Originally Posted by strange_brew
[not an expert]
I have looked into the "Hum" issue quite a bit. ebr is correct, it occurs when the "system" has more than 1 path to ground and balanced connections will not fix it. This most often occurs with the cable line and can be best solved with one of these:http://www.partsexpress.com/pe/showd...number=180-075
However, another path can exist if you have interconnected equipment on different circuits. Common examples are subwoofers and projectors since they tend to be in different locations from the a/v equipment and many of them have a ground plug. A good (inexpensive) cure for this kind of thing is to use the Ebtech HumX:http://www.zzounds.com/item--EBTHUMX
You can also get isolation transformers (Tripp-Lite for example) that you plug your equipment into, but that is an entirely different snack bracket (another potential option on audio-only equipment is to use optical vs. coax if that is the only line connected to the pre-amp since it doesn't introduce another electrical ground path).
To figure out where your problem is, start by unplugging everything from your pre-amp and amp, including the amp/pre-amp interconnects. Turn on the amp. There should be no hum since it has only one path to ground. If you DO have hum, then you likely have equipment in a metal rack that is grounding the amp to the other equipment in the rack, creating a separate path to ground. Use of plastic washers on the rack ears, or rubber isolators on the feet may fix that. Now connect the pre-amp. Hum should still be gone (assuming you have them on the same circuit, or the same side of the panel). Now start connecting your sources until you find the hum. The way you remedy it depends on what the root cause is. In all cases you should be able to fix it without removing the ground on the equipment. That is just asking for trouble - a lot of equipment can't cope without the ground. I believe it helps stabilize the voltage or some such but I'm a little fuzzy there.
Pro amps should not, in and of themselves, cause hum. It is the circumstances in which they are installed that cause the problem. The reason a lot of consumer gear does not hum is because it isn't grounded (2 prong plug). Any time you have a ground on your equipment plug, you have the potential for a ground loop.
In my case, I had 2 problems. My gear runs off a sub-panel in my equipment room. While its still grounded to the main panel, the ground potential is different. So the other things grounded directly to the main panel (Projector outlet), or directly to house ground (cable line) had different electrical potential and, therefore, it hummed. So I used the Dayton isolation transformer on my cable line, and an Ebtech HumX on my PJ outlet. Problem gone.
Knowing what I know now, I would have made sure ALL of my interconnected gear ran from dedicated circuits on the same panel or, at the very least, all of the equipment requiring a ground was on the same side of the panel on dedicated circuits.
Another related point is where you put the circuit on your panel - you want to stay away from putting your dedicated A/V circuits on the same side of the panel (bus) as motor driven devices (e.g., fridge, freezer) or dimmers such as the Lutron GrafikEye - these things can also introduce noise into the system that isn't easily remedied.
Again, how you solve it really depends on the root cause of the problems. Understanding the theory behind it will help you solve the problem without spending unneccessary time or money.
Hope that helps.
[/not an expert]