Finally, how do we troubleshoot a system?
Isolation, Isolation, Isolation
We have to separate each piece from the system and test it. Consider the system below:
[Source (PC, HU)] ----> [amplifier 1,2,3...n] ----->[passive crossover] ----->[speakers]
We just installed the pieces making sure to properly ground all devices and keep signal path away from EMI emitting devices. We have discovered that we still have a ground loop somewhere. How do we find it?
Lets get that ground loop isolator out and plug it in at any of the -----> points. This will tell us where the ground loop is and help us determine which components to look at.
My personal preference is to break the chain right in the middle. I will unplug the RCA's going into the amplifier and plug in my shorting plugs (see my faq on troubleshooting tools). You can try to use an ipod as well, just make sure its battery powered and you keep the RCAs away from power. Is the sound still there? If yes, we know we have a problem (may not be all of the problems) with the amplifier or the speakers. Next step is to disconnect the speakers and grab a test speaker. A test speaker can be any old working full range speaker. Even factory speakers will work. Attach the test speaker and listen for the sound. If the sound is gone, the problem is in your speakers or passive crossovers. Check to make sure you have not put a screw through your speaker wires and they are not touching a metal ground somewhere. Relocate your passive crossovers as they may be picking up the sound. If the noise is still there, Its your amp. You may want to try mounting the amp to metal, relocating the amp or changing your ground. If none of these work, I would say you have an issue with the amp and get it looked at.
If no whine is detected from the above, its the RCAs or the source. You can use your shorting plugs on the source ends and determine if the RCAS that are picking up interference. If they are, look for a screw that has gone through the RCAs or consider rerouting them. It could be a lousy pair of RCAs so you may want to have another pair to test with.
Lets suppose that the RCAs are not the culprit. That leads us to the last unit, the PC. Keep in mind that the PC was NOT intended for the environment we are putting it in. There are MANY factors that can cause havoc with the pc and you may NEVER eliminate the noise (component related). Plug a set of headphones into the output of the sound card. Do you hear the noise? Well then we know that there is something internal to the PC that is picking up EMI or a ground trace is burned up. We can check the later by swapping sound cards. If its EMI, you may need to relocate the PC or ground the chassis.
I have found low quality sound cards will cause this issue as well as power supplies. I recommend picking up a Xfi as it seems to be well isolated but for many this is not an option. My message is try not to skimp on this area. I had noise issues with my integrated sound card that never went away until i upgraded. Its due to improper shielding and sometimes you just cant get it far enough away from EMI sources. I have also found many power supplies are not designed to properly shield the EMI they produce. The first DSATX caused this but have subsequently been redesigned.
This system is simplistic as its more of a methodology than a checklist. As you add on USB sound cards, things get much more complex. There are several posts to help out so search. Hopefully you will have an idea on the methodology behind troubleshooting and you can adapt my methods to help you locate and isolate the problem. Once you have isolated it, you can further search the forum for more details on how to solve it.