Canadian Musician - November/December 2018 | Page 60

LIVE SOUND Drew Robertson is an audio engineer, live sound tech, and educator. He is based out of London, ON, is the co-owner of Prosperity Audio Works, and also co-hosts The Double Platinum Podcast. He can be reached at [email protected]. By Drew Robertson Get the Hum out of Your PA Today T here are very few things that can make a front of house engineer cringe like bringing up the mains or stage monitors only to be greeted by the grating, mind-numbing buzz of AC hum. It’s bound to happen from time to time to even the best and most experienced of us because we can’t always be in control of what happens in a venue – especially one we don’t work out of on a regular basis. Re- member that, when working in any kind of live situation, there is no problem you cannot make worse with hasty decisions made in panic. Your most valuable skill is troubleshooting, so think logically about the problem and work from the ground up. Time is valuable, and you don’t have all day to stand around asking questions. Determine which speaker(s) the hum is coming from and work quickly and me- thodically until you’ve solved your issue. Work from the Wall The first instinct many of us have once we’re done cringing is to turn the vol- ume down and start unplugging things from the desk, but hold those horses and instead take a look at where your stage gear is plugged in. More often than not, AC hum is caused by a ground loop, which occurs when one or more pieces of gear are connected to each other but plugged into different AC outlets on the same circuit, thus creating a loop. The solution here is rather simple: get your gear plugged into the same outlet. You’re not going to trip the circuit as you were already running off the same 60 • C A N A D I A N M U S I C I A N one to begin with; you were just using different break-in points. You can also invest in a power conditioner for those instances when the venue just has “dirty” power. These handy power strips come in a variety of formats, from tiny wall plugs and stan- dard-looking power bars to rackmount- ed affairs with built-in lights and digital power management and will act as a filter for any 60 Hz AC hum caused by a venue’s less-than-stellar wiring. The Problems with Parallel Let’s say you’ve got all your gear plugged into one outlet and everything is run- ning off of power-conditioned strips but you’re still hearing buzz from your mains or monitors. Knowing that, it’s time to take a look at what’s happening on stage. Start by making sure that none of your audio cables are running in parallel with any cable that transmits power. I really cannot stress this enough. Any cable that carries electricity will emit electrical noise in a circular field around its cable and transfer some of that noise into any adjacent cables. Now because audio cables can and do carry an electric signal, they will pick up that noise, which gets amplified when it reaches your audio desk’s preamp. It’s one of the reasons we use balanced cables whenever possible. The addition of a ground pin helps keep electrical noise to a minimum, but all the grounds and shields in the world won’t help if you are running both ca- bles parallel to each other. So, make sure that any power and audio cables cross each other at a right angle to minimize contact and reduce the chance you’re getting noise that way. Troubleshoot the Troublemakers on Stage If you’re still getting hum, it’s a good bet one of your instruments is the cause. The simplest thing to do here is ask the guitarist and bassist to unplug their ca- bles and see what happens to the buzz in the speakers. If it stops or changes for either of them then you’ve found your culprit. Sometimes it’s as simple as the cable not being fully seated in the jack, or simply a bad cable. If there are pedal boards involved, check them as well for the same reasons. A bad power supply or connecting cable can really ruin your day. If after all that you still have trou- ble, it could be a channel on the mixing desk itself, so have a back-up whenever possible. The Takeaway Buzz and hum in your PA system can commonly be generated from a hand- ful of sources like equipment, cables, and instruments. Don’t panic and think logically. If you’ve got a good set-up routine that involves making sure the gear is plugged in appropriately and cables are run smartly, you can eliminate half of your troubleshooting right out of the gate and focus on the instruments. Work smart, work fast, and above all else, work with confi- dence and you’ll be back in business in no time.