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[TheBoy]

Not Omega. A Mini. A proper one.

Stereo > Amp (under seat) > Speakers - "alternator" type whine, pitch changes with revs. Perfect with engine off.
Stereo > Speakers - OK (sound clearly isn't as good without external amp).

Good gauge 12v run from battery. 0v is to chassis, near amp, and apparently a good connection.

New suppressor fitted to alternator.

I've only briefly looked, in passing.

Any ideas I can forward to him - left him to use jumpleads to provide good 12v and 0v direct from battery to amp

[TheBoy]

Of course, it could be noise from points, etc

[Kevin Wood]

If it's a whine it will originate in the alternator, not points.

The problem is likely a ground loop between the head unit and amp. Since alternator whine is superimposed on all the 12v supplies when the engine is running, if there is a difference in potential between the head unit ground and the amp ground this will result in the whine getting into the input to the amp.

Best way to diagnose this would be the try feeding the amp with a "floating" music source such as an iPod. If the whine has gone, try either a "ground breaker" (basically an audio transformer) between head unit and amp or try grounding the head unit to the same ground as the amp.

Of course, it could be that the ground connection isn't as good as it looks.

[iainb]

Resistor spark plugs are a must have item on old mini's to shut up the back ground noise.
So if you have Champion N9Y try RN9Y version.
Ive always had acceptable results with them.

[TheBoy]

Thanks Kevin Wood and iainb for your input

Kevin Wood's iPod test was successful, which I assume rules out the plugs/leads iainb suggests, and puts it down to earthing issue?

Asked the guy to test by running a wire from stereo 0v to amp 0v, to hopefully bring them in line with each other.

[zirk]

As you've suggested a good quality +/- 12v feed directly from the Battery, try it with both local and non local Amp to Ground.

Good quality RCA cables, preferably screened and foiled, run the RCA leads away from other cables behind dash.

Run the 12v feed and RCA leads as far as possible away from each other.

Try grounding / un-grounding the components at various points, ie, HU, seperate 0v cable from HU to Amp.

On the Engine side, Main Battery 0v lead to Body good?, Alternator bushes on their way out?.

Chris.