Ok, after searching through the Car Audio forums and reading a lot of threads on ground loops and trouble shooting them, I have some questions.
I have my carPC installed using a DSATX dc-dc converter. I am using a PCI audio card (SB Audigy 2 ZS) for the audio. From the PCI card I am using (2) 3.5mm phono plug to two male RCA connecter cables to go out to my front and rear amps (David Navone AMP-100).
When I power the PC on (without the car running) I hear some aweful noises that I can only assume are coming from the HDD and other components from the PC. If I plug a set of headphones into the PC instead of going to the amps, I do not hear the noise. I then took an old Walkman and plugged it into the same phono-rca cables that go to the amps, and everything sounds fine (no noise). Therefore, I am pretty sure that the problem has something to do with the PCI card or the PC itself.
I am using a distribution block for my power as well as a small one for my grounds. All of the grounds for the amps, PC, and a power relay for the PC are tied to the load side of the distribution block and the line side is tied to a bolt that has had the paint stripped off from underneath it using 8awg wire. I also tried to run an 8awg wire directly from the ground distribution block to the negative side of the battery, but that did not change anything. I also grounded my PC case (DIY homemade case) by grinding off some paint and using a ring terminal to go to the GND terminal of the DSATX. That also did not help the noise problem any.
I picked up a Scosche Ground Loop Isolator (ES034) and hooked it in between the PC and front amp (through the RCA cable) and it got rid of the noise completly. I would rather not leave this in there as a permanant item since I see these as just a band-aid.
What else can I do to eliminate the noise? Is it in fact a ground loop? Is it possibly coming through the sheild wire on the RCAs?
you could try grounding one of the rca negative leads-- in most cases, this gets rid of the noise.