A ground loop is the most common type of noise. It is a difference of resistance between your source and your amplifier. Easiest way to tell is if you get a hum while the car is running, and it increases in pitch when you rev it.
First things's first. The ground wires to everything. Check and make sure you have a clean solid ground from your battery to the body of the car, as well as a good solid clean ground from your battery to your engine. Almost all cars have rubber motor mounts, hence the two grounding locations. Test the grounds with a multimeter set to ohms. Put the one end of the meter at the one end, then the opposite lead of the meter at the other end of the wire. Which side you put the leads on doesn't matter. Anything less than 4 ohms is generally acceptable, but the lower the better. My grounds all have a .3 ohms or less resistance.
After you check that, we start troubleshooting the inside components. If you have a pioneer deck, they're notorious for blowing the pico fuse. ground the outer RCAs to the body of the radio. This will usually remove all the noise.
Start with each component to troubleshoot if you do indeed have a noisy component. Take a cheap pair of rcas and cut one of the ends off. strip and twist the two wires together. This will make you a muting plug. Fire up your car, turn the radio on, and plug in the muting plug directly into the amp. Check all of the channels of the amp. If there's noise from the speakers, then the problem is with your amplifier. Check the grounds for the amp, and if that doesn't cure it, replace the amplifier.
Next check any signal processors/line drivers/equalizers. Use your muting plug on the input of the device, never the output, unless you want a paperweight. If you do have noise coming from one of these components, take the ground of it and run it to the exact location of the ground for your amplifier. If you have no noise from these, we go on to the next step.
Pull your radio out. If you're using the factory ground, take a run of 12 gauge wire right back to the ground where the amplifer is grounded to. Better yet, if you can, run the wire right into the terminal of the amplifier. Upgrade your power wire as well, and be sure to fuse it.
To test if it's a bad radio or not - try another radio (duh) or use an mp3 player and plug it into either your eq/processor/driver or if those don't exist right into the amp. You can also hook up your head unit to a battery or a jump pack that isn't connected to your existing electrical system. If there's noise, then it's a bad unit. If not, keep reading.
Check and make sure your rcas aren't close to power wires. Check and make sure your speaker wires aren't sitting on long runs of power wire either.
The last thing that it could be is your alternator. Alternators make, by their nature, AC voltage. There's diodes or a rectifier inside of an alternator to change the AC into DC current. If your diodes are going bad, then this will then introduce noise. Get your alternator tested.
After all of this, and you still have noise, the next recourse is a ground loop isolator. It removes the DC noise from the outer shell. Be sure to get an isolator that uses transformers. The cheaper isolators will remove your midbass.
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