How to Eliminate Ground-Loop Hum in Live Event Productions

To maximize the quality of your audio when you’re producing a live event, it’s important to assess your signal carefully all the way through the chain, as I discussed in my May/June 2015 article How to Master Audio Mixing for Live Production and Streaming. Another challenge when using analog video for streaming, live audio, or even recording, is dealing with ground loop hum in your recorded or broadcast signal. It could be a low hum, or a highly audible and annoying buzz. Ground loop hum can originate in different ways, but regardless of what it sounds like or where it comes from, the bottom line is you need to kill it.

The key reason for eliminating audio hum (other than it’s annoying to listen to), is because it changes your signal-to-noise ratio. While the hum may not be particularly loud in your signal, it raises the noise floor and creates a perceived lowering of the dynamic range of your program. If your program content—typically the people who are speaking—happens against a very quiet aural background, it’s easier to understand everything the speakers are saying, including the parts where their voices are quietest. However, if the signal has some sort of hum or buzz, the speakers becomes considerably harder to hear or understand.

Most often, a ground loop hum comes from different parts of the audio chain being plugged into different places, or having different ground paths or electrical potentials. This is why they call it ground loop hum. The interference results from excess energy trying to level itself out, and doing so by passing through your audio path. I won’t specifically go any deeper into the physical and electrical principles that create this phenomenon because were more interested in solving it and getting good production audio. There are several different ways that you can eradicate the hum — both electrical, and in the audio path itself.

Electrical Approaches

If you have an audio mixer on the stage and an analog snake or long audio cables going to the back of the room where there’s a second audio board for your production or video village—or even just a camera—you’ve got the potential for audio hum. If that roughly describes your setup, try this first: If possible, plug both ends of that audio path into the same power source. You might be able to eliminate the hum that way. However, you don't want to run the AC cable right next to the audio cables because that, in itself, can cause hum in your audio.

If that doesn’t work or isn’t possible with your setup or venue, the next step to try eliminate the grounding issue is to use a ground lifter like the one shown in Figure 1 (below) to lift the ground on one or the other side of the audio signal path. To be honest, this is a quick-and-dirty solution that is not the best long-term way to solve the problem; it’s a simple Band-Aid to let you get on to other things that you need to do before your event begins. Then you can come back and revisit the grounding issue and solve it in a better way.

Figure 1. A Pro Co Sound GLX In-Line Barrel Ground Lift

Here’s another common ground-loop hum scenario cause by electrical problems: The onstage mixer is plugged into a power strip and that power strip plugged into a ground lift adapter. The problem with this is it opens up the potential for other electrical issues because now your audio gear is no longer grounded properly. This creates the potential for equipment that’s not working properly to pass an electrical signal into the audio path itself.

While you may never have done it, there are times when a singer’s lips may touch a microphone and it feels like there’s a tingle. That’s because there's extra voltage on the line going into the microphone and actually being felt by the singer as their lips touch the microphone. This won't be an issue if all of your presenters are wearing lavalier microphones on their jacket or blouse, but it’s still something you don’t want to have happen. You really do want to keep things grounded properly throughout the entire electrical system.

A better and more permanent solution is to use transformers in the electrical path to isolate the ground from the house to your equipment, but still provide a safe dispersion of extra electrical voltage (Figure 2, below). These devices were often used in analog video days when the video itself would have a visible “hum bar.” You could use an isolation transformer for an entire rack of gear and ensure that there were no grounding issues between that rack of year and something else elsewhere in the studio.

Figure 2. A Tripp-Lite isolation transformer, front (left) and back (right)

These transformers tend to be fairly big and heavy because they use large transformers of heavy copper to pass 120 V AC on through to the equipment. In comparison, you can pick up an audio transformer, which will also do a good job of isolating you electrically, while still allowing you to pass a signal. Audio transformers are considerably smaller and lighter than power transformers. I'll cover them in the next section.