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.