Did you get a loud hum coming through the PA system at your last show? Or is there a persistent ‘zzzz’ sound in your rehearsal room, especially when plugging in certain bits of equipment? If so, you are experiencing ‘earth loop’ or ‘ground loop’ hum. All musicians recognise this sound (especially guitarists) and the way most people remedy the problem is dangerous.
‘Grounding’ is the practice of ensuring the electrical current has a safe path. A safe path to ground for electricity is away from your body and confined within whatever piece of electrical equipment you’re using. A ground loop is common in any audio system containing balanced and unbalanced cables, or connecting grounded equipment – two guitar amps joined by an A/B box, for instance. In unbalanced audio cables (guitar cables, for instance), there is a single insulated core surrounded by a screen. Grounding or earthing the screen prevents other electrical interference. However, in a system containing mains-powered equipment joined by cables, the signal screens and mains ground wires interconnect. The two contacts of each cable create a difference in voltage as it flows through these screens , and signal contamination takes place—an audible low-frequency hum and/or a high-end buzz or “rizz.”
The simplest way to reduce ground loop hum is to remove the mains earth/ground from one piece of connected equipment. This is also the most dangerous remedy because your equipment is now not grounded. Ungrounded faulty equipment, or equipment introduced to liquids, may cause you to suffer a fatal electrical shock. Electricity naturally flows to the ground through anything that will conduct electrical current including the human body, sometimes with fatal results. If your guitar amp, mixing console, or keyboard is faulty or has a shorted wire, for example, the electrical current may try to find another path to ground—and that path may be you. A better way to break the ground loop involves disconnecting everything and building up your system one piece at a time, checking for hum at every stage.
If, after connecting a particular piece of equipment, something hums, then try:
- Checking to see if the last equipment added is plugged into the same power strip/wall socket as the other equipment. Plug it into the same socket if it was not at first. This may involve buying a new mains plug board – one with enough outlets for all the connected equipment.
- Taking rack-mounted equipment out of its rack and/or place it in a different rack. Equipment with large transformers (power amps for instance) can emit a strong hum field, which will be picked up by neighbouring gear in a rack. Don’t place anything electrical on top of mixer power supplies for the same reason.
- Avoiding running mains and signal cable side by side.
- Checking to see whether equipment has a ground-lift switch and use it.
- If all else fails, buy ground-lift connectors or isolating transformers. You can use your soldering skills to build your own ground lift cables.
Health and safety
So, don’t remove the mains earth from the plug to get rid of ground loop hum. And while you are at it, perhaps invest in a simple mains power tester (search ‘ground tester’) and a Residual Circuit Breaker (RCB) for when you perform live.The ground testers should check the state of the mains power before you plug your gear in at sound check, and an RCB will protect you if there is a grounding incident caused by introducing liquids to your equipment.