I've installed an audio auxiliary input unit for my car stereo, but I'm encountering a problem. After driving for a few minutes, I start to hear a low pitched "throbbing" sound from my stereo that gets loud enough to drown out what I'm actually trying to listen to.
I got a tip that I may not have a good ground connection – I was tapped into the car's electrical wiring instead of bolting the ground wire directly to a large chunk of metal. Once I remedied that problem I was able to drive for a longer period of time without hearing the noise, but it still came back.
If I turn the unit off and on again, the noise goes away but comes back quickly. If I leave the unit off for a few minutes and then turn it on again, it takes maybe 10 or 20 minutes for the noise to show up again.
What electrical principles are causing this problem, or what can I do to diagnose/fix it?
EDIT: The auxiliary input unit is a separate box plugged into the radio's antenna jack. When the unit is turned on, I tune the radio to a certain frequency and hear music (until the throbbing starts). When I turn the unit off and go back to regular radio, I don't have any problems.
Best Answer
The top three suspects I'd round up for questioning are the following:
Temperature-related component variations are having a large effect on one of the gain stages in your stereo. The first step to identifying and maybe fixing this would be to lower the volume on each stage independently, as in lower the line-out volume and try it, then return it to normal and lower the car stereo volume and try it. Look* for cracked and dry solder joints in the ground return lines. Also ensure the ground for the added input and the stereo preamp/receiver connect to the car ground/chassis at the same place.
Low-level oscillations already present in the system are exacerbated by the nature of the auxiliary connection, giving them a gain slightly over one. Since the observed feedback frequency is quite low, it would have to be two or more higher frequency artifacts combining to produce a low frequency beat. This can be identified by probing the audio jack while connected and while unconnected. The first fix to try would be to again reduce the gain of different stages. Speculating, this may also be caused by a gradual phase drift between speakers. If it only occurs when using the auxiliary input, I would suspect temperature variations in only one of the channels near the audio jack (bad soldering, overdriven components) or a flaw in the auxiliary audio player -- try a different one.
Demonic infestation. Sell the car.