Car DC Voltage supply filter

[igorbog]

Hi to all
I have connected an preamp for mic in my car to dc source. when I connect preamp out to aux I hear a lot of noise. The preamp work good outside of a car. how can I design filter for car dc noise?

[phil_jp1]

What kind of preamp do you have? Is is special car stuff, or something else?
You hear a lot of noise when the car engine is working or not?

The easiest thing you can try is to put about 470-1000uF 50V low-ESR capacitor in parallel with 0.1uF 50V ceramic cap before your preamp (as close to it, as possible).
The other way is LC filter.

But that's just my guess, because I don't know what kind of preamp you have (how much current it draws, etc..).

[sonicj]

i would try to eliminate the noise prior to filtering it.  does everything in your signal chain reference the same ground? ie: car chassis, battery neg
-sj

[Psi]

An audio ground loop isolation transformers between the preamp output and aux in may fix your problem.

Or you could try an isolated powersupply module to run the preamp.

[igorbog]

I have connected this device http://www.ebay.com/itm/ws/eBayISAPI.dll?ViewItem&item=220837381675&ssPageName=ADME:X:AAQ:US:1123
through usb to dc car converter to car dc supply.

[phil_jp1]

I have connected this device http://www.ebay.com/itm/ws/eBayISAPI.dll?ViewItem&item=220837381675&ssPageName=ADME:X:AAQ:US:1123
through usb to dc car converter to car dc supply.

Those DC-DC car power supplies (that converts car voltages to 5VDC) usually have _very_ poor output filtering, and because of that they have a really huge output voltage ripple.
I guess - that's the cause of your problem.

I guess your amplifier don't consume that much current, so the ideal decision is to build linear DC-DC converter using LM317. With heavy output filtering.
A little bit easier way is to use 7805 voltage regulator (almost the same as LM317, but with fixed output voltage and a little bit lower ripple rejection ratio).

And the easiest way is to put LC filter right before your mic amplifier. (I reckon you should try that first) I guess 1uH-10uH inductor and 100uF low-ESR cap would be enough.

And for building linear DC-DC converter you can watch the video I made(I explained all the concepts there): http://jumperone.com/2011/08/lm317-adjustable-psu/
Just don't pay attention to mains transformer and you can put fixed resistors instead of potentiometer.
And use about 50V (at least 35V) input caps in car electronics. There could be a huge voltage spikes.