A phenomenon that occurs when an audio (or video) system has multiple paths and path lengths to ground. When a ground loop occurs in an audio system it will manifest itself as a hum. The hum occurs at 60 Hz (Hertz) or some multiple of 60 Hz because that is the line frequency of AC (in the US). In video a ground loop can often shows up as rolling bars (sometimes quite faint) in the picture. The severity of the hum will vary depending upon numerous factors; sometimes it can be quite faint, while other times it can be so loud that it overloads an amplifier. Ground loops have been a problem since the very early days of connecting audio equipment together and there have been volumes of materials written on how to prevent and solve them. Any time one connects multiple pieces of audio gear together that are each plugged into an electrical outlet and connected to each other via signal cables, and possibly even electrically connected by their cases touching or being in the same rack, the risk of ground loops increases. There are many valid schools of thought on stopping them, some of which we will get in to in the coming months. At the most basic level, however, the only way to prevent or get rid of ground loops is to make sure that all of your equipment has only one path to ground. One way people sometimes accomplish this is to utilize AC plug ground lift adapters (a.k.a. widow-makers) on offending pieces of equipment as determined by troubleshooting the system, but this is considered unsafe and we highly recommend against it.