How to Eliminate Ground-Loop Hum in Live Event Productions
Every live producer knows half of good video is 90% audio, and intrusive ground loop hum can ruin even the most visually pristine webcast. Here we'll look at how to identify the source of the problem and detail two possible solutions.
Audio Solutions
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 one is giving you this hum. Sometimes it may even be more than one.
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, below). 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.
Figure 3. A Boss Audio Systems audio isolator
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.
Placement
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 may mean multiple microphones or multiple line-level sources need to be filtered before going in to the mixer (Figure 4, below). However, that may not always be 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.
Figure 4. An Ebtech line-level hum eliminator
Then the solution then 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 solo'd channels), but using these tools and practices can help you ensure that your final program has crisp and clean audio.