Ground loop audio noise is something that every audio engineer experiences at some point in his working life. A combination of cabling, equipment interaction, and environmental conditions (temp, humidity etc) can cause ground loops to occur in one scenario but not another.
Ground loops can happen when multiple devices are connected to a common ground via different paths. This is more problematic when unbalanced audio feeds are used; an unbalanced feed uses ground for the audio signal—in contrast to balanced connections, which 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.
Below are a few things you can try to 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.
- 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.
- Use a Direct Injection (DI) box.