator whine with the engine running but I'm not bothered about that here (unless I need to be!).

I've managed to make a cumulative but worthwhile improvement (see below) but the noise is still there and it's starting to bug me!

In the hope that somebody reading this might be able to offer some tips I've listed below things I can be fairly certain of:

FACTS
• Present with engine off
• Level is constant i.e. does not vary with Empeg's volume setting
• Same either in Pause, zero volume or AUX input (no aux device attached) mode
• Disappears when the 4 RCAs are disconnected (from either end)
• Worsens slightly when RCAs are slid back a little so they don't ground
• No difference with Empeg & sled on seat (in case of inadvertant grounding while in dash)

ACTIONS
• Relocated input leads away from power & ground leads
• Checked for other ground paths in the speaker wiring - none present.
• Rewired the grounding (seat mounting point: paint stripped & cleaned) for Empeg & amp using a *star* configuration (slight improvement)
• 3 ground leads of equal length/guage to: Amplifier, Empeg harness & docking sled (Grounding the sled gave a slight improvement)

OBSERVATIONS
• Grounding an RCA tip to the Empeg casing will produce this noise.
• Squeezing top & bottom at the sled pips - improves vastly (depending upon pressure)
• Grounding an RCA tip on the Empeg harness ground - no noise.
• While using the detached sled ground as a flying lead:-
>Grounding pin-5 (gnd) on the RS232 plug - disappears
>Grounding the Empeg chassis (much less so against the top cover) while in the sled - disappears
>Grounding the AC RCA outters - disappears
>Grounding an earth braid at the docking connector - slight improvement


Crutchfield's troubleshooting guide (indirectly linked from the excellent Empeg FAQ) suggested a power filter - I don't own one/haven't tried one and need to be convinced further.

Referring back to my Observations the solution would appear quite simple; All I need is a good ground to the Empeg chassis/signal ground.
While I don't trust my Digital VoltMeter with readings less than 1ohm I firmly believe there is a very good contact (and I've cleaned them both) between the sled and the body of the Empeg. I know that squeezing the two together improves the connection but I know that persuing this further (E.g. ad-hoc earth contact springs added to the sled etc.) must surely be sidestepping the root cause.

Suggestions (please)?

Edit: Typo's & grammatical errors