Virtually every audio engineer experiences ground loop noise at some point in their working career. It can be caused by many things and happens at the least expected times. Here’s a guide to what causes ground loops and how to get rid of them.
A ground loop occurs when there is more than one ground connection path between two pieces of equipment. The duplicate ground paths form the equivalent of a loop antenna, which very efficiently picks up interference.
As a consequence of ground-loop induced voltages, the ground reference in the system is no longer stable and signals ride on the interference. This interference becomes part of the program signal.
Ground loops are a major cause of noise, hum and other interference in audio and video systems. Such loops can be caused by a combination of cabling, equipment interaction or environmental conditions such as temperature or humidity.
Wiring practices that protect against ground loops include ensuring that all vulnerable signal circuits are referenced to one point as ground. Of course, removal of safety ground connections to equipment in an effort to eliminate ground loops also eliminates the safety protection it is intended to provide.
Ground loops are more problematic when using unbalanced audio feeds since an unbalanced feed uses the ground for the audio signal. Balanced connections, on the other hand, do not.
When a ground loop occurs, the cable’s ground conductor (often the shield) ends up carrying both the audio ground and hum/noise caused by power flowing through the ground connection. Ground loops can be improved by using balanced connections, or, if that is not possible, by isolating the signal/power grounds between the devices. Stay with balanced connections when you can.
Below are a few things that can reduce or eliminate ground loops. There is no definitive set of rules which can resolve every ground loop situation, but trial and error often leads to a solution.
First, use short, high quality power and audio cables to ensure low impedance connections. Where possible, use balanced cables and audio devices with transformer-balanced inputs and outputs. Electrically isolate the metal casing of one device from another (to eliminate the loop). Use inline ground loop isolators to connect audio outputs to inputs or use a Direct Injection (DI) box.
The ultimate solution to ground loop noise is to break the ground loop, or otherwise prevent the current from flowing. Create a break in the signal cable shield conductor. The break should be at the load end. This is often called “ground lifting.”
This is the simplest solution, since it leaves the ground currents to flow through the other arm of the loop. Some modern sound system components have “ground lifting switches” at inputs, which disconnect the ground.
One problem with this solution is if the other ground path to the component is removed, it will leave the component ungrounded, or “floating.” Stray leakage currents will cause a very loud hum in the output, possibly damaging speakers.
In this case, put a small resistor of about 10 Ω in the cable shield conductor, at the load end. This is large enough to reduce magnetic field induced currents, but small enough to keep the component grounded if the other ground path is removed. It can prevent loud hum.
Another possible solution is to try a ground loop isolation transformer in the cable. This is considered the best solution, as it breaks the DC connection between components while passing the differential signal on the line. Even if one or both components are ungrounded (floating), no noise will be introduced. The better isolation transformers have grounded shields between the two sets of windings.
Radial ISO Isolator
A technique used in recording studios is to interconnect all the metal chassis with heavy conductors like a copper strip, then connect to the building ground wire system at one point. This is referred to as “single-point grounding.”
Remember, 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 other variables. No matter how small, the resistance can always allow an electrical voltage to exist across it when there is any current flowing between those grounding points (and most of the time there is some current flowing).
Audio-frequency ground loop problems are typically in the low millivolt range, so it does not have to be much interference in grounding system to cause problems in audio systems. The ground wires between wall sockets and power company transformers are not perfect conductors. If they were, ground loops would not be a problem.
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