If doing the right thing fails...

So let's imagine you've taken my suggestions, done all you can to fix a ground loop hum issue at its source and you still have hum. What then? Then you have to take a different path: break the ground loop. To do this you will need to 'unground' the offending piece of equipment. This can be accomplished in any number of ways depending on what country you live in. But here's the thing. You must know it's not recommended to unground a product from the wall socket. Why? In the unlikely event a connector or wire with live voltage should get loose in the chassis it could be dangerous. Lethal, in fact. So this is one reason we suggest you fix the problem of the different ground in the first place. But I have seen many instances where this is not practical. In those cases I sometimes remove the ground pin from the power cable or use a simple 'cheater plug', available at most any hardware store for this very purpose. Once connected back in the system, the offending unit is still grounded, but it's grounded through the connecting cables. It's not up to code and it is not advised the YOU do this. Legally I cannot and should not encourage you to do such a thing and I won't. In fact, don't. One thing you can try is to use a ground loop buster which claims to be effective and safe. Click here. I haven't any experience with this because I live a dangerous life on the edge. I take all sorts of risks. It's just in my nature. If one of the components in my chain has a ground loop issue, it's almost always the power amp. I am not sure why this is, but that's been my experience. If I can't fix the ground then I cheat this plug and leave every other piece of equipment in the system grounded. This solves the hum problem by 'floating' the power amp, relative to ground. The power amp is, in fact, grounded. Only, instead of being grounded through the third prong on your AC socket, it's grounded through the connecting cables to the preamp's ground, which is connected to the third prong ground on your AC connector. But don't listen to me. Certainly do not do what I do.