Audio Systems: bad sound from speakers, rockford fosgate cd, rf amp

Question
I have a problem with bad sound (scratch, popping)coming  from my speakers. I have a Rockford Fosgate cd player connected to a 4 channel RF amp.  The two front 6x5 pioneer speakers are connected directly to the head unit while the back two  6x9 pioneer are bridged to the amp.  When the car is off the sound is fine, but when the car in on the front speakers sound horrible.  The back two sound fine.  I have changed speakers and tested the old ones to another unit and they are fine.  I also tried reversing the connection using the rear speaker output wires instead of the front while using the front output RCA connection to the amp but still the same.  The ground and power connection seem to be fine.  HELP!!!!

Answer
I cannot be sure what your testing was and what the results were.

If you can disconnect the front speakers and power them off the amp that normally is driving the back speakers and still the fronts sound bad, then it has to be the speakers.  If these same speakers sound good in another installation or when driven even from an external amp then they must be screwed down too hard causing severe voice coil rub.  The baskets of the speakers must not be distorted or they will cause coil rub everytime.  Try loosening them up.  BUT: that does not account for them sounding ok when the engine is OFF.

Are you sure the fronts are bad only when the engine is running?  How about when the ignition is on, but engine is NOT running?  If only when engine is running you may have engine interference.  Does the noise increase in frequency as the rpm of the engine goes up?  If so, it may be related to alternator interference.   

You may want to test with the engine running but the alternator belt loosened temporarily such that the alternator is running.  Then you will know if it is alternator or ignition noises - or something else.

You may also want to power the back speakers from the front head amp outputs to see if they behave badly.

So, some testing to do, but with the process of elimination you should be able to pin it down and solve the problem.

Please come back to me if you get stumped with all the above testing procedures I layed on you.

Cleggsan