Studio Hum Busting, as easy as 1, 2, 3!!!
~A practical way to get rid of hums and buzzes in your studio.
I've been at it for 30 years and spent a lot of time fixing hums and buzzes. From Westlake Studios in LA, where we had the power company install an isolation transformer on the the entire building, and the production assistants would water the ground stakes everyday; to Brentwood Baptist Church in Tennessee where a cell phone tower hidden in the steeple wrecked havoc on the audio; to a theater in Huntsville Alabama where we found a power strip full of phone charging wall warts near the console was making everything hum. Throughout the years I have found there are only ever three things that make a system hum. Forgive me if this information is redundant to you, but I want to be thorough.
Hums are 50 or 60Hz related to the AC power from the power company, buzzes are multiples of those frequencies. Other noises like hiss, frying, crackling, popping, etc... are usually problems in the electronics of the audio system itself. However, hums are usually a problem in the way things are installed, and buzzes are usually a problem in the power supply. If you are hearing a buzz, then you can probably skip to step #2 below.
1. The usual problem is ground loops. If the ground was absolutely, perfectly 0 volts because you paid to install superconductors throughout your building and to the power company, then ground loops would never be a problem. No matter how much current circulated around in ground loops, the difference in potential between any two pieces of gear would never be more than zero, and there would be no hum. Having said that, it follows that making sure the building itself is wired with good grounds goes a long way to reducing ground loop problems. However, all commercially available, real world wiring has some resistance, and if current starts to flow around a ground loop, according to ohms law, that resistance times current creates a difference in voltage, and that means hum in your audio.
The only sure way to prove if your hum is due to a ground loop or not, is to physically unplug every cable from the console so that there is absolutely nothing (not even speakers) connected. With only the power supply connected, listen with headphones to the noise floor. I learned this lesson the hard way as a young tech in the late 80's. I was called out to fix hums at a newly installed control room at Cimarron Studios in Hollywood. I was geeking out because when I got there they were recording voice overs for Alvin and the Chipmunks. They were on a tight deadline and I thought unplugging everything would be a waste of time. So I went about unplugging one thing at a time and plugging it back in if the hum didn't go away. After going around the mountain once or twice and finding no one thing made the hum go away. I finally unplugged everything to find out the console alone was dead quiet.
All you should hear is hiss. However, with the inputs not terminated, they could pick up hums and buzzes, so to make this a fair test, make sure the input gains aren't all maxed out. If there is still a hum with everything unplugged go to step #2 or #3 below. But normally, you should be able to leave up an average sort of mix on the console, crank up the control room volume and get nothing but hiss in the headphones. Hopefully that is your case, then you can start plugging in one cable at a time, listening after each connection to see if a hum has returned. I like to start with the speakers, so I can get away from using the headphones as soon as possible.
Don't be surprised if connecting a cable to an input makes an output hum even with the faders off and muted. Ground loops can go around faders and switches, and take funny pathways (so plugging in an aux could make a group start to hum). If the hum comes back, unplug the last cable and investigate if that piece of gear has a problem or needs to be ground lifted or have an isolation transformer installed. And so on, down the line until everything is hooked up again. Later down the road, if you add new gear, check for hums one connection at a time.
Going about it this way, if the hums were gone, but then getting all the connections to be quiet continues to be elusive; and no matter what grounding, lifting, isolating, etc. you try just doesn't do the trick, the building itself may have an electrical or ground problem. Any sort of bad connection between the console and absolute ground at the service ground stake could be an issue, there could even be ground loops in the wiring of the AC outlets themselves.
If the building grounds are proven to be good then this won't really matter, but it doesn't hurt to do this: Most buildings have two or three phases of AC power, you should try to keep all the audio gear on the same phase. That usually means every other breaker in the breaker box. From top to bottom they usually go phase A, phase B, phase A, phase B, and so on (that way you can use a double wide breaker to send 240 volts to an Air Conditioner or whatever). Check that all the outlets you are using for audio go to the same phase of power, skipping over every other breaker. Use the opposite AC power phase for lights and microwave ovens, etc...
