I get more questions about balanced circuitry than about any other aspect of audio amplification. What does it do? Does it really make a difference? What are its advantages? Disadvantages?
Before answering those questions we will do well to step back and describe what we mean by a “balanced” circuit. And, that description will make more sense if we take a second step further back and examine the single ended or “unbalanced” circuit. A single ended circuit is one in which there are two conductors, one that carries the signal (i.e., the music) and a second that is tied to ground. The signal conductor (typically a wire or printed circuit board trace) carries the signal in the form of a varying voltage that replicates the music. The ground conductor has an unvarying voltage of zero.[1] To measure the signal in a single ended circuit, one measures the difference in voltage between the signal conductor and the ground. The signal is amplified on the same basis. The input to a single ended amplifier is the difference in voltage between the signal conductor and ground, and it is that difference that is amplified.
This concept is key. A single ended amplifier “sees” at its input and does its best to make an exact, but bigger (amplified) copy of the difference in voltage between the signal conductor and ground.
This arrangement works perfectly so long as the circuit is not exposed to any unwanted disturbances, such as stray electromagnetic fields (e.g., from radio, computers, power lines, electric motors, etc.). But, these sorts of disturbances abound in our world. When a single ended circuit encounters them, the signal conductor picks up some amount of voltage which gets added to, and modifies, the musical signal. But the ground conductor is not influenced by these disturbances; it remains at zero voltage[2]. And that’s the problem. Remember the single ended amplifier acts on, and amplifies the difference between the signal conductor and ground (i.e., zero). Because only the signal conductor picks up the disturbance, the difference between it and the ground now includes that disturbance. The music has been irretrievably changed.
This phenomenon came to pose a considerable problem in professional recording studios where it is necessary to run long lengths of signal carrying wire. Those long wires acted as antennae that broadcast and picked up each other’s signals, thus causing significant degradation of signal quality. The solution proved to be balanced circuitry.
In a balanced circuit there are not one but two signal carrying conductors, and a third conductor that carries the ground. The two signal conductors carry the same musical signal in the form of a varying voltage, but they carry it in opposite phase. Thus, when a peak on one conductor is positive in voltage, the same, corresponding peak on the other is negative. The signal voltage is the difference between the voltages on the two signal conductors. If the two signals were in phase (both going positive at the same time), there would be no difference. But because they are in opposite phase, there is a difference.
To amplify such a balanced signal, one needs a differential amplifier that “sees” at its input the difference in voltage between the two signal carrying wires and amplifies that difference.
But how, one might ask, does this business of using two conductors help with the problem of picking up noise from outside disturbances? Doesn’t having two conductors subject to those influences just make things worse? The answer lies in how the two conductors interact with typical electromagnetic fields. When two conductors pass though the same electromagnetic field, they each tend to pick up the same interfering voltages. Because the conductors are close together both the phase and the strength of the voltage they pick up will be similar. Thus, to oversimplify, if one wire picks up +1 mV (one millivolt) the other will also pick up +1 mV. And – this is the essential point – the difference between the two conductors will be unchanged. Because the musical signal is, as we noted above, the difference between the two conductors, it remains unchanged.
Similarly, the differential amplifier sees and amplifies only the difference between the two conductors. It thus ignores or rejects the +1 mv change that happened to both conductors and amplifies only the musical signal. It is this ability of differential amplifiers to reject the unwanted signals that are common to both conductors while amplifying the difference between them that makes balanced circuitry and differential amplification valuable.
But there is much more to the story, many additions, refinements and qualifications. In future installments we’ll cover some of them, including a question you may be asking yourself: if single ended circuits are so vulnerable to interfering signals from stray electromagnetic fields, how can so many high end audio components using such circuits sound so good?

[1] Only a theoretically perfect, ideal ground has an unvarying voltage of zero. In any real world ground there are always small variations in voltage from one place to another in the material that makes up the ground. These variations can cause problems (e.g., noise, hum) in audio circuits, and we will discuss some of these problems in future installments. But for present purposes, it will advance an understanding of the single ended and balanced circuitry to think of an ideal ground carrying an unvarying voltage of zero.
[2] At least our theoretically perfect ground does. In reality, the ground may pick up minute amounts of voltage, but the levels are so much smaller than those picked up by the signal conductor that for practical purposes we can treat them as being zero.