Car Stereos: interference, power line noise, speaker wiring

Question
hi, do you think it could be to do with the power line to the headunit runs from the engine bay, where fuses are?  also the headunit gets its power after its gone through the display module, where it shows the track number and mpg, this was the follow-up from the yaris interference. i just read on the net its power related. could this be right?  other people still get the noise after fitting a new headunit.  thanx

Answer
Danny,

I'm looking for a smart way of saying "I got nothing".

You're thinking of "induced noise", which is what happens when a conductor is affected by a changing magnetic field.  There are plenty of devices in a car that can generate such magnetic fields; in fact, since current flows right through the vehicle chassis itself, it's hard to place any conductor where it won't be affected.

The thing is, induced noise is generally only a problem in a conductor that carries an audio signal.  A power wire doesn't qualify.  The speaker wiring, between the head unit and the speakers, carries an audio signal; but if induced noise on the speaker wiring was the culprit, it shouldn't be affected by the treble setting on the head unit.

Excessive AC ripple on the power line can find its way into the audio signal, which is why any component designed for car audio usually includes power line noise filtering.  If you add an additional noise filter to the power line, it certainly won't hurt anything.  I just don't know that it will help either, because I don't know how excessive AC ripple can get into the power line when the engine isn't running.  With the key in the accessory position, there usually aren't too many vehicle systems operating that can generate noise to be induced into the power wiring or the signal wiring.

I personally haven't heard of the problem with the Yaris sound system that you're finding commonplace; possibly it's only an issue in the UK or European version of this vehicle.  I can only speculate as to the cause; my best guess is that it's an internal problem inside the head unit, not an outside interference generated by some other component.  One experiment might be to disconnect the radio from the vehicle's power wiring, and power it directly from a separate 12v power supply (such as a motorcycle battery).  If the noise persists, that's a pretty good indicator that the problem is internal  to the head unit.

In your particular case, I'd say go ahead and install the power line noise filter.  It certainly can't do any harm, and might do some good.  

Sorry I can't be more helpful!

Brian