Adventures In Audio

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

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.

Featured image by Soundcontrol, Hamburg

Saturday November 3, 2012

Like, follow, and comment on this article at Facebook, Twitter, Reddit, Instagram or the social network of your choice.

David Mellor

David Mellor

David Mellor is CEO and Course Director of Audio Masterclass. David has designed courses in audio education and training since 1986 and is the publisher and principal writer of Adventures In Audio.

Learn Pro Tools with our amazing range of video courses

Pro Tools video course catalog

Browse Pro Tools courses...

Learn Logic Pro with our amazing range of video courses

Logic Pro video course catalog

Browse Logic Pro courses...

Learn Cubase with our amazing range of video courses

Cubase video course catalog

Browse Cubase courses...

Audio Masterclass gives you all the technical knowledge and skills to bring your musical dreams to life

The Audio Masterclass Music Production and Sound Engineering Course

Get the most from your studio with the Audio Masterclass Music Production and Sound Engineering Course.

Learn more...

More from Adventures In Audio...

Hi-Fi comfort OVER your ears? TRUEFREE O1 detailed review

Get the tube sound in your system with the Fosi Audio P3

Any studio you like, any listening room you like - For producers and audiophiles

Hidden Hi-Fi - The equipment you never knew you *didn't* need - Fosi Audio N3

Adding tubes to a jazz mix with Freqport Freqtube

Adding tubes to a rock master with Freqport Freqtube

Adding tubes to female vocals with Freqport Freqtube

Adding tubes to male vocals with Freqport Freqtube

Adding tubes to real drums with Freqport Freqtube

Adding tubes to a bass guitar with Freqport Freqtube

Adding tubes to speech with Freqport Freqtube

Adding tubes to an acoustic guitar with Freqport Freqtube

Parabolic reflector microphone - Sound On Sound latest issue

Your power amp is average - Here's why

MANCAVE REVIEW: In-ear monitors - Better than earbuds?

Can this tiny amp really produce 600 watts? - Fosi Audio V3

MANCAVE - Recreating Olivia Rodrigo's 'Vampire' vocal

Why does this song sound so bad?

Audiophiles - You're all wrong!

MANCAVE RE-REVIEW: OpenRock Pro earbuds in language Audiophiles can understand

MANCAVE REVIEW: OpenRock Open-Ear Air Conduction Sport Earbuds

Can lossy digital audio be better than lossless?

Man-Cave: Microphone mysteries revealed

How I improved my audio - From the Mancave