A ground loop is a problem in audio equipment where a buzz noise is added to the audio signal because of how your equipment is grounded. It's generally caused by equipment that doesn't share the exact same ground connection and at the same time are passing an audio signal to each other. I'm no expert in ground looping, nor am I an electronics engineer, but today I hope to share what I know about ground loops.
Explaining Ground Looping and what causes it:
You may think that the ground connection in your building is 0 but it's not; there can be 1 or 2 volts difference from wall socket to wall socket. The difference is caused by resistance varying on cables/connectors/wall plates from your fuse box.
This difference effects amps in audio equipment as they often use the ground connection as a reference in order to amplify sound. Any changes in the ground resistance will add noise to a dry audio signal. This difference can cause a buzz throughout the entirety of the sound spectrum which makes the buzz impossible to remove with any sort of equalizer effect.
The buzz will most likely happen when more than one item of equipment is connected via audio cable to one another and there is a difference in the ground connection. However adding a projector and computer connected to each other by a video cable (VGA/DVI/HDMI/RGB) and sharing the same ground with audio system may also give you a ground loop buzz.
Figure 1. Example of situations where ground loops may occur
What does a ground loop sound like being amplified out of speakers? It's a buzz that is louder than a general speaker buzz but not as random as a broken cable sound.
You can generally diagnose a Ground Loop by:
1. Going through all the channels on your mixer and pressing the mute button until the buzz stops. This will help in finding the channel that has the ground loop.
2. Then change the cabling between the input source and the mixer with cables that you know works.
3. When you un-mute the channel and the buzz is still there than you most likely have a ground loop.
Solutions to stop or minimise ground loops
Before we look at solutions we first need to understand balanced audio versus un-balanced audio. We will also look at the difference between stereo and balanced as there is a difference between the two.
In a balanced audio system's I/O and cable have 3 connections that need to be made and are used for different things:
Ground (not required)
Audio Signal positive
Audio Signal inverted
Figure 2. Balanced connectors
Basically because there is an inverted signal it is possible to remove noise picked up over the length of the cable. For example positive 5 signal noise plus negative 5 signal noise equals 0 signal or no noise.
Generally all balanced audio is mono and is used for long cable runs.
Un-balanced audio system's I/O and cable has only two connections that need to be made and are used for two different things:
Ground (required)
Audio Signal positive
Figure 3. Un-Balanced connector
Because there is no system in place to remove added noise, un-balanced cables pick up extra noise and can cause ground loops. For these reasons un-balanced cables should only be used on small cable runs and I'd recommend using them as little as you possibly can.
Stereo is not the same thing as balanced. Yes there is the same amount of connections required, but they are used for completely different things. To cut the explanation short most stereo connections are two un-balanced Audio Signals that share the same ground connection:
Ground (required)
Audio Signal positive (left channel)
Audio Signal positive (right channel)
Figure 4. Stereo Connector
Devices like computers, CD players, radios and projectors; that have sound capabilities, all have stereo connections that are unbalanced.
Solutions:
The first solution is to get rid of the ground connection; from the power that audio equipment has. Hang on, don't go away and do that because it's extremely dangerous. Removing the ground connection from the power will mean that if your equipment shorts or surges it will ground the excess electricity through the metal casing of your equipment and you could become electrocuted and killed.
So there is a better way and that is to remove the ground connection in the audio cable.
With unbalanced cable I have had success with Audio Ground Loop isolators. However I have not been able find enthusiast grade or live event's audio grade Ground Loop Isolators but only ones that are used in car audio systems.
These car audio system Ground Loop Isolators often don't have quarter inch connectors and as such need adapters or require connector modifications to become compatible. I have heard about Line Level Ground Decouple Boxes that are used for live audio and I imagine they work in a similar way as a ground loop isolator but I can't find them in the audio stores in my area.
From what I have seen they work on both balanced and un-balanced connections but your experience may vary from mine.
Rather than running un-balanced audio; which is not a good practice, it is possible to convert an unbalanced signal to a balanced signal with a DI Box (Direct Interface). This means that we can now remove the ground connection in the audio connection because balanced audio does not require a ground connection.
A lot of DI Boxes also have a ground cut switch so there is no need to attach an extra ground pin removal adapter or modify a balanced cable to remove the ground connections.
Zach Radloff lives on the Gold Coast and is studying IT and Multimedia at university.
Zach Radloff's previous articles may be viewed at www.pressserviceinternational.org/zach-radloff.html