How to eliminate ground loop hum in live event productions.
The key reason for eliminating audio hum (other than it's annoying to listen to) is that it changes your signal-to-noise ratio. While the hum might 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 become 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--which is why it's called 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 we're 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.
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 setup sounds familiar, 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, try to eliminate the grounding issue by using a ground lifter, such as the one shown in Figure 1, to lift the ground on one side of the audio signal path. To be honest, this quick and dirty solution isn't the best long-term answer; 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.
Another common ground loop hum scenario is caused by electrical problems: The onstage mixer is plugged into a power strip and that power strip plugged into a ground lift adapter, which means your audio gear is no longer grounded properly. Equipment that isn't working properly could then pass an electrical signal into the audio path itself.
While you might 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, which the singer can feel. This won't be an issue if all of your presenters are wearing lavalier microphones, 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). These devices were often used in the days of analog video 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 gear and something else elsewhere in the studio.
These transformers tend to be fairly big and heavy because they use large transformers of heavy copper to pass 120-volt 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.
The first test you should perform if you're having a ground loop hum in your audio feed is to pull out all of your sources. If you still have hum, the noise is coming in through the power. If the hum vanishes when the audio sources are pulled, then add them back, one by one, to see which source--or combination of sources--is generating the hum.
One solution is to lift the ground pin from a balanced microphone feed so that the grounding lead of the signal does not make it to the receiving mixing board. This would also be a quick fix or a test to see if it is coming in along that microphone cable. There are commercial inline XLR adapters that do this.
A better solution is to use isolating transformers for audio that can readily be found at any pro audio or video store (Figure 3). These work on the same principle as the power transformers I mentioned earlier, but because the voltages here are so much lower, the transformers can be integrated into some very small enclosures--small enough that you can always keep them in your audio kit so they're available when trouble crops up.
Typically, audio transformers take an unbalanced signal, but you can also find higher-end boxes designed to take XLR or line-level 1/4" inputs and pass the signal on to the rest of your equipment properly.
The best placement of these isolation transformers is before the mixer. This way, while listening to and adjusting each of your sources, you'll be hearing clean program audio. This might mean multiple microphones or multiple line-level sources need to be filtered before going into the mixer (Figure 4). However, that isn't always possible--especially if you're dealing with a lot of wired audio and a diversity of other inputs. Or maybe there's just noise in the house AC that isn't part of your chain, but a problem with their transformer that simply will not be fixed in time.
Then the solution is to isolate the output of your mixer that feeds your recorder(s) and streaming hardware. You'll have to deal with hearing the noise while mixing the show (if you have headphones in the mixer so you can hear soloed channels), but using these tools and practices can help you ensure that your final program has crisp and clean audio.
If you can use wireless mics, that can often help avoid mic-grounding, as can operating your gear off a battery. But often, with a bigger show that's going to be going all day, for several days, you'll need to use wired receivers and multiple mixers, and that's when the grounding issues start to appear.
Sometimes, if you have one piece of hardware that always buzzes when you attach it to the mixer, such as a consumer video player, you can try running a "ground line" between that piece of equipment and the mixer--usually by finding an unpainted screw in the metal enclosure of both items and connecting a thin copper wire (such as speaker wire) between them. This gives the excess energy a path to travel that is not your audio path, similar to a grounding strap for working on computers.
I've had this happen with a laptop--I connected the headphone out to the rest of my gear and got a buzz regardless of whether the laptop was running off its own battery or plugged in. But when I connected a wire between the screw hole for the VGA video out and my audio mixer, suddenly the buzz was gone. It doesn't have to be thick wire--it just needs to give the electricity a different path around your audio chain.
I hope these techniques and solutions will help you conquer the vast majority of grounding issues you'll encounter. It's important to remember that the ground loop hum you're experiencing could arise from any number of individual possibilities, at different stages in the audio path. Don't limit yourself to bringing only a ground lift and assume you'll have it licked. Sometimes a new development can trip you up where you least expect it.
I recently did audio for a two-DSLR shoot. I used a battery-powered mic, a battery-powered mixer, and a battery-powered audio recorder, and the audio sounded good. Then the producer decided that he wanted me to feed the audio out of the recorder into the battery-powered Canon 5D DSLR. No problem, I thought. All seemed well and good, but in post he reported that there was a little buzz on both channels on the DSLR. Thankfully, the audio in to the audio recorder was clean.
Nevertheless, I'm now buying and adding another audio isolation transformer to my gear bag to isolate my audio recorder from whatever it feeds after me (Figure 5).
There's always something new to learn and adapt to. Good luck!
Anthony Burokas (firstname.lastname@example.org) has provided corporate communication services and consulting through IEBA Communications for 20-plus years. His award-winning video has been seen on PBS for more than a decade, he helped build in-mall advertising, and he is currently transitioning to 4K. Comments? Email us at email@example.com, or check the masthead for other ways to contact us.
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|Date:||Apr 1, 2016|
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