From Lawrence L. comes the following question:
"What is a ground loop, and how do you get rid of it?"
A simple question Lawrence – unfortunately, there is not a simple answer!! Many, many of you have written with similar questions, specific grounding problems, and asking for advice on grounding gear. We’ll be addressing these topics in future TTOTD’s, but for now:
A ground loop is a problem that exists when a device sees more than one path to ground (or "earth"). It manifests itself as a 60 cycle hum in the audio system. One way this can happen: The metal chassis of a piece of gear is normally grounded to the device’s AC plug. There is also a ground connection at the shields of the device’s various audio connections. If a shield ground finds another path to ground (through another connected piece of gear’s chassis, for example), a hum can result.
How to get rid of it – well, that’s the trick! The short answer is: "Break the loop!" Again, we’ll be offering more in future Tech Tips, but a few things you might try to accomplish this:
- If the gear is rackmounted, make sure that the loop is not being formed by the interaction of metal gear chassis and metal rack rails. Several companies market non-conductive rackscrews and spacers which address this problem.
- Stacking or setting gear so that the metal casing of one device contacts the metal case of another can sometimes cause loops. Try slipping something non-conductive between these items to separate them.
- Plug all of your gear into a single AC circuit.
- Telescope (or float) the shields on your cabling. (More on this in a future TTOTD)
- If you can’t find the source of the loop, take your system apart, and put it together again piece by piece, checking for ground loops after installing or connecting each item. Time consuming and painful, but eventually you will find which item or connection is the source of the problem.
- If you have a patchbay, try wiring your system without it. A patchbay brings together a lot of different grounds, and can increase the possibility of problems.
Other points:
- DON’T lift the ground on gear with those little 3-to-2 prong AC adapters (or widowmakers as our Service Department refers to them). This is very unsafe, and there should be other solutions. (TTOTD 2/24)
- Be sure the hum you hear is actually a ground loop, and not induced noise being picked up from other sources like nearby transformers, AC cables, etc.
