The "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 end.
Most of the time, the hum is due two two or more paths to the house ground 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 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.
each one of these leads to a circuit breaker.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 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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