Audio Troubleshooting 101
OK.. Thanks for the comments.. however, I have to take exception to the last reply... Muting the amplifier = supplying a "known good", which is a signal of 0 Volts with a source impedance of 0 Ohms.. It doesn't get much better than that for a pure signal with a low source impedance.. If a muted amplifier produces anything but 0 Volts at the output, then this part of the audio system must be addressed before continuing the troubleshooting. It's important to understand that Muting the Amplifier is not turning off the amplifier as was mentioned in the previous post.
The reason that it's important to understand muting is that once the amp/speaker part of the audio system is proven free of noise, then we move our muting plug back up the signal path... A muting plug can also be called a dead short. At the bottom of the home page on my website:
David Navone - Car Audio Engineering there are a couple of video productions on the construction and use of a muting plug...
One last point.. The term "high quality RCA cables" is totally meaningless to the world of automotive audio. Marketing has produced very expensive triple shielding signal cables that actally pick up more low frequency inductive interference than basic UTP cabling with no shielding.. A good parallel is to look at the telephone and ethernet wiring in your home/office. How much shielding does your telephone system use? The answer is none...
Why is telephone and car audio similar? Because the noise lies in exactly the same bandwidth as the signal. Rectification is not required for noise to be produced in a car audio system.. In fact, most of our audio systems have low pass filtering on the inputs to eliminate the type of noise that shielding would benefit. My partner, Richard Clark, and I have a patent on a particularly low noise signal cable, Delta Factor... and we have studied this subject for many years.
I'm not saying that troubleshooting a car audio sytem is trivial... on the contrary, multiple amplifiers fed by various processors driven by several sources (i.e. iPod, cell phone, CD Changer, AM/FM/Satellite, HU, etc.) can lead to a complex system. The way to handle such a system is to start by Muting the Amps... This basic step is important to logical troubleshooting... And I've been doing this type of work for 40+ years... in avionics, ham radio, home/pro audio, and car audio. Mute the Amp!