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Reprinted from harmonycentral.com with the permission of the author and publisher Craig Anderton
In today's studio, you're likely to find some gear that has balanced analog inputs and outputs, some with unbalanced inputs and outputs, and some with both. What's the difference? Does it matter? Which is most appropriate to your setup? Let's investigate.
The typical musical instrument cable is an example of an unbalanced line that has two conductors. One wire is the ground line, which is also referred to as common, earth, shield, or DC return. The other is the signal line, often referred to as the "hot" line. The hot line carries the signal, and the ground acts as a voltage reference.
Unbalanced systems typically use two-wire, 1/4" phone connectors, or RCA jacks and plugs (as used in hi-fi gear). Almost all guitar processors and amps use an unbalanced input to be compatible with conventional guitars; computer audio interfaces often include an unbalanced, guitar-compatible input so you can plug your guitar directly into the interface without needing any other preamps or level-matching boxes.
The Mackie Onyx 400F is just one of many computer audio interfaces with a 1/4" input that can be optimized for guitar.
A balanced system adds another signal line (called the "cold" line) to the "hot" signal-line-plus-ground combination in an unbalanced system. The basic principle here is that the two signal leads carry signals that are identical, but 180 degrees out of phase. Thus, as a signal increases in voltage along one line, its mirror image on the other line decreases.
This type of signal has to feed a special type of input, found in transformers and certain types of amplifier designs, called a differential or balanced input that responds to the difference between these out-of-phase signals. The differential input is a somewhat unusual beast that rejects in-phase signals that flow into the line, such as hum and noise. This is because the kinds of noise and garbage that get into a balanced line generally spill over both signal lines equally. As the differential input responds only to the differences between the two signal lines, when the same signal is present on both lines, then there isn't any difference, they cancel out, and the differential input ignores them.
Some differential amplifiers can reduce these interfering signals (technically called common-mode signals) by over 90 dB—enough to make the interference fade way into the background, assuming that there's the same amount of interfering signal on both lines.
Mic connectors are almost always balanced, as evidenced by an XLR jack (a common giveaway that the circuitry is balanced).
A typical XLR input, as found in the mic preamp section of DigiTech's GNX4.
However, 1/4" phone jacks can also be balanced, even though they appear to use standard 1/4" guitar cord-type jacks. This is because the jacks are actually stereo jacks, but instead of being wired for stereo, they're wired for balanced operation: The tip is the hot, the ring is the cold, and the ground is ground. This type of jack is called a TRS (tip-ring-sleeve) connection and is common in pro audio gear.
But what if you run 1/4" unbalanced lines, but your gear has 1/4" balanced connectors? No worries. TRS jacks are electronically balanced, so if you plug in a mono plug, they simply turn into an unbalanced output. (The one place where you don't want to plug a mono plug into a stereo connection is the headphone out.)
Combi jacks are a welcome addition to the connector scene. These combine an XLR connector and 1/4" TRS connector; the 1/4" connector is located inside the XLR section. This is a truly universal solution, as you can interface XLR balanced, 1/4" balanced, and 1/4" unbalanced lines with one connector.
Yamaha's GO46 mobile interface has two combi front panel input jacks. Note how the pins from an XLR connector can fit in the three small holes bordering the internal, larger hole, which accommodates a 1/4" plug.
In a small studio or onstage setup with relatively short cable runs, unbalanced connections work just fine — although you'll still probably want to use balanced connections for your mics. Most of the time there's no real need to rip your setup apart and rewire it with balanced connections. Where balanced lines shine is in situations with long cable runs, especially ones carrying low signal levels that are subject to noise or interference.
The bottom line is that both technologies have their uses. But balanced lines tend to be more expensive, so if you don't really need to go balanced, you're probably okay sticking with unbalanced lines.