Explanation of ground loops and alternator whine

Saw talk of ground loops so figured I'd write up a breakdown explaining it and how to prevent it.

Voltage gets lost in the chassis as current travels back to the battery negative post, so measuring battery negative post to different points on the chassis will give you different voltage readings. Audio signal is some X amount of volts over ground reference. When there's a difference in ground reference between two pieces of equipment linked by a conductor (an RCA), the grounds attempt to balance themselves, sending a current between the conductor to the better ground. So long as this balancing current remains the same, DC, 0hz, you won't hear it. It'll just be a small loss in volume.

Now for alt whine... your alts diodes put out ripple into the system, which switches between high and low current at some frequency determined by engine speed. The back and forth AC ripple current spat out by the alternator diodes increases and decreases the chassis ground reference voltage as it cycles (more current at the same resistance = higher voltage)

So what was constant offset in the first example now becomes fluctuating offset depending on the frequency of the alternator (determined by engine RPM). Fluctuating voltage is AC, which is picked up by the amp as signal and sent to the speakers

Grounding everything to one point on the chassis means that all the equipment has the same ground reference.

With numbers:

• Say the headunit has a ground reference of 0.1v and the amp has a ground reference of 0.3v.

  • The headunit outputs a signal with a strength of 2v in reference to its ground, so 2v above 0.1v; 2.1v output on the RCA positive pin, 0.1v on the ground shield, 2v difference.
  • The amp gets this 2.1v and compares it to its own ground reference, which is 0.3, so the signal is 1.8v above ground, not 2v, a difference of 0.2v
  • Basically the HU sent 2v and the amp received 1.8v, so there's a 0.2v error.

• Through the conductor (RCA) the devices try to balance. If the 0.2v error tries to drain out through the better ground.

• If the ground points never fluctuate, then that 0.2v error just becomes a gain loss, its a little bit quieter. But no noise, as the voltage difference on the signal measured is always the same. There's no noise as the freq of a DC current is 0hz.

• Once the alternator comes in with its ripple, high / low current switching at a frequency, the ground references begin to fluctuate too, at that frequency. As an example, when the AC current hits its peak, the references both go up up by 0.2v. So it will jump from low ground potential difference to high; 0.2v to 0.4v. The current that went through the RCA to balance the two grounds begins fluctuating at the frequency of the alternator, back and forth. Back and forth current is AC. Amp sees this AC and can't differentiate it from the music signal, so it amplifies it and plays it through the speakers.

• You now have a whine that increases pitch with RPM, as higher RPM increases the frequency of the alternator ripple, which increases the rate at which ground potential on the chassis fluctuates back and forth, which determines the rate at which the balancing current on the RCA switches between high / low.

If you have a better understanding of this and spot a mistake in this, please let me know