Noise only when Engine is running (Buzzing sound - increase with gain boost)

RonRizz

If the "noise" is actually slight ripples in voltage, it will smooth them to a certain degree, but this is not "Audible noise".
I have not seen a single article that states that a stiffening cap will help reduce noise. Not even from a manufacturer trying to sell them.
I would think that if it were in fact a benefit of using a capacitor, every manufacturer would shout it from the rooftops to try to sell one more unit.
In my opinion, Stiffening caps are snake oil, and if your sound system is ridiculous enough that you need additional help, a second battery, or upgraded alternator may be in order.

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