I am a newbie here...just bought a 98 328iC and I have been reading up so I can start my install on my BMW. I was a car stereo installer for 10 years was deep into the car stereo world on a technical level. I have designed and built numerous amplifiers and have a degree in electrical engineering. I have a vast amount of knowledge on installs and I am ready to contribute
So...thought I would kick out a few secrets to give something back!
Lets start with noise:
The biggest problem in car stereo is noise. To start we are dealing with an extremely noisy power source...the evil alternator
: ! To make things worse we have all kinds of unprotected motors and relays that are not filtered adding to the mix. To put the icing on the cake they all use a common grounding source...the frame...creating multiple ground paths that cross everywhere.
All devices in a factory car have a switching power supply...the underlying purpose in a switching power supply is to completely isolate the device from it's power source and to insure that the power output never varies even if the input falls below the required voltage it needs to operate. Car amplifiers and most all high end audio devices have them so they are protected as well...so where does the noise come from? The RCA's!
The end all cure is to use fiber optics...but this isn't available on budget systems so we'll keep our scope on RCA's. There are several methods to reduce or eliminate noise from the RCA's.
First do not route any power cable near the RCA cable. If you must cross one do it perpendicularly. Use good quality RCA's. An RCA is a single tiny little power wire that is surrounded by a shielding wire. The thicker the shield the less likely you get noise from EM transfer.
Next is the ground loop. This is the number 1 cause of all system noise. Noise can be picked up when ground paths cross in the chassis. Ground loop RCA filters can be used but these are notorious for adding DC bias into the line and can cause gain problems and distortion at high levels as well. I found the best way is to run a grounding strap to every device in the system. It only has to eliminate noise and not carry any real current so I like to use something around a 10 gauge wire. You can use a smaller wire but if the ground gets loose or cut on a high power amplifier it will use this wire instead and melt everything. A 10 gauge wire will handle the current and you'll notice a weak amp before it has time to get really hot.
I start from the battery and then to the head unit and then on to the back to any processors, crossovers, and amplifiers. I make sure that I connect it to the device chassis as well as the ground lug/wire itself. This sounds like overkill but I have NEVER had a system with this strap installed have any ground loop noise EVER!
The next hidden source of noise is the amp turn on wire. Your radio uses a switching power supply so it is clean inside...what you don't know is that noise can travel from the amplifier/processor back through that wire to the radio. You have two choices to defeat this. The best way is to use the amp turn on to power a relay and feed the amps/processors from the battery 12+ rather then the radio. This takes a load off the radio (making it's power supply work better at defeating noise, less heat, etc) and it also isolates it from the rest of your system. On smaller single amp systems you can also use a regular barrel diode which only allows power to travel one way. It does allow a little to feedback but not as much as not having one.
Another good way to attack the engine noise is to place a capacitor at the battery. Yep..I said at the battery. The noise comes directly from your alternator to your battery first. The battery absorbs the majority of this noise, but not all. I like to stop it there before it gets into the rest of your system. If you are going to have a capacitor in your system I always like to put it there. Any real amplifier with a good power supply section will not benefit from a capacitor....unless you have puny supply wires coming from your battery. Keep in mind this won't eliminate noise from other motors in the car...but it does help with the alternator...which is the number one noise supplier. If you feel the need for more caps then add all you want! They certainly don't hurt anything.
Your battery is your best ally against noise. Buy a good quality battery...that will ensure a lower internal resistance. Also make sure you keep your battery terminals clean. I don't opt for the gold plated crap. Gold plating tends to flake and they cost more. If you are really anal get some of the brass battery terminals used in boats. Typically the lead ones work fine. If you make your own power cables like I do, I recommend welding cable with copper lugs. Welding cable costs a fraction of the store bought wire, has tougher coating, and has a much higher strand count. When you solder them on cover all exposed copper with high quality tin solder including the lug. Silver solder will not corrode (take a look at any outdoor PCB like an antenna amplifier...the hole PCB is tin coated like this). The one thing I highly recommend on any connector exposed to the elements is stuff called Corrosion Block. Spray it on those terminals and then forget about it! Lastly...some cars have chinchy little ground straps from the battery. (haven't looked at the Bimmer yet). They also like to run a big one to the engine for the starter and then a small one to the chassis for the rest of the car. I like to run one from the ground directly to the frame...a shock mount is usually right there handy to bolt to.
Okay...my fingers hurt...hope this helps. More later
I want to thank this board for all of the information I have found so far...it's really the reason I bought this car...nothing beats support!!!