The War on Hum
How to Maintain a Hum Free Audio System
"war on hum" is a 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 simple
practices 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
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
Ebtech HUMX Voltage Hum Filter
Behringer HD400 2-Channel Hum Destroyer
Ebtech HE2PKG Hum Eliminator (2-Channel)
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 loop hum, may be 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. Like it or not, your studio is connected by wires to nearly
everyone on the continent.
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.
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
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 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 ground 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.
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"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)
Always, my friends, enjoy silence.
Tweak's Articles on Essential Studio Concepts
Hooking Up Audio
The Many Functions of MIDI Data
The Audio Interface
Signal Flow Computer-based Studio
Signal Flow of an MPC Hip Hop Studio
Signal Flow of a MultiTrack Studio
Assembling Your Studio Rig
Studio setup in a Nutshell
5 Hot Tips
Building a Quiet Room
Understanding MIDI Interfaces
The War on Hum
Multiple Video Displays
Latency and how to Deal
Everything About Cables
Digital Audio Converters
Bit Depth and Sample Rate
Impedance for Musicicans
How to setup a Patchbay
Room Acoustics Basics
Studio Monitors Price List
Catalog of MIDI Interfaces