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Ground lift - What is it? What do you do with it? What do you do if you haven't got it?

There's a monstrous hum in your system. And a little switch might be able to cure it, but where do you find this magical ground lift switch?

'Ground lift'. It sounds like some kind of excavating process. Or perhaps it protects against sea level changes caused by global warming.

But in sound engineering, ground lift means breaking a connection to electrical earth. That's odd, because earthing or grounding (same thing) electrical equipment is an important safety measure.

The reason why equipment is earthed is so that if a fault develops and the casing becomes live, a high current will instantly flow to earth and blow the fuse, thus protecting the user.

Where equipment requires an earth, it is vital to life not to disconnect it.

Some equipment is so-called 'double insulated' and is allowed not to have an earth connection.

But if you have two pieces of equipment in a sound system connected to earth through their respective mains cables, it can cause an 'earth loop', sometimes called a 'ground loop' or 'hum loop'. This in turn causes a 50 Hz or 60 Hz hum (depending on your locality) that is sometimes irritating, sometimes deafening.

So, for instance, you connect a keyboard instrument that is earthed to a mixing console that is earthed. You have a prime earth loop scenario going on and, yes indeed, there is a hum. What do you do?

The answer is to interpose a DI (direct inject) box between the keyboard and the mixing console.

But it still hums.

Well of course it does. Both pieces of equipment are still connected to mains earth. But if you look carefully at the DI box you will see a switch labeled 'ground lift' or 'earth lift'.

Flick that switch and the hum will disappear completely. It breaks the earth connection between the keyboard and the mixing console, but both pieces of equipment are still independently earthed, so they are safe to use.

In practice, engineers don't spend time thinking about this. They use a DI box automatically and flick the switch when they hear a hum. They don't even think which way they are flicking the switch - they just know that it works.

One interesting point is that some guitar amplifiers have built-in DI's without ground lift switches. The reason for this is that an electric guitar or bass guitar would not be connected to mains earth, so there is no need.

It is also possible to lift the earth without a DI box. To do this, you need to disconnect the earth at one end of the cable connecting the two items of earthed equipment. Clearly this isn't as convenient as flicking a switch.

In summary, hum is always around and waiting to become a problem for you. But usually the solution is as simple as to use a DI box and flick the ground lift switch.

By David Mellor Saturday November 3, 2012