I don't mean to brag, but the world of hum has been an absent partner for all my recent gigs. While I tackled hum back in the "good ol' days" at my local soundco, I now look back on how I was inflicting hum and noise problems onto myself. This diatribe is meant to bust the demons of hum out of your system, but also outlines how to take preventive measures and minimize its occurrence. Preventative Measures
Most hum problems revolve around a power distribution faux pas, or cabling or gear that needs repair or is too cheap. The first rule in knocking out hum is the proper usage of balanced cables. While not a 100% cure-all, twisted pair wiring with a shield is a great start. This means XLR to TRS interconnects throughout. With the cost of balanced patch cables not much more expensive than unbalanced cables (signal/hot and shield/cold), using unbalanced cables at any primary signal path is just foolish.
Figure 1 (see magazine) shows a typical electrical schematic diagram with a microphone source to mixing console to drive signal processor path. Smart people will indulge in balanced cabling between each piece of gear. I show (in red) the cold wire in the console to processor cable interconnect, to indicate what the difference in balanced to unbalanced cable paths would be like. In balanced connections, the hot (pin 2 XLR) and cold (pin 3 XLR) carry the signal in and out of polarity forms. If hum currents couple through via the shields or grounds, both the hot and cold wires get nearly identical hum noise. Each piece of gear receives balanced cable signals and does "difference amplification." This means the hot signal and noise are subtracted from the cold signal and noise, and the differential signal from the
two wires remains.
Because hum and noise are common to both hot and cold wires, the differential amplifier subtracts the signals, resulting in the cancellation of the hum and noise. This leaves twice the individual wire signal as the output. So not only are you busting hum, but getting a higher signal capability as well. And if you are ultra-cautious like I am, almost all your balanced cabling is configured as "star-quad" type.
Star-quad balanced cabling uses redundant hot and cold wires for a four-wire plus shield cable design. The hots and colds are interleaved to further reduce coupled hum, audio noise and radio-frequency noise on the transmitting and receiving pieces of gear. Although terminating star-quad cable with XLR connectors is more of a chore, I choose to do this for additional peace of mind.
Now back to Figure 1. If that cold wire were missing, as in the case of unbalanced patch cabling, the shield serves as the cold path and ground reference. Because the shield also has hum and noise currents from power supply redundant paths–rack chassis, adjacent processor chassis, distro branch circuits, etc.–all the hum is injected into the signal path and not rejected at the difference amplifier.
Getting Cheap
So why did you use unbalanced cables? Because you were cheap, the cables were handy or you did not understand the benefits of balanced cables. Short two-, six- or 10-foot XLR patch cables are not something most of us have hanging around the shop, and we generally have to make the time to solder up these specialties. Most hum problems are created by just not knowing the advantages of balanced signal transmission or not having the balanced patch cables handy to make it happen.
Getting cheap also extends to the gear you're using. While most live sound gear can receive and transmit balanced signals, some pieces make a halfhearted attempt. Servo balanced transmits and differential receives are the expectation, but sometimes gear is provided with "impedance-balanced" transmit (output) circuits. In this case, the cold wire connection is just terminated with a resistor to ground, instead of a proper drive circuit. While it is better than a grounded cold wire, the ability to reject hum and noise is less than servo-balanced drive circuits. So when choosing your next purchases, note the balanced drive circuit types.
Afflicted
Even with the best intentions, you may be doomed to battle hum in your sound system. Your best bet is to dust off your
troubleshooting skills and start the divide and conquer process. Typically, my routine is to check the console inputs first to see if I have a single input "hummer." For example, a recent hum intrusion for me revolved around the bass guitar direct input. After muting the channel strip and assuring that I had isolated the culprit, the next step was to troubleshoot further onstage. Upon checking for worn cables and DI issues, I found the problem–the bassist had plugged his amp in to a forbidden receptacle, instead of the handy stringer receptacle box I had conveniently located on the floor near his amp.
If your hum is not at the console, but further down the drive line, two possible sources should enter your mind–worn or broken wires in cables, or differing sources of power to the power amp racks and the drive racks. Simple troubleshooting is again the obvious choice. Turning the humming power amp's attenuator down should drop the hum. If not, the amplifier or amplifiers are the issue. Going back towards the console, you can check cabling by dropping amplifier gain (increasing the attenuators) and opening the patch cables and snakes along the way. With balanced cabling, an open balanced connection should result in very quiet power amplifiers upstream. On the other hand, a half-busted balanced wire or an unbalanced patch can result in a very noisy affair.
If you have hum problems, and you have to live with distro connections or other bad signal flows, you should have a last-ditch solution in the form of isolation transformers. Isolation transformers break the pin 1 (ground) connection and insert an audio transformer into the hot and cold connections. The industry standard for isolation transformers is the Sescom IL-19, which comes in a handy barrel-XLR format to break into XLR patch cables. Because of its small size, the smaller internal transformer can only handle modest line signals without distortion. But I will take lower drive levels over hum any day. And serious sound companies always have a pair or more of IL-19s (or the equivalent) stashed away for just in case (JIC) usage.
I hope this provided some common sense notes on hum busting when under the time pressure of a gig, or when configuring systems back at the shop. If you are taking out a new batch of gear for the first time, consider doing a dry-run setup back at the shop first. Yes, it is time-consuming, but I would rather tackle hum and problems at the shop than last minute at the gig. Remember, it is the sound company's duty to be so prepared that it puts the Boy Scouts to shame.
