Ground loops

Ground loop or problems and how to get rid of them

Why grounding is so important ?

Improper grounding or earthing can create a lethal hazard. Correct grounding is essential for correct operation and safety of electrical equipment. Most countries have strict regulations about how electrical installations are earthed for safety. Earthing can solve many problems, but it can also cause new ones. One of the most common problems is called the "ground loop" or "earth loop".

Why a ground loop is a problem ?

Ground loops are a mystery to many people. Ground loops are the most common cause of AC line frequency hum in sound systems. A ground loop in the power or video signal occurs when some components in the same system are receiving their power from a different power source from other components. This may cause your system to have different ground potentials. This becomes a problem because amplifiers and mixers need an electrical earth connection to their chassis to provide screening against interference. Audio input leads are screened against interference and the screen inevitably connects to the same earthing point in the amplifier / mixer. Audio signal (from a guitar for example) is a variable voltage relative to earth. When the earth potentials of two or more pieces of equipment are different, the relative signal is less stable, screening is compromised and interference manifests itself as hum.

Ground loop is a common problem when connecting multiple audio equipment together. Considering the quantity of equipment used on a typical stage it is surprising that ground loops are not more common. Ground loops commonly cause a humming noise to audio signals and interference bars to picture. Ground loop makes the system sensitive to pick up interference from mains wiring which can lead to erratic operation of the equipment or even damage to the equipment. Audio-frequency ground loop problems are typically in the low millivolt range, but there does not have to be much interference in a grounding system to cause problems in your audio system because of the hugh amount of amplification applied to microphone and instrument sources.

Remember that there is no absolute ground. There is a certain amount of resistance to electrical current between all grounding points. This resistance can change with humidity, temperature, connected equipment and many other variables. No matter how small, the resistance can always allow an electrical voltage to exist across it. The ground wires between wall sockets and power company transformers are not perfect conductors and neither is the shield of your signal cable. If they were, ground loops would not be a problem. Certain types of cable screening can give better results because they cover the inner signal cores completely eg foil screening in multicores and braid screening on good quality XLR microphone leads.

Home audio and video systems

• Ideally, use a mains distribution board to connect all your audio equipment to a single wall power socket.
• Keep signal cables as short as possible between sources and amplifier.
• Keep signal cable routes away from mains power cables wherever possible.

Professional audio systems

• Use a professional mains distribution system to connect all your audio equipment to a single power source of an appropriate power rating for all your equipment. This gives the best chance of avoiding ground loops as the professional mains system is wired in a "star" format with all earths leading back to the same point.
• For small systems which will all run on a ring main, make sure that all equipment is plugged into the same stage circuit.
• Front of house (FOH) control equipment must be powered from the same stage power circuit. Normally this is achieved by running a long mains extension alongside the multicore.
• Never remove earth wires from plug tops or mains distribution systems.
• Use earth lift switches on DI boxes to isolate equipment which is causing loop problems.
• Make up a bunch of dedicated short XLR link cables with the earth (screen) connected at the female end but not at the male end. These can then be inserted onto any signal lead which is humming, thereby avoiding having to permanently damage your XLR leads by cutting off the screens. Make sure these link leads are clearly marked up to avoid any mix ups and do not use them on phantom powered microphones.
• All long microphone and instrument leads should be balanced to give maximum defence against induced pick up of interference.