What Beerman is saying is that a ground loop isolator is a bandaid for a headwound. System noise is caused by a limited number of things...they may be great in number, but theres only so many things it can be.
What you guys need to do is short the RCA inputs on your amps. You can do that by making muting plugs which involves soldering the signal pin to the outer shield. Stick them in the RCA inputs to the amp. Then turn things on. If you still have noise, you have a ground problem at the amp that is unrelated to the signal chain. Things to check are your ground connections or the traces on the board inside the amp. Alot of times, things can come loose when you're toolin arou nd with whatever and if hte ground connection is shitty, you get noise. If you have the problem of noise despite the RCA inptus being shorted, try grounding the heatsink of the amp. If the noise goes away, you have a broken trace on the board.
If the noise isnt there when the RCA's are shorted, you need to trace the signal chain. The best thing to do is get a different set of RCAs and see if replacing them fixes the problem. Alot of times, RCA's will get smashed and the conductors inside will come into contact with each other, resulting on noise being put into the signal, as the noise cancelling/blocking ability of the cable is lost. If the RCA's solve your problem, great.
If not, you need to start looking at individual components. First, check your head unit ground. If its through the stock wiring harness, you're a dumbass and you should be beaten with a blunt object :twak If its grounded to the firewall, you should also be beaten with a blunt object because alternator whine loves to be transmitted through the firewall. What I do is get a ground distribution block that is grounded to good, solid metal that isnt anywhere near interfereance from heater motors, alternators, spinning disco balls, etc. :histerica Good, so your HU ground is good. Next, if you have processors inline, try eliminating them and see if that makes it go away. If not, and you've already muted the amplifier and the noise disappeared...your head unit is probably to blame. If your HU grounds are good, the RCA's are good...you probably ahve some kind of internal head unit problem. In this case only, I would use a ground loop isolator, HOWEVER, personally, i'd get a new head unit because Ground Loop isolators kill sound quality....audibly! I realize most of you guys dont care what you have for stereo as long as it plays but I'm a pretty picky dude.
So, thats my noise tutorial. remember: Ground to the frame for your amplifiers, ground your alternator to the frame directly, and make sure that your other auxillary grounds are good...99% of the time, good grounds solve all noise problems. :thumbup
