Car Stereos: speakers hissing, power line noise, power line filter

Question
hi, i contacted you before about this problem as its very annoying lol.  I have a toyota 2005 yaris and using the stock headunit, when the engine is off and cd is on at high volume i get a hiss from all speakers. but today when i turned the treble down the noise was going away, many people have had this problem, i have a power line noise filter which i havent installed yet, do you think this could work?  ive tried regrounding the headunit but still no good. the noise must be from the car so would the noise filter be the only fix?   thanx

Answer
Hi Danny,

To be honest, I doubt that the noise filter will be a fix either.  The purpose of a power line noise filter is to remove alternator "ripple".  An alternator produces power as AC current, but it's converted to DC using rectifying diodes.  The capacitance of the battery also helps smooth the leftover AC ripple, but components designed for car audio still have to include power filtering to eliminate alternator noise.  A power line filter usually includes a capacitor and a coil, both of which can prevent AC current from passing.

The trouble is that your problem is occuring when the engine isn't running.  That means the power is coming directly from the battery, and it's pure DC.  There's nothing there for the power line filter to remove.

It sounds very much as though the noise originates inside the head unit itself, where it can't be filtered. It might be a defect, or it might be poor design; but if you have a noisy audio component, there's really no way to quiet it.  (You might be able to install a parametric equalizer on the output of the head unit, and adjust it to cut out the offending frequency; but that would require dropping the head unit's speaker outputs to line level for the EQ, then amplifying the EQ's output to drive the speakers).  Ultimately, I think you're going to end up replacing the head unit.

Sorry I can't be more helpful!

Brian