The subject of "balanced" versus "unbalanced" circuits in audio seems to come up again and again, and raises some interesting questions. Do twisted-pair balanced lines provide better noise rejection than coaxial cable? Can balanced sources be connected to unbalanced loads with good results, and vice versa? To help make some sense out of this, let's take a look at the issues.
First, what do "balanced" and "unbalanced," as referring to audio circuits, really mean? In the world of transmission lines--whether we're talking about a radio antenna feedline, a high tension power line, or a simple analog audio cable--a "balanced" line is one where two conductors, neither of which are grounded, carry the signal. In some references you'll see these referred to as the "signal" and "return" wires, but those names are a bit deceptive; electrically, the two wires do exactly the same job. A balanced line may or may not be shielded; but if it is shielded, the shield is not joined to either of the two signal conductors.
When one of these two conductors is carrying electrons to the source, the other is taking them away; and as the current in the circuit alternates, the conductors, in unison, alternate from positive to negative. The polarity of the two conductors, at any moment, is always opposite; they never both flow to the source, or to the load, at the same time.
One simple way to visualize this is to think of a rope with a weight on one end, looped over a pulley overhead. When you pull down on the end of the rope, the rope on your side of the pulley flows down; at the same time, as part of one and the same action, the rope on the other side of the pulley flows up. Whenever there is a force operating on one side, in one direction, there is an equal force operating on the other side, in the opposite direction. This condition of equal and opposite forces on the two conductors creates a "balanced" system.
In contrast to this is what are called, not surprisingly, "unbalanced" circuits. In an unbalanced circuit, there are still two conductors required to convey the signal; however, one of these conductors is attached to an external reference point--what we call "ground." Although in practice electrons may flow along the ground conductor, it's helpful, for present purposes, to think of "ground" as a vast reservoir of electrical charge which dwarfs, absorbs and dissipates any charge which hits it. In an unbalanced circuit, we don't rely on the ground as a signal conductor in quite the same way; we rely on it only to ensure that the circuit is complete. Instead, all of the work of delivering the signal really takes place on the other conductor. Where the balanced circuit involves two conductors in a mutual push-pull arrangement, in an unbalanced circuit we can think of all the pushing and pulling of electrons as occurring along the signal wire.
To return to the analogy to a rope and pulley for a moment: while a balanced circuit is like a rope running over the pulley, an unbalanced circuit is more like a rope with a weight being lifted and lowered by someone standing on a high place; there's no mutuality of action, and all of the information is contained on one straight run of rope.
From the above, it should be apparent that whether a connection between two devices is "balanced" or "unbalanced" depends entirely on the devices, and not on the cable. If one side is grounded, the connection is unbalanced. If neither is grounded and both have equal impedance to ground, the connection is balanced. However, we also speak of types of cable construction as "balanced" or "unbalanced" because there are two very different ways of making cable for these two types of connection.
Balanced cables, as one might expect, usually have a symmetrical design. Examples include twisted-pair telephone cable, CAT 5 computer network cable, and old TV antenna "twin-lead" cable. Professional audio cables made for balanced audio are usually composed of twisted pairs of insulated wire, usually with an external shield over the pair. Unbalanced cables generally take the form of coaxial cable--so called because the two conductors in coaxial cable share a common axis. In the center is a signal conductor, coated with an insulating dielectric; outside of the dielectric is a shield, consisting either of a wire braid, foil, or a combination of braid and foil, and this shield is used as the ground conductor.
Perhaps by now you're thinking: fine, but who cares? Whether a signal is balanced or unbalanced, it gets where it's going. Indeed it does, but there's one important difference in terms of what happens along the way. Balanced and unbalanced lines use very different strategies for noise reduction, and this difference accounts for the widespread use of balanced circuits in professional audio gear, as well as for the differences in construction between balanced and unbalanced lines.
The man with the foil on his head has at least one thing right: all around us there are unseen waves of electromagnetic energy, traveling through the air. Any cable is exposed to this energy, which comes from other cables, from power sources, from fluorescent lights, switches, and a thousand other places. When this energy meets a cable, the cable will absorb some of it, which causes the unwanted phenomenon we call "induced noise."
