lets look at the electrical system
charge from alternator (car on) to battery. + terminal wired to fuses to the car then to equiptment. - terminal of battery grounded to chassis of the car. all equiptment grounded different points of the car chassis.
the entire car chassis can produce noise. grounding it to the chassis i like grounding it to the (-) terminal of the battery.
Almost all car audio is based off of 12v dc which requires a circuit to operate. What it means is that you need to have 12v coming in from the alternator/battery and it must return to the battery via a ground. (Purists: Yes, 12v actually runs from - to + in US vehicles, but lets not confuse the point). Electricity is sneaky, and rather moody. It will try to find the easiest path to create this circuit. We want to ensure that its easiest and most efficient route is through the ground wiring of the unit. This brings us to our first potential cause for your noise.
Most modern cars are built on the uni-body concept. Metal portions are tacked together with high power tack welders. The welds are awful for grounds and can cause a real hassle.
care should be taken for your 12v+ wiring. Make sure you use good crimps, adequate gauge wiring and LOTS of contact at every connection. That means your connection to the battery should have a ton of surface to surface contact. The more the better as I have seen numerous people just shove a little bit of a ring terminal into the battery terminal. This does NOT make the proper type of connection. I usually tug on my wiring after i connect it. Give it a decent tug. If it comes loose, it wasnt a good enough connection/crimp.
Our next potential cause of noise is something called Electromagnetic Interference...
This stuff is everywhere. Radio Frequency Interference (RFI) is electromagnetic radiation which is emitted by electrical circuits carrying rapidly changing signals, as a by-product of their normal operation, and which causes unwanted signals (interference or noise) to be induced in other circuits. This interrupts, obstructs, or otherwise degrades or limits the effective performance of those other circuits. It can be induced intentionally, as in some forms of electronic warfare, or unintentionally, as a result of spurious emissions and responses, intermodulation products, and the like. It is also known as Electromagnetic Interference or EMI
What this means is that if you are not careful with the quality of your components and where you place them, they hold a strong chance of picking up EMI. Shielding on cables can help reduce this effect but the cost of cables is rediculous. A standard RCA cable from a decent manufacturer purchased outside of Radio Shack should be sufficient. Looking for single shielded cable. Plan on spending about $1.00 to $1.50 a foot. These cables are very important to your system and should not be the weakest link. Make sure you route your cables as far away from RFI sources as possible. Your power cables are a source, your PC is a source, strange electrical component boxes around the vehicle are sources. Basically anything that carries an electric signal can produce this unwanted nuisance. Keep your cables as far away as possible from them all.
Grounding a components chassis can help create a shielding barrier by passing the interference back into the ground. Depending on how the system is designed, this can be good or bad as it can create a ground loop.
Some amplifiers will also pick up EMI. Usually these are inexpensive amps where they cut corners in the design. If you have determined your amp is the problem, try mounting the amp to metal. It may help the situation. HUGE NOTE: grounding the chassis of an amp can cause the exact symptoms we are trying to avoid, so only try this in your trouble shooting process. I usually recommend AGAINST mounting directly to metal.
Keep in mind that those external crossovers (passive AND active) can also pick up EMI. Be careful where you place them.
Finally the most famous culpret... The Ground Loop.
In electrical engineering and electronics, a ground loop refers to an unwanted current that flows in a conductor connecting two points that are nominally at the same potential, for example ground potential, but are actually at different potentials. In english, this means that one component has found it easier to ground itself through another component.
If a component does NOT have a proper ground, it will look to ground itself through the easiest means. This is by grounding through the next component down the line. How does it do this? Through those nifty RCA cables. The outer shielding of the RCA cables is actually a common ground (in most cases). If your PC's ground is not so great, it will try to ground itself through your amplifier's ground by passing its ground through the RCAs and into the amp. This is where all that noise comes from. This can happen if the ground potential of any one component is higher/lower than another. We combat this by making sure our ground are sufficient
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]
from your test just isolate the rca wires... DONE
Ride: 2001 330i
Radio: Clarion NX500
Amps: Diamond Audio
Speakers: DLS Audio
Subwoofers: (2) 12" MB Quarts
MODS: Coming Soon