Throughout your audio life, you’ll hear the terms “balanced” and “unbalanced” in reference to the audio I/O on a device. What exactly does this mean? Let’s take a look.
Audio signals are subject to a variety of nasty interference from sources such as TV and FM radio broadcasts, microwaves, stray magnetic fields, nearby power cables or electrical service, cell phone signals, CB and ham radio signals, lightning, static electricity and probably myriad other electronic signals that the government doesn’t want us to know about. These signals are referred to as “RFI” (Radio Frequency Interference), and they can easily leak into audio cables, causing problems. Sometimes, they manifest as hum, while other times you may actually pick up a broadcast. Regardless, they have no business being in our audio path.
RF signals are most problematic with low-level sources such as microphones or electric guitar and bass. As other devices such as audio interfaces, outboard processors, mixing consoles, drum machines, samplers, keyboards, etc., output a stronger signal, they are somewhat less susceptible to RF — but certainly not immune. Speaker level signals (e.g. the cables connecting a power amp to a passive speaker) are typically strong enough to overcome RF, but in really bad cases of RFI you could pick up a neighbor’s CB radio over a speaker cable. Electric guitar and bass are particularly susceptible to RF, as they output a relatively low signal level and are also “unbalanced” (stay tuned…).
Agents of Shield
To guard from RFI, audio cable has a wire “shield” of some sort that surrounds the “hot” conductor (see Fig. 1). An unbalanced cable has two conductors: a shield that’s connected to the sleeve and a hot conductor that’s connected to the tip of a TS connector.
This shield can be made of foil, braided wire, or stranded wire that is simply wrapped around the hot conductor. Cheap cable may use unbraided, stranded shield that does not completely surround the hot conductor and might let RF leak through. When cable is made using a foil shield there is usually an uninsulated “drain” wire which connects to the ground contact of the connector (see Fig. 2).
When you purchase premium cable, part of your money pays for a more robust shield. The shield is supposed to be connected to ground (literally), the idea being that if there is strong RF in the vicinity, the shield acts like an antenna, picks up the signal and dumps it to ground where it is harmless to the audio signal.
Two Out of Three Ain’t Bad?
If there is RF in the neighborhood of an unbalanced connection, the shield picks up the signal and dumps the RF noise to ground. But as the shield is also the negative conductor, there’s a chance that the noise will find its way into your audio path. (See Fig. 3) This is clearly demonstrated when using a guitar in an area where there’s a lot of RF. Simply moving the guitar around produces varying amounts of hum because the guitar pickups and cable act like antennas. When you move the guitar to a certain position, the hum goes away. This is usually the position where the guitar player can’t see the band and is as uncomfortable as physically possible.
Three Out of Three Is Better
A balanced cable has three conductors: a + (positive or hot) and – (negative or cold), surrounded by a shield wire which is connected to ground [For the record, the AES standard for wiring a three-pin XLR connector is pin 1 = ground, pin 2 = hot (+), and pin 3 = cold (-)]. An electronic circuit or transformer inside the sending device is used to duplicate the + signal and reverse the polarity of the duplicate. As shown in Fig. 4, we get the original signal and a mirror-image, “negative” duplicate of that original (that’s why it’s called “balanced”.)
The shield protects the + and – wires from interference by capturing RF and dumping it to ground where it is harmless — but sometimes RF can get through the shield. As the + and – wires are so close (they are usually twisted tightly together, thus the term “twisted pair”), noise that does sneak through the shield will be the same in both wires. When the signal hits a balanced input, the polarity of the negative signal will be inverted back to normal and added with the positive signal, which cancels the noise while also increasing the signal level.
This balancing act is a function of the device, and not the cable, i.e. connecting an unbalanced device using a balanced cable will not balance the signal.
The most commonly used connectors for balanced lines are XLR and TRS, but DB25, TT or EDAC connectors can also be used. The most common unbalanced connectors are TS and RCA, but we don’t use RCA connectors much in pro audio because they’re pretty unreliable.
Whether a device is balanced or unbalanced doesn’t necessarily correspond with the price tag: there are cheap devices with balanced I/O and expensive devices with unbalanced I/O. The effectiveness of the circuitry depends more upon the circuit design and execution, so don’t be put off by a device just because it’s unbalanced.
What happens if you connect a balanced output to an unbalanced input? Let’s say that you’re using an XLR to TRS cable to connect the balanced output of a digital reverb to an unbalanced input on a mixer. Internally, the mixer will probably “float” the ring (leave it unconnected) but it might tie the ring to the sleeve. A trip to both manuals will help you determine whether to float pin 3 or connect the cold wire of the XLR to the ground (sleeve) on the TS connector. If the ring of the TRS is wired to pin 3 (cold) there’s probably no problem. But if the ring is accidentally wired to pin 2 (hot) of the XLR then the hot pin could short to ground, making the digital reverb very unhappy. Best case scenario: noise and distortion; worst case: permanent damage to the output circuitry of the digital reverb.
On the other hand, connecting an unbalanced output to a balanced input typically does not present major problems, though the signal level will be lower than it would be if the same device fed the input with a balanced signal (that’s because there’s no “-” signal to add with the + signal). Some inputs will be fine if you “float” (omit) the cold connection, while others will need the cold pin tied to ground at the source. Once again: if in doubt, RTM! (Read The Manual!).
Steve “Woody” La Cerra is the tour manager and front of house engineer for Blue Öyster Cult and Jon Anderson & The Band Geeks.
