The cables used in an audio system—and how they’re put together—can have a major impact on the sound of the system. Every cable in an audio system has the potential to add noise and to compromise the sound quality of the components it connects, so it’s important to use the right cable for the right job. That starts with understanding the kinds of signals the cables are carrying.
There are two main issues to consider here: the level of the signal and the signal type. We’ll set aside signal level for a future post and focus for now on whether the signals are balanced or unbalanced.
Unbalanced Cables and Signals
An unbalanced cable consists of two connectors with two conductors each, connected by two wires inside the cable—a signal wire and a ground wire. You can quickly (in most cases) identify a cable designed to carry an unbalanced signal by its connectors: because each wire has to terminate at the connector with its own contact point, an unbalanced cable requires only two conductors at the connector. A standard TS (or “tip-sleeve”) guitar cable is the unbalanced cable you’ll run into on stage most. Standard RCA cables used for many AV components are also unbalanced cables.
Inside the cable itself, the signal wire is typically in the center of the cable with the ground wire surrounding it. The ground wire serves two functions—it carries part of the audio signal and serves to shield the main signal wire to some degree from outside interference from noise such as the hum from lights and transformers, as well as RF (radio frequency) interference that comes from TV and radio transmissions. It does a decent job of rejecting noise, but unfortunately, the wire itself also acts like an antenna and picks up noise.
Unbalanced cables work great for connecting a guitar to an amp, for instance, but because they are not very good at suppressing noise from outside interference, unbalanced cables should have a maximum length of 15-20 feet (4-6 meters), especially when used in noisy environments and with signals that are low level to begin with, such as those from keyboards, guitars, MP3 devices and so on.
Balanced Cables and Signals
A balanced cable, by contrast, has three conductors in the connector and three wires in the cable: two signals wires plus a separate ground wire. As in the unbalanced cable, the ground wire still surrounds the signal wires and is used as a shield against interference. But what makes a balanced cable special is the way the gear utilizes that extra signal wire.
Balanced signals are what is called differential. The two signal wires both carry a copy of the signal, but the two copies are sent with their polarity reversed. If you sum two signals that are identical but are reversed in polarity, the signals cancel out, leaving you with silence. (Just like adding positive and negative numbers: +15 added to -15 equals 0.)
So why would you want audio gear that flips the polarity of your signal? In this case, because the receiving gear will flip the inverted signal back into its original orientation. But because both copies of the signal picked up the same noise as they traveled along the cable—and that noise is identical on the two wires in the cable—flipping the polarity of what arrives at the receiving gear will produce the original signal intact and noise which now has reversed polarity. Summing that gives you a welcome result: signal that’s preserved and noise that’s canceled.
Because of this, balanced cables can support much longer cable runs; 50 to 100 feet (15-30 meters) is not uncommon, though even shorter runs will often use balanced wiring to protect against noise. The wiring for microphones, and the interconnect cables between consoles, signal processors, and amps, etc., in a pro sound system or recording studio environment are typically of the balanced variety. Standard connectors designed for use with balanced signals are XLR and TRS (or “tip-ring-sleeve”).
Use the Right Cable for the Right Signal
It’s important to note that using a balanced cable on an unbalanced signal gives you no benefits. The jacks on the gear on both ends of the cable must be designed for balanced signals as well; otherwise there’s no circuitry to do the polarity inversion that produces the noise cancellation. On the flip side, however, using an unbalanced cable with gear that expects balanced signals will “work” (in the sense that audio will go from point A to point B), but the signal will be unbalanced and susceptible to the same noise as any unbalanced signals. Check the documentation of your gear (or the labels on the gear itself, near the jacks) to determine what type of signal a given jack is designed to support, if you’re not certain.
So what do you do when you need to go a longer distance with an unbalanced signal? In some cases, a wireless rig is a great (though potentially pricey) option. The other option is a direct (or “DI”) box. Learn more about direct boxes in this post.