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The War on Hum
How to Maintain a Hum Free Audio System
By Tweak
The
"war on hum" is a major battle every home
studio has to wage. I have been battling it since I started recording,
sometimes winning, sometimes losing. Currently, I am winning the war. The
more gear you have, the more likely you are to encounter hum. Like it or
not, it's a war you have to fight. Fortunately, observing a few
principles can fix many situations.
This is deliberately a non technical article. You probably don't want a
lecture on the nature of ground loops and electrical systems. So I won't
get too deep into it, but I will link you to some excellent sources of material
at the end.
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Furman
AR15 Series II 15-Amp Voltage Regulator
The 15 amp AR-15 II delivers a stable 120 volts of AC
power to protect equipment from problems caused by AC line voltage
irregularities such as sags, brownouts, or overvoltages -- all of which
can cause sensitive electronic equipment to malfunction or sustain
damage. AR-15 II accepts any input voltage from 97V to 137V and
transforms it to a constant 120V, +/-5V. Voltages beyond that range may
also be converted to usable levels, depending on the range variance.
Behringer DI4000 Ultra DI Pro 4-Channel Direct Box
MXR
M135 Smart Gate Pedal
If you're addicted to the indispensable juice of a high
gain amp or a string of stomp boxes, you need this pedal. Because along
with your hot-wired tones, you're probably getting a generous helping of
noise. Equipped with 3 selectable types of noise reduction, Hiss, Mid,
and Full, the Smart Gate bites down on sizzle and hum but lets the
smallest detail of your playing through.
Mogami Gold Studio Microphone Cable
Virtually every major recording facility in the world is
wired with Mogami cable. This means that just about any music you choose
has been recorded using Mogami in the audio chain. This cable is famous
for unmatched accuracy, extremely low noise and remarkable flexibility.
Top engineers rave about its amazing clarity and silent background. If
accuracy in reproduction is your goal, this is the cable for you.
CBI
BLUA Ultimate Series 1/4 in. TRS-TRS Cable
Belden/CBI MLU "Ultimate" 20-gauge cable with braided
shield. Neutrik Nickel 1/4 in. connectors; TRS - TRS male.
Neutrik NYSSPPL 48-Point TRS Balanced Patchbay
New, economic, and versatile describes the 1/4 in.
modular Patch Panel. 48 balanced channels in one rack space and just 1U
high.
CBI
8-Channel TRS to TRS Snake
CBI cables give you quality connections without hissing
or cut-outs. Without the high price of other big named cable brands, CBI
cables will provide you with clean, crisp sound. And with a variety of
colors and connections, you can pick up the perfect cable for your
individual needs and looks.
Ebtech HUMX Voltage Hum Filter
Behringer HD400 2-Channel Hum Destroyer
Ebtech HE2PKG Hum Eliminator (2-Channel)
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Basically, the hum you hear is typically a bass tone at 60Hz (or 50Hz, if you
are across the pond), along with its
harmonics at higher frequencies, which may sound like a buzz. Because this
hum and buzz creates noise throughout the audio spectrum, its almost impossible
to filter it out without totally wrecking the audio signal. Causes can be
many: ground loops, often caused by different electrical pathways to the house ground, TV cable
lines, bad or shorted audio cables, old equipment with damaged power supplies,
equipment with poor or broken internal grounds, and cables that travel near magnetic
fields. There are plenty of other sources of
noise too--electric motors, radio stations, even your neighbor using power
tools. In some cases, the electricity supplied by the power company may be
erratic.
1. Make sure your audio gear and all devices that connect to audio gear are on the
same house circuit,
observing the specified limits of the circuit. That
is your audio interface, monitors, mixer, and gear connected to the mixer.
Trouble shoot your house circuit breaker so you know which switches go to which
outlets in your house and most importantly, your studio. You want all the
gear to use the same path to ground.
| Before you touch your home's
electrical equipment, Beware! Messing with electricity
can be dangerous, even fatal. If you don't know what you are
doing, don't do it. |
While this is not a magic formula to
cure all ground loops, it can get rid of many preventable ones.
2. Use balanced gear with balanced TRS and XLR cables. If you have to use
unbalanced gear, keep the cables short, under 10 feet if possible. Long RCA and
TS (two wire)
cables are highly susceptible to picking up hum. For those of you using
mixers, this is really important. One poorly grounded device or poorly
situated unbalanced cable can infect the whole mixer with hum. Those with
a lot of vintage synths (which are nearly always unbalanced) will certainly run
into this problem. I've had real good luck with
Behringer direct boxes
plugged into and powered by the board's mic preamps. You can input TS line
level and output a hefty XLR balanced signal. Touch it up with the gain
and you have a clean sounding vintage synth. You can't just use TRS cables and
expect your unbalanced gear to be balanced. It does not work that way.
You can use a
line level shifter to do the job though.
