I think I have what might be some sort of interference problem that has been driving me crazy. It has actually been a problem for quite some time, but only recently since troubleshooting the issue has the problem become severe enough to merit putting everything on hold until the issue is figured out. The problem was a high-pitched buzz (not low enough to be a ground loop) I kept getting from a Yamaha S90ES keyboard. I finally realized I was only getting noise when I had the USB cable from my computer plugged in to the S90ES; unplug the USB cable and the buzz was gone. I use the USB functionality for sequencing, so it is a must to get this figured out. The interesting thing was that even after disconnecting power from the S90ES and the computer, the buzz continues as long as the USB cable was plugged in to both devices. I tried several different USB cables with no resolution. I suspected a Furman power supply in the rack that holds the computer, as it has given me trouble with other devices in that rack in the past, so I started going through the rack putting isolating washers on every device. I also tried a ground lift plug on every device in the rack, as well as the Furman so that the entire rack had a ground lift on it, but the problem continued. At the same time, I pulled the computer out of the rack (which is in an Antec rack mount case with an Antec power supply) to check and make sure the motherboard was properly grounded to the chassis. While I had the computer out of the rack, I attempted plugging it in to another wall outlet to determine if the problem was with something in the rack, connected up the S90ES via the USB cable, and this time I received an obviously loud ground loop that I was able to resolve with a ground lift. This started me wondering if something is up with the computer and/or power supply. I put the computer back in the rack, connected up the power and USB cable to the S90ES, and the same high-pitched hum was there. I backed the computer out of the rack just enough that the rack ears didn’t touch the rack rails, and voila, hum gone. I continue hooking up everything to the computer, and as soon as I hook the two monitors (LCD) up to the computer, an awful high-pitched squeal came out of the S90ES. The squeal goes up and down as the monitors turn on and off, so it’s quite obvious THOSE are causing the interference now. Again, remove the USB cable that connects the computer and the S90ES or remove the monitors' DVI cable from the computer, and the noise goes away completely. I decided to try my Access Virus TI which also connects to the computer via USB, but had the exact same problem. I switched over to using balanced cables on the Virus TI, since it has balanced outputs, and the issue was resolved completely. I tried the same with the S90ES, only to find out it does not have balanced outputs. I’m really pulling my hair out on this one. Is it possible there is a grounding or some sort of interference issue in the computer, and how does one go about resolving such a problem? Secondly, how do you deal with a device that does not have balanced outputs (i.e. a keyboard like the S90ES) that must use balanced connections due to interference issues such as this?