2. The next most usual problem is noise coming from the power supply. This will usually be a buzz because the way they rectify AC to make DC typically creates 120 Hz ripple. That should normally be filtered out and regulated by the power supply before it gets to the signal components. So if there is a problem in the power supply, it could be letting buzz get to the audio path.
If you unplugged everything in step #1 above and still have a 60Hz hum, maybe the power supply is missing a ground. Any discussion of the power supply includes all the intervening cables and connectors between the inside of the power supply and the inside of the console. If a spare power supply is handy try swapping it to pin down or eliminate the power supply quickly. I would usually never trust a switch-mode power supply, they are notoriously noisy and the power is typically soft (which makes for poor audio bass response), if you don't have a linear power supply, getting one should be a high priority.
If the power supply checks out, an inspection of the power and ground wires going up to and inside the console needs to be done. Look out for browning or brittle connectors and wires. That is a sign of heat, which is a symptom of a bad connection. If you find any discoloration in the wires or around the connectors, they need to be replaced or hard wired. Also, it is a good idea to re-flow the solder on the connectors to fix any possible solder fractures, just be careful not to bridge any pins together! And, look at the traces leading out of the connectors to make sure they aren't damaged by heat. In addition, if there are dark streaks along the ribbon cables that carry power, they could be failing or causing problems.
3. Assuming the power all checks out and you still have hums and buzzes, the only remaining source of noise is from the environment (including the cosmos). There could be high current wires running through the walls of the building; or computer monitors, florescent lights or amplifiers in the vicinity. These things could be putting out an electromagnetic field that can cross-talk into the console circuitry or the audio snakes. Don't just turn those things off, unplug them, wall-warts too! A lot of things still generate hum fields as long as they are plugged in, even if they are switched off. Also, make sure the console's power supply is not directly under the console!
If there are high voltage power lines or cell phone towers near the building, that could be a source of hum. You might get lucky and make the hum go away, or at least change it, by moving the console around in the room or taking it to another room. If physically moving things around makes the hum change, you definitely have an environmental hum... One way to be absolutely sure is to unplug the console, take it across town to a friend's house, and listen to it there with headphones. Chances are, in a completely different environment you will not get the same sort of hum. Just make sure your friend isn't in a worse environment!
4. There is no four. If it is still humming, then there is probably still an esoteric ground loop (maybe the metal chassis of something was still touching the console), or there is an esoteric problem with the power getting to the console, or something strange in the environment.
The circuits themselves are incapable of generating a hum, they can make frying and popping sounds. I've seen them oscillate and sound like an AM radio station (a little out of tune and heterodyning). If capacitors are bad that could make ground loops, power problems and environmental noise worse, but in 30 years I've only seen that happen once, and it was a console that was way past needing to be recapped. At this point, if you are no closer to solving the hum problem, then all I can suggest is bring it in and if it still hums here we can make sure it gets fixed.
Be sure to contact Creation Audio Labs (ph: 615-884-7520, email: info@CreationAudioLabs.com) if you’re interested or have any questions!
Alex “Skip” Welti is currently the V.P. of Research at Creation Audio Labs, Inc. a Nashville based pro audio service facility which also designs custom circuits and upgrades for pro audio gear and manufactures a line of products for guitar and bass. Skip is an abbreviation of "Skipper", a nickname Alex earned as National Service Manager for Soundcraft USA where he worked for 10 years before opening Creation Audio Labs. Prior to Soundcraft, he was with Westlake Audio in Los Angeles for eight years. As Technical Supervisor there, Alex helped out on projects such as Michael Jackson’s “Bad” and Paul Simon’s “Graceland”, among many, many others. A true techno-wizard, Skip the mad scientist is ever at work in his laboratory; dreaming about cheap clean energy, zero noise and even-order harmonic distortion.