Mysterious hum in system...

I hope that someone might be able to advise me. I have a buzz in my system that I cannot identify. First let me say that I had to make a compromise with the wife to get my 2 channel set up. I have an ARC LS3 preamp, Bryston 4bst, Naim CDX and Revel F30’s for my 2-channel set up. I also have a home theater setup that is a Pioneer Elite VSX-41 receiver and a Pioneer Elite DVD/CD player. I also have my High Definition cable TV box connected into the receiver via a digital coax cable.

The compromise is this: I had to have this system do ‘double duty’ as both 2 channel and home theater to keep the peace at home ( I am sure many of you can sympathize with this ). To accomplish this I have connected the Elite receiver’s ‘preamp out’ connection into the Bryston’s XLR inputs by way of Audioquest Diamondback RCA interconnects and a set of Cardas RCA/XLR adapters. When using the HT I set the switch at the back of the Bryston to the “XLR’ setting, otherwise for 2 channel listening I leave it set to the RCA setting.

The problem is that I have a hum in my system when I have it set to 2 channel RCA setting. The humming disappears as soon as the XLR's are taken out of the Bryston. I have put cheater plugs on the receiver & DVD player but it didn't help, nor does the ground loop switch on the back of the Bryston (it does make it buzz less though). When the XLR is disconnected my system is dead quiet though. Any advise short of the obvious (scrap the bloody theater system!)?
It's probably a difference in ground potential between your electrical service and TV cable.
One solution that I have found to work nicely is to install a Panamax line conditioner.
At least in my my case(Panamax 5300), this completely rectified a persistent ground buzz in the system.

It also protects your components from damage due to variations in voltage and/or lightning.
This is a common problem. It usually happens when the coax is connected to the system from a cable box or satelite receiver. Disconnect the coax from one end and see if the hum goes away. If so, you need a ground break.
Try to put cheater plugs on all of the components. If that works, try to eliminate them one at a time to see which component is causing the problem. I had a similar problem with humming too. I have a bryston 4bsst and that I'm using for 2 ch. stereo as well as for home theater to drive the
front speakers and the hum was a lot louder when switched to the rca inputs. I only needed to add one cheater plug on the CD player and the hum has totally disappeared. Good luck.
First, Try what Rwwear said. Coax can be very "dirty." Personally I dont even let the coax cable get close to my audio stuff, much less route it through a piece.

Secondarily, I am unsure technically, where is Sean when you need 'em, but it could very well be that the Bryston doesnt like all its holes filled at once. You could run the Pioneer's Preouts to an input or tape loop on the ARC, assumming it has one. Even better if the ARC has a Processor loop, I dont think it does. You will have to select the input and tweak the volume on the Preamp when watching a movie. Overall it shouldnt be any harder than reaching behind the Bryston to flip the XLR/RCA switch. This way the amp has only one set of inputs and your preamp has a clean signal path.

Hope this helps.
I had this same problem. I traced it to a ground loop hum coming from my cable line. I eliminated it by using a MIT ISO-Linq ground isolation filter. Mondial's Magic Box would probably have worked just as well. I've heard that Radio Shack sells such a device, but the 2 stores I went to didn't have one. Maybe you'd have to mail order it. One way to check is to disconnect your incoming cable or sattelite feed, and see if the hum goes away. If it does, then you'll need a ground loop isolation filter.

Good luck,
There are a ton of $100 boxes out there that IMHO do not work any better than a $5 device available from Xantech. The part you want is a Xantech 634-00. Call a local custom install shop or a retailer that does some custom and ask them if they've got one, or have them order and mail you one.
I believe Distortion is right. I did the same connection before on my Theta Dreadnaught which has both RCA and XLR inputs. Flipping the input selection switch did not completely switch out one of the input and it caused hum. It sounds like the Bryston has the same design. Try what Distortion suggested and see if that solves the problem.