Alternator whine is the rising and falling tone that tracks engine speed. It is not a fault in your amplifier and it is not something an expensive RCA cable will cure. It is a ground loop, and there are four places it comes from.
What is actually happening
Two components in the system are grounded at points that sit at slightly different electrical potentials. That difference appears as a small voltage across the signal cable’s shield, the amplifier faithfully amplifies it, and you hear the alternator’s ripple. Fix the potential difference and the noise goes with it.
Work through these in order
In order of how often it works
- Ground the amplifier to bare, sanded chassis metal, under 18 inches, same gauge as the power wire
- Ground the head unit and the amplifier to the same point where practical
- Move the RCA cables to the opposite side of the car from the power wire
- Check that the RCA cables are not run through the same grommet as the engine harness
- Only then consider a ground loop isolator
Why an isolator is the last resort
A ground loop isolator works by breaking the shield connection through a transformer. It does remove the noise, and it also removes some low frequency content and adds a small amount of distortion. It is a patch over a wiring fault, not a repair. Use one when you genuinely cannot re-route or re-ground, which on most cars means almost never.
If a new ground point cures the whine, the old ground was also costing you output. The noise was the visible symptom of a connection that was not carrying current properly.
Marcus Reed, founder and editor-in-chief
Preventing it during the build
Decide the ground point before you mount the amplifier, not after. Route power down one side of the car and signal down the other from the start. Use a grommet everywhere the cable crosses metal. All three are in the subwoofer installation guide, and doing them the first time is far quicker than diagnosing whine afterwards.
