Ok, anytime you rectify ac you get dc with what is called "ripple". This is pulsing voltage on top of the dc. When you rectify standard single phase ac, like house current, the voltage goes to zero twice per cycle. So, you will have the brief dips in the dc.
To help prevent that and to make the alternators more efficient, they wound them to produce three phase. The third phase "fills in" the zero voltage dips, so it does not go all the way to zero.
But it still dips. It does so at an audio frequency. Thus the whine.
Dc powered gadgets don't like noisy dc, so in the several power supplies I built over the years, I put in capacitors. A capacitor acts like a water tank in a water pump system.It charges up to the maximum voltage of the power supply. Then, when the voltage starts to dip in ripple, it supplies the needed current to fill in the dip. Thus you get a nice clean output.
But, its a fact of life that the more current the power supply has to provide, the more ripple you get and the bigger the capacitor you need. So, trying to eliminate the ripple from an alternator that is putting out 50 amps or so, would require a massive capacitor.
But, in addition to audio frequency ripple, alternators also can put out radio frequency noise unrelated to the ripple.
Capacitors are shorts to ac, but block dc. So, putting a relatively small capacitor across the output of the alternator will short any rf to ground. That is the purpose of the capacitor on the alternator.
To short the ripple to ground under high current, would require a huge capacitor. Which the battery tends to act like. I have seen bad battery connections and even batteries that are going bad cause whine.
In your case, I think its a poor or defective filter system in the power input in the amp. So, we need to help it with an external capacitor. The one you posted might work, but its primary use is to supply current during high amp loads.
Most amps don't need it, but if you are running one of the big 500w or up amps, you probably do. There are two ways to get rid of the noise. Short it to ground, in which case a small cap, say 47microfareds might work, or try to fill in the ripples in which case the bigger .5 fared one might work.
I wish I could give you a size, but whatever you use is going to become part of the filter system inside the amp, and that will affect its behavior.
Do you have a local source of supply for capacitors? They need to be rated at about 35 volts. They sell 16 volt ones, but that is pushing it. With Radio Shak gone, most of the stuff nowadays is mail order. And you really need an assortment to try out.
Any radio guys in your area? Remember if you use an electrolytic one, it will have one end marked either plus or minus. That end MUST be connected to the correct wire.
And the voltage rating is kinda critical. It can be way above the voltage you are hooking it to, but cannot be below it, and should not be too close. So, a 1000mf at 16 volts is risky, while 1000mf at 250volts is fine.
Even one of the little cans like go across old time points might help. Just hook the wire to the positive side and the can to the negative. Assuming it did not come off an old positive ground car or tractor......
If you can find some capacitors, just hook up one and try it. If the noise goes down, try a bigger one.
The only good thing about trial and error with capacitors is that they are usually fairly cheap.
For instance.
https://www.alliedelec.com/illinois-capacitor-inc-108ckh035m/70112171/
Notice the arrows on the side of the cap with a minus sign in them. They are pointing to the terminal that has to be hooked to the negative lead. Some capacitor have the same arrows with plus symbols in them. Naturally they hook to the positive side.
Axial leads might be easier to hook up, such as this one.
https://www.alliedelec.com/bc-components-vishay-mal202117102e3/70122976/
The difference in price is due to the higher rated voltage.
Note in the specs where it says ESR. The lower that number the better.
I'm afraid I have not helped much. Post back with other questions or results. But I'm pretty sure your problem is in the amp.
