Part 7: Balanced and Unbalanced Lines

Line-level interconnects come in two varieties: balanced and unbalanced. A balanced interconnect is recognizable by its three-pin XLR connector. An unbalanced interconnect is usually terminated with an RCA plug. Balanced and unbalanced lines are shown in the photograph at the beginning of this chapter.

Why do we use two incompatible systems for connecting equipment? At one time, all consumer audio hardware had unbalanced inputs and outputs, and all professional gear was balanced. In fact, balanced inputs are often called "professional inputs" to differentiate them from "consumer" unbalanced jacks. Balanced connection was considered both unnecessary and too expensive for home playback systems.

The emergence of high-end audio changed that thinking. Instead of using the least expensive connection method, high-end product designers started using higher-quality balanced lines and terminations in consumer gear. The better the equipment, the more likely that it has at least some balanced connections in addition to unbalanced jacks. Moreover, more and more manufacturers are offering balanced connections on their equipment. This is why we have two standards-balanced and unbalanced. The technical and sonic advantages of balanced connection, once the exclusive domain of audio professionals, are becoming increasingly available for home playback systems.

But what exactly is a balanced line, and how is it different from a standard RCA cable and jack?

In an unbalanced line, the audio signal appears across the center pin of the RCA jack and the shield, or ground wire. Some unbalanced interconnects have two signal conductors and a shield, with the shield not used as a signal conductor. If this unbalanced interconnect happens to be close to fluctuating magnetic fields-an AC power cord, for example-the magnetic field will induce a noise signal in the interconnect that is heard as hum and noise reproduced through the playback system's loudspeakers.

In a professional application, this hum, buzz, and noise is unacceptable, leading to the development of an interconnection method that is immune to noise interference: the balanced line. A balanced line has three conductors: two carrying signal, and one ground. The two signals in a balanced line are identical, but 180° out of phase with each other. When the signal in one of the conductors is at peak positive, the signal in the other conductor is at peak negative (see Fig.11-3). The third conductor is signal ground. Some balanced interconnects use three conductors plus a shield.

When the two identical but opposite polarity signals carried on the balanced line are input to a differential amplifier in the component receiving the signal, noise picked up by the interconnect is rejected. Here's why: a differential amplifier amplifies only the difference between the two signals (see Fig.11-4). If noise is introduced into the line, the noise will be common to both conductors and the differential amplifier will reject the noise. This phenomenon of rejecting noise signals common to both conductors in a balanced line is called common-mode rejection. Differential inputs are specified according to how well they reject signals common to both conductors, a measurement called Common-Mode Rejection Ratio, or CMRR. Note that a balanced line won't make a noisy signal clean; it just prevents additional noise from being introduced in the interconnect. If the noise is common to both halves of the balanced line, however, common-mode rejection will eliminate the noise.

In professional applications, a transformer sometimes serves the same function as the differential amplifier, passing only the difference between the two signal conductors and rejecting signals common to both conductors. Consumer audio equipment uses differential amplifiers, not transformers, but the concept is identical.

In a balanced line terminated with XLR connectors, pin 1 is always signal ground. There is, however, no convention for which of the two signal conductors carries the non-inverted signal and which carries the inverted signal. The non-inverted conductor is often called the "hot" conductor, with the inverted conductor designated "cold." After decades of no clear standard, the Audio Engineering Society recently adopted the North American tradition of having pin 2 carry the non-inverted ("hot") signal, pin 3 the inverted ("cold") signal.

How the balanced line is wired (pin 2 or pin 3 "hot") can determine if your playback system is inverting or non-inverting of absolute polarity (see the section on absolute polarity in Appendix A). If your system is non-inverting-that is, a positive-going signal on the LP or CD produces a positive-going signal at the loudspeakers-substituting a pin 3 "hot" balanced input power amplifier for a pin 2 "hot" power amplifier will make your system inverting. When switching balanced components-digital processors, preamps, or power amplifiers-you should know if the new component's XLR jacks are wired the same-either pin 2 or pin 3 "hot"-as the existing component. You can also change your system's absolute polarity by rewiring the balanced interconnect to swap the wires going to pins 2 and 3. It's far better, however, to simply reverse absolute polarity by switching red for black and black for red on both your loudspeaker cables if you want to change your system's absolute polarity. The freak interconnect with pins 2 and 3 switched at one end may wind up in another system or application where you don't want the polarity switched.

Quite apart from the advantage of noise cancellation in a balanced line, a balanced connection often sounds better than an unbalanced line. A system connected by balanced interconnects can, however, often sound less good than one connected with unbalanced lines. Say you have a digital processor that takes an unbalanced signal from the digital-to-analog converter chip and converts it to a balanced signal so that the processor manufacturer can tout the product as having "balanced outputs" (see the section on balanced digital processors in Chapter 8). Inside the digital processor, the unbalanced signal is converted to a balanced signal by a phase splitter, a circuit that takes a signal of one polarity and turns it into two signals of opposite polarity. Phase splitting subjects the unbalanced signal to an additional active (transistor- or op-amp- based) stage and puts more circuitry in the signal path.

The balanced digital processor output is then input to a balanced-input preamplifier. Because all but the very best balanced preamplifiers convert a balanced input signal to an unbalanced signal for the preamplifier's internal gain stages, the preamplifier's input converts this balanced signal to an unbalanced signal-adding yet another active stage to the signal path. After the unbalanced signal is amplified within the preamplifier, it is converted back to balanced with another phase splitter.

The preamplifier's balanced output is then sent from the preamplifier output to the power amplifier's balanced input where it's-that's right-converted to unbalanced with yet another active stage. The result of these unbalanced/balanced/unbalanced/balanced/unbalanced conversions is additional electronics in the signal path-just what we don't want. This is why you can't assume that balanced components sound inherently better than unbalanced ones. Magazine reviews of audio components should include musical and technical comparisons of the product's balanced and unbalanced modes.

Some products, however, are truly balanced and don't rely on phase splitters and unbalancing amplifiers. For example, a digital processor may create a balanced signal in the digital domain (at no sonic penalty but, indeed, a sonic gain) and convert that balanced signal to analog with four digital-to-analog converters and analog output stages (left channel + and -, right channel + and -). Similarly, some preamplifiers are truly balanced and have double the circuitry to operate on the non-inverting and inverting signals separately. You can tell a truly balanced preamplifier by the number of elements in the volume control. A preamplifier that operates on an unbalanced signal internally will have two volume-control elements: one for the left channel, one for the right. A fully balanced preamplifier will have four elements: ± left channel and ± right channel. The signal thus stays balanced from before the DACs inside the digital processor all the way to the final stages in the power amplifier.

As in all things audio, the proof is in the listening. When shopping for a component, listen to it in both balanced and unbalanced modes. Let your ears decide if the component works best in your system when connected via the balanced or unbalanced lines.

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