DC Filter

I would like to know the best way to filter DC current in car. What I currently have is a power supply based on switching step-down converter to convert on-board voltage to 5V. This power supply was used for charging my PDA in car, but as it has a very big power reserve (max 2A@5V), I connected some additional devices to it as well.
One of these devices is a Bluetooth stereo receiver which is connected to AUX input of head unit (HU) and is used to stream music from PDA to this HU. But the problem as we know is that in-car DC voltage usually has much AC noise. I can hear engine revs through speakers, fuel pump or something similar (constant noise while engine is on).
Whay I want to know is a best way to filter all this junk. It definitely comes from power source, because I also tried a mains AC-DC power supply and there was no noise at all while receiver was placed in the same place in car.
What do you suggest? Ferrite beads? Chokes? Power supply shielding?
I suspect best way would be to use choke designed for audio frequencies, but where to put it? Before power supply or after? Or maybe in both places to have a better efficiency.

I've run in to this with amateur radio equipment. Normally auto electrical systems have a lot of noise - no suprise there. I've used inductors in series (must handle the current) but found large capacitors to be relatively effective. What I didn't do - but will do if I ever get back into this spot, is isolate the capacitor via diode - as otherwise it's paralleled with the rest of the systems - and not something I like to have.

Back to "normal" - by looking at the waveform on a scope I was able to determine that a rectifier in the alternator failed. That created a level of noise that was nearly impossible to filter out. Once I replaced the alternator there was still plenty of noise - but it was such that I could filter it out.

The car is only 2 years old, so it would be strange, that something went wrong with alternator rectifier or brushes (which may also cause interference). The noise is not so bad being impossible to filter out I think. Most annoying noise comes from the alternator, as it has the highest pitch.
Will 100uH inductor rated at 1,1A in series before the power supply do any good? The power supply as I mentioned before has max 2A output current at 5V so input it should not exceed 1A. Or should I better do filtering at the secondary side (5V). It is easier to mount the inductor at the 5V side.
And one more question. If I connect 100uH inductor after the power supply, can it do any harm to it when power cuts out (turn-off surge). Should I connect a diode in series as well to prevent this surge?

I have a basic theoretical knowledge, so this is a time I start to learn something in practice. Your help would be appreciated.

Hi,

The automotive supply is typically filtered at source ie. within the device you attach (stereo for instance).
You should try a bulky LC low-pass filter.
1/ 2*pi sqrt LC.
I have also attached a document which may be useful to you - it was useful for me when I recently designed an in-car accessory.

*Actually, this is my first post here and I cannot figure out how to add a file

Thanks,

S

A big LC filter may benefit the rest of the devices plugged into the system but if it's just the one receiver you're worried about use a linear regulator with modest buffer capacitors just for the receiver. You may need to get pretty grossly large with an LC filter to get rid of low frequency noise where as a linear regulator can be made in a MUCH smaller space, as long as it's a lower current just for the one device.

Agreed although if you 'isolate' it with a series diode before the LC (car supply side) you should be ok.
A linear reg is also a good option assuming it can handle the potentially large transients and the load current is not too large (i suppose you could use an LDO to reduce dissipation) etc - see the ST RBO40 device for protection.

Ok, so what do you suggest? Should I better connect an inductor in series (choke) or use a proper LC low pass filter. Mybe 1st order RC filter will be enough?
There are relatively cheap noise filters available on the market, but the only ones I can find where I live are for 20-30A, so they are bulky. I need one for max 2A, so maybe I should solder one myself, but then I will be needing circuit diagram with component nominals.