DC-DC isolation for my car stereo... Where do I start?

Question

I recently noticed that my jeep's stereo whines with a frequency that seems proportional to the rpm of my engine (and alternator), and is louder when my headlights are on.

If my stereo was powered by AC electricity it would be no problem to add an isolation transformer, but since I am working with DC I really don't know how to isolate the stero from the rest of my vehicle's power system.

A quick eBay search came up with some small ICs that appear to be DC isolators but none of them exceed 1W - an I have a 200w stereo.

How should I go about isolating my stereo from the rest of my Jeep's electronics?

Alternatively, What if I were to add a huge cap in parallel with my car battery, would that help keep things smooth? If I used five of these 3V capacitors in series I would essentially have a 4F, 15v capacitor. Would that be able to prevent noise from my alternator and headlights from affecting my stereo?

Answer 1

Headlights are DC. So I'd guess that the alternator is noisy [under load], and your stereo is picking that up.
You could try beating the noise at the source, or focus on the stereo. If you want to rip off & re-terminate your stereo DC plug, you could wind a couple of turns on a toroid [ring] core. If playing with plugs doesn't appeal, try winding the DC cable around a ferrite rod. Hold it in place with electrician's plastic insulating tape.

Answer 2

Your alternator generates noise on the power line. Better car radios have internal filters for the power supply (a big choke and some caps). You can also buy those filters separately (for example this one) or just build your own - a big choke in series with the positive wire followed by a cap in parallel to the radio's power input.

Answer 3

Prevention is better than cure or so the doctors tell us so I'd be inclined to sort out the generator's noise rather than try and cure the amp. I'd also look at the cables feeding the amp and if necessary wire then directly from the battery rather than let them share possibly contaminated current paths with other circuits.

As a last resort I'd filter the power to the amp. I wouldn't want to isolate the amp if there's a radio because (from memory) the radio's antenna needs to be grounded and isolation might cause more problems in that area.