They're 12v, not 13.8v.
Also, switching power supplies are known to produce RFI.
Good luck!
I have had relatively good luck doing this also. Seems most computer power supplies are not too noisy. Sometimes you will find one that puts a noise level in your receiver. I can tell you from experience an alternator whine filter will not get rid of noise from a switching power supply. Unlike in the case of the alternator where we want to filter out relatively low frequency AC ripple, the switching supply creates RF hash. This RF is actually radiated from the power supply circuits and is not easily filtered out. It's usually picked up by the antenna and removing it will confirm it's radiated RF and not ripple on the DC line.
Good points... Maby extra power supply case shielding? there not very good stock.
They got so many holes in them I aint surprised they have RF noise.
T23
I would think the computer PS would work well, as a computer needs good clean DC to function. The computer PS is a switcher type and the switcher's frequency S/B above the human audio range. The only drawback is the 12V vs 13.7V. You should be able to adjust the voltage by changing the resistive divider, or adding a silicone diode in series with the sensing voltage pick off in the +5V side of the PS. The +5V is the only voltage that they regulate, the 12V and the rest follow the +5V. Be carefull the PS has high voltage (as high as 340VDC via a voltage doubler with capacitor storage), can be lethal!