3. Keep audio cables away from wall warts, those power supply adapters that so
many pieces of studio gear use. A cable resting on a wall-wart on the floor can
pick up hum. Also don't let the AC cables run parallel to audio cables. If they
cross, do it at 90 degree angles. This happens because of magnetic fields
that form around the the power cables and adapter. Electrons don't always
stay inside the cable jacket. Don't look now, but are they jumping
all over the place in that mess under your desk? Back in my 8 bus mixer
days, whenever the hum started to rear its ugly head I would go under the desk
and fix the cable paths. Result: Less Hum. It can help significantly.
If you do all the above and you still have hum, it could be that a particular
unit is causing the problem. This happens a lot with old gear, whose power
supplies may be weakened from years of use. Troubleshoot by disconnecting
everything and plugging in items one by one until the culprit reveals itself.
Note that for some gear, ground loops can persist even when equipment is not turned on.
This is important to know when troubleshooting. You may have to disconnect
the power cable from the wall as well as removing audio cables to ensure
that a piece is not causing trouble. That piece may need separate treatment if you need to continue using it. I've
had good luck with the
Ebtech hum eliminator and with direct boxes that have
ground lifts.
Hard Questions and Answers
Q) When I pan my synth to the left, its clean. When I
pan to the right I get HUM. What is wrong with my mixer?
A) Nearly always that is a cable problem. Swap them to
see if the problem is reversed. If it is that confirms it is the cable or
the gear connected to the mixer, not the mixer. You then might try
connecting the gear with different cables. If the problem continues, it is
likely that the problem is in the gear itself.
Q) Here's a really strange problem. When I raise the
fader on my mixer, hum disappears! When I lower it, it comes back. WT heck
is going on?
A) That's a tough one. You have to go into
advanced troubleshooting mode. It's quite possible that the problem is on a
different channel than the one you are boosting. Start disconnecting audio
cables on other channels. When the problem stops, you have found the
villain. Now peek under the desk to see if any cables are touching wall warts.
Also check for an impedance mismatch where a +4 output is going into a -10
input.
Q) I hear "digital hash" in my audio. It's not hum, but almost sounds
like shortwave radio interference.
A) Here's a cool experiment to make you more aware of magnetic
fields. Connect a TS cable to the input of your mixer or audio interface.
Turn up the volume.
Don't plug in the other end, but use it as a sensor and point it towards each
piece of gear. As you get closer to the gear, within 1 inch, you will hear
the digital clock signals bleed into your audio, especially when you get near
the LCD. That's digital clock noise. Now look for a cable that
strayed too close to one of these electronic fields. Many times, if the
gear is balanced, using balanced cables will knock this right out.
Q) I put a hum eliminator on the outputs of my mixer but the
hum is unchanged! I thought these items always worked!
A) They do work if you know their limitations.
First you have to find the device causing the ground loop and put the hum
eliminator on that device. You can't use it "downstream" as hum has
already become part of the audio signal earlier in the chain. You must
apply the hum eliminator to the source of the problem, before the gound loop
becomes a hum problem.
Q) When I connect the audio outs from my TV cable box
to my audio interface it hums so bad I can barely hear the program.
A) Common problem. The TV cable itself may use a
different ground path than your studio equipment, especially if you have a lot
of TVs in the house. For me, a Hum Eliminator completely fixes the
problem.
Q) What types of rigs are best for avoiding issues with hum?
A) Avoid cheap unbalanced mixers. Those with large
mixers and a lot of gear know that ground loop hygiene is crucial to keeping the
board hum free. Those folks have to be especially vigilant to win the war. Going
mixerless can help. Plugging direct into an audio interface gets rid of a
lot of cables, and as we have seen, cables can cause problems. Don't plug
in gear that you know is problematic. Instead, connect it only when you
want to record it, and run through helper devices like noise gates and the ones
mentioned earlier in the article. Audio interfaces like the Tascam FW1884
and Project Mix can help because they incorporate what would be several separate
pieces into one box with one ground. I'd also avoid using cheap laptops
for audio. We have had several problems documented at studio-central where
ground loops could not be corrected. Likewise, adapting an audio chain to
an unbalanced 1/8" stereo line input is just begging for trouble.
Q) Does one ever actually win the
war? Can you truly ever totally eliminate ground loop hum in a
home studio?
A) Technically, the answer is a matter of
degree. The ground differential potential that creates ground loops is always
present, but you can succeed in getting rid of its artifact, the hum.
The enemy is still there, waiting for you to let your guard down. The
way homes send current to ground will always have the potential for ground
loops, but with good gear and following a few simple practices, you won't hear
it. When you turn up your monitors all the way (without playing anything
but with all your gear connected) and all you hear is sweet white hiss, you know
you are, for now, winning the war.
You can leave feedback and discuss this topic here:
http://studio-central.com/phpbb/viewtopic.php?p=286223#286223
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Cool Quote:
"Music and silence … combine strongly because
music is done with silence, and silence is full of music.."
Marcel Marceau
(US News & World Report 23 Feb 87)
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