Unbalanced circuits rely upon a simple and straightforward approach to noise: "keep it out." The signal conductor is protected by the shield from interaction with outside noise, and as the grounded shield plays no direct role in conveying the signal, it doesn't matter how much noise hits it--the noise is safely shunted away to ground. In an ideal world, the shield works perfectly, and the noise never makes it to the center conductor. The world being a tad less than ideal, some of the noise may make it through, and once it meets the signal conductor, there's no way to separate it from the original, intended signal.
Balanced circuits behave in a different, and somewhat surprising way. First, of course, a balanced line may have a shield to carry noise away to ground. But when noise does reach the inside, it strikes two conductors instead of one. Because the conductors are closely physically associated with one another, the noise reaches them more or less simultaneously, and affects them in the same direction. Where the polarity of the intended signal on these two conductors is opposite--one conductor being negative, and the other positive, at any moment--the polarity of the noise is the same.
Now, electricity flows from negative to positive--and it flows so as to resolve the difference in voltage between two points. This is why a bird standing on a high-voltage power line is, generally, safe as long as he stays on a single wire--there may be lots of voltage under his feet, but his feet don't have a voltage running between them. If he steps across two wires of different potential, it's a whole different story. This difference is not only important to a bird on a wire; it is also important when noise gets into a balanced audio cable. The audio signal in the balanced line shows opposite polarity on the two sides of the line, and so current flows through the load. But the noise presents a voltage--whether positive or negative is unimportant--which is equal on both sides of the line. While the signal is pushing and pulling on alternate sides of the line, the noise is pushing or pulling on both sides at once, and that doesn't cause any current to flow through the load. It's as though the noise met itself at the load, and cancelled itself out, without ever managing to pollute the signal. This phenomenon is called "common mode noise rejection," and it's a decided advantage in balanced circuits.
So, what's it all mean to audio cable design?
First, a little bit of thought about common mode noise rejection will explain why it is important to use balanced cable for balanced circuits, and unbalanced cable for unbalanced circuits. If we were to make the mistake of using coaxial cable in a balanced audio circuit, we'd get plenty of noise--because coax is designed to focus as much of the noise as possible onto the shield conductor and as little as possible onto the center conductor, and common mode noise rejection only works if noise affects the two conductors equally. If, on the other hand, we were to use an unshielded twisted pair to convey signals in an unbalanced circuit, figuring that twisted pair cables provide common mode noise rejection, we'd get a rude surprise. Common mode noise rejection won't work in an unbalanced circuit, because we're not getting signal current flow in the ground conductor; meanwhile, by dispensing with a shield, we've given up the only protection that an unbalanced circuit can provide against noise.
Confusion of these concepts is fairly common, and understandable. One will often hear in audio discussions that "twisted pairs provide superior noise rejection," because it's often assumed that it is the cable construction itself, rather than the balanced or unbalanced nature of the circuits, that accounts for common mode noise rejection; as we've discussed above, it's really the combination of the two which account for the phenomenon. This misconception sometimes leads to people using shielded twisted-pair balanced audio cable as an unbalanced interconnect; they will ground one of the two signal wires at both ends of the cable, and then ground the shield--sometimes at both ends, but sometimes only at one end, causing a loss of shield effectiveness. The problem with this sort of construction is that the shields are often not quite so good on balanced cable stocks and the capacitance, when one conductor is combined with the shield like this, tends to run high. Instead of just the capacitance between the two conductors, one now has the total of (1) the capacitance between the two conductors, and (2) the capacitance between the signal wire and the shield. As capacitance in audio cable is very definitely an enemy, this is a serious sacrifice to make, especially when there is no noise rejection benefit. Coaxial cable, not twisted pair audio cable, is the right choice when connecting unbalanced components.
Of course, most consumer electronic devices support only unbalanced audio lines, usually through RCA jacks. But some high-end devices also offer balanced audio connections, usually through XLR connectors. Should you use them?
The answer, if you can run from a balanced source to a balanced load, is usually yes. Balanced circuits do provide a degree of noise rejection which unbalanced circuits cannot, and so if you have balanced connection options open to you, by all means use them. If, however, you have balanced connections available on only one end of the line, and will have to run from balanced to unbalanced or vice versa, it's better to pass, so long as an unbalanced-to-unbalanced option is open to you. (If you have no choice, then think about using a balun -- a balanced-to-unbalanced transformer -- to make life a little easier.) Grounding one side of the signal will unbalance the line; you won't get common mode noise rejection; and mating balanced and unbalanced circuits has been the cause of many a headache, because of the incidental problems, such as hum from ground loops, which may arise.