A few summers of my misspent youth found me working at an outdoor concert venue on the local crew. The local crew helps the show’s technicians — don’t call them roadies; they hate that — put up the show. You unpack the trucks, put up the lights, fly the sound system, help run the show, and put it all back in the trucks at the end. It was grueling work, but a lot of fun, and I got to meet people with names like “Mister Dog Vomit.”
One of the things I most remember about the load-in process was running the snakes. The snakes are fat bundles of cables, one for audio and one for lighting, that run from the stage to the consoles out in the house. The bigger the snakes, the bigger the show. It always impressed me that the audio snake, something like 50 yards long, was able to carry all those low-level signals without picking up interference from the AC thrumming through the lighting snake running right alongside it, while my stereo at home would pick up hum from the three-foot long RCA cable between the turntable and the preamp.
I asked one of the audio techs about that during one show, and he held up the end of the snake where all the cables break out into separate connectors. The chunky silver plugs clinked together as he gave his two-word answer before going back to patching in the console: “Balanced audio.”
I didn’t really know what balanced audio was back then, at least from an electrical point of view. I’ve since learned what the difference between balanced and unbalanced audio is and why balanced audio is the way to go for pro audio applications.
The first and most obvious difference between balanced and unbalanced audio is that while an unbalanced line, like that RCA cable behind my stereo, has two conductors — signal and ground — a balanced line has three conductors. A clue to what’s going on lies in how the conductors are labeled. There’s a ground conductor, of course, as well as two input conductors, generally labeled “IN+” and “IN-” but often referred to as “hot” and “cold” respectively.
These two wires carry audio signals that are identical except for polarity — the cold line carries an inverted version of the signal on the hot line. This looks like the perfect setup for a differential amp, and that’s typically what lies behind the jack on pro audio gear, although transformers are sometimes used instead. The differential amplifier outputs the difference between the hot and cold signals, which gives the audio signal amplified by a factor of two.
The noise rejection trick comes from the fact that both the hot and the cold lines are balanced, or have the same impedance referenced to ground, which means that any noise that makes it into the cable will be added to both lines equally. Since this will increase the voltage on each line by the same amount, the differential amp will subtract out the noise on the receiving side. Here’s a good demo of how it works using an audio mixer app.
Crosstalk and Phantoms
Common-mode noise rejection is not the only benefit of balanced audio. Another one is the cross-talk reduction offered by the twisted-pair hot and cold lines canceling out each other’s electromagnetic field. This ends up being really important in those thick snakes with dozens or hundreds of channels snuggled up against each other. There’s also noticeably better performance at the higher audio frequencies in long cable runs if the audio is balanced.
Another advantage of balanced audio circuits is the ability to overlay a DC bias voltage between the hot and cold circuits and the ground line. This is often called phantom power and can be used to power microphones at the other end of the line. For pro audio gear, 48-VDC phantom power is typical, and a switch is generally provided at the connector on the audio console to control whether a mic gets power.
As for the hardware at the end of that audio snake that long-ago day, it fairly bristled with the de facto standard for pro audio connections, the XLR or Cannon plug. The XLR has been around since James Cannon, after whom the connector gets its other name, invented it in the 1950s. The XLR has proved itself a sturdy and reliable connector that can handle almost anything, from audio inputs to connecting speakers to amps and even DMX digital lighting control.