R F / E M I S h i e l d i n g
Sometimes simply moving a component or a cable will affect the background noise. Because components and cables can act as receiving antennas, reorienting this antenna can sometimes eliminate or greatly reduce the background noise signals.
Also grounding can make a big difference here. Different grounding schemes for the components can be tried to determine which one works best. While you are doing this, you can also measure the voltage potential on each chassis and reverse the AC plugs to try to minimize any voltage differences. Do not do this with polarized or grounded plugs unless a qualified individual has determined that doing so will not create an electrical hazard in that particular piece of equipment.
Sometimes you may find that you need to lift an AC ground on a particular component in order to achieve lowest noise. While we cannot recommended this as the best course of action, it is a way to troubleshoot problems. Unfortunately it may not be practical to reengineer equipment or AC line grounding schemes, making the lifting of AC grounds a common practice.
You should be aware that a system with lifted grounds relies upon the interconnects between the components to maintain the safety of the AC ground. Never lift all the grounds; always leave at least one piece of equipment grounded. While we are unaware of anyone being injured from this practice, the potential exists for electric shock in the event of equipment failure.
Another approach to reducing grounding problems can be using a star grounding system, where all grounds terminate at one common point. Depending upon your system, this may be difficult to achieve.
If your system uses balanced audio interconnects, you may find that lifting the ground at one end of the interconnect (usually the source end) will reduce extraneous noise. This is know as "telescoping the grounds." If your system uses single-ended cable, this technique can't be used. We suggest using telescoped grounds only as a last resort. We recommend that you try other ways of addressing any problems first.
Because each system/location combination is unique, there is no one answer for everyone. However any problem can be solved if enough time, energy, and creativity are applied. And remember: there is no substitute for good design!