ground loop

Suffering from mysterious hums and buzzes in your studio? This down‑to‑earth explanation will help you identify and solve the problems — or hopefully avoid them in the first place! Some of the most frequent calls for help in the Sound On Sound Forum concern unwanted hums or buzzes emanating from studio monitors, usually with a statement that “it’s something to do with grounding.” That assumption is usually right, too — it often is something to do with grounding! But there are many ways that incorrect grounding/earthing can cause problems, and different fixes are required for different issues. It’s also possible, of course, for there to be multiple different grounding issues within the same system at the same time. Figuring out the problem(s) requires some understanding of what grounding/earthing is supposed to do, as well as how different equipment is designed to be earthed (or not). But with the right approach, it’s usually straightforward to work out the most effective way of interconnecting different kinds of equipment correctly and, even more importantly, safely. That last point really cannot be overlooked — people have died trying to fix grounding problems in some really stupid ways! Solving grounding issues only requires a little basic fact‑finding about the equipment involved, a plan of what’s connected to what, some logical thinking and, it must be said, plenty of persistence and patience. It can often be a confusing, frustrating and iterative process, but I guarantee that following the concepts and techniques I explain below will eventually get you there. Most unwanted noises in an audio system are caused by some form of external electrical interference getting somewhere it shouldn’t. Sometimes that interference gets in via the mains cable, but often it’s picked up as RFI (Radio Frequency Interference), so let’s deal with that problem first. The best preventative solution for RFI is to protect sensitive electronics within a conductive box. This is called a Faraday Cage (or Faraday Screen) and derives from the work of Michael Faraday in 1836. In essence, it’s nothing more than a conductive (metal or metalised) enclosure, which naturally stops varying electromagnetic fields (like radio transmissions) from passing in or out. Audio equipment, like effects processors, mixers, interfaces and microphones, usually has a metal case that serves as a Faraday Cage, protecting the audio circuitry inside from external interference. A Faraday Cage doesn’t have to be solid metal, either; a metal mesh works too, provided the holes are smaller than the wavelength of the highest radio frequency to be excluded. Screening the electronics inside a unit is great, but the signals coming in and going out need protection from interference too. To that end most cables have a conductive outer shield of some type enveloping the signal wires, and that shield is normally connected to the equipment’s metal enclosure when the plug is inserted. In this way the cable shield extends and links the Faraday Cages of devices when they are connected together. Although a Faraday Cage doesn’t necessarily have to be earthed (grounded) — it isn’t in the case of an aeroplane, for example — most are earthed, for reasons I’ll explain shortly. The cases around outboard gear and mesh grilles surrounding mic capsules act as Faraday Cages, which protect the signal and circuitry from RF interference. Putting this into a practical context, consider a balanced microphone connected to a battery‑powered digital recorder. The metal case of the recorder forms a Faraday Cage protecting its electronics, and that Faraday Cage is extended by the shield of the mic cable to the microphone itself, where it is connected directly to the mic’s case and pop‑shield grille. External interference is therefore kept out of harm’s way as the Faraday Cage protects and envelops all of the mic’s and recorder’s circuitry and associated wiring. As the recorder is battery‑powered there is no