Audio Wiring and Connections

The "audio chain" analogy is an especially good one when talking about wiring. Like a chain, a sound system is only as good as its weakest link. The kinds of cables used and how they are connected can often be the difference between a great system and a useless one. Most experienced audio professionals can tell stories about contractors who have saved a few pennies on installation and wiring costs, only to spend costly hours back on site correcting noise or other problems later.

The kind of wire to use will vary depending on the kind of signal it will be carrying, as well as the environment it will be used in. For most commercial installations, wiring will be "jacketed," meaning that the insulated conductors will be bundled together, often in twisted pairs, inside an overall jacket for extra protection.

One of the challenges in sound engineering is to avoid the introduction of unwanted electrical noise and interference into the system. Unwanted noises enter the system in one (or both) of two ways:

Induced noises can come into the system from sources that are not directly connected, much as radio waves can be picked up at a distance. In fact, radio waves are one of the main sources of induced noise (this type of noise is called radio frequency interference, or RFI). Induced noises may also be the result of inductance or capacitance between cable conductors and other conductors nearby (often called electro-magnetic interference or EMI, and electro-static interference).

Common sources of induced noise include electric motors, radio transmitters, some types of lighting equipment, digital circuits, all kinds of power supplies. Indeed, in microphone applications, if you use the wrong cable, then just about any circuit where AC current is flowing could be a source of induced noise. The good news is most induced noises are easy to control by choosing the right type of cable and input/output circuit.

Ground loops come from ground reference mis-matches, which are a function of the power source(s) used for the sound system. If a mixer/amplifier is plugged into one AC outlet, and the input signal comes from a source that is plugged into a different outlet elsewhere in the building, the ground wires at the two outlets might have slightly different voltage potentials with respect to ground (and more importantly, with respect to each other).

If the signal ground is tied to the AC mains ground, as is commonly the case in unbalanced audio circuits, then connecting the audio cables from the source to the mixer/amplifier will complete a circuit through which will flow a voltage equal to the potential difference between the two AC mains grounding points. This circuit is called a ground loop. The main symptom of a ground loop will be a 50 Hz hum in the sound system, often with harmonics above this at multiples of 50 Hz.

There are three ways to alleviate ground loops, or avoid them altogether:

1) Use the same AC outlet for all equipment in the system. This may be impractical, if distances are great, or even inappropriate if the current draw exceeds the rating of the AC circuit.

2) Use transformer isolation between sound system components.

3) Use a "floating" balanced line for the audio signal, so that neither leg of the signal is tied to ground. Often, methods 2 and 3 are combined with the use of transformer-balanced inputs and outputs.

The two most popular methods to reduce the pickup of induced noises through sound system wiring are the use of twisted pair wiring, and the use of shielded cable.

Twisted pair wiring is just what it sounds like: two insulated conductors are twisted around each other over the length of the cable run. The twisting has the effect of rejecting certain types of induced noise, since each half-turn of the wire exposes it to the noise source with the opposite polarity of the preceding half-turn. The effect also works in reverse: twisted pairs generate less noise than pairs run in "flat," untwisted wire. This fact helps to reduce the effect of "crosstalk" between pairs when multiple lines carrying similar signals are bundled together. Twisted pairs have been used by telephone companies for the better part of a century to carry voice communications, and are now the standard type of cable for Ethernet networking and other data transmission protocols (for example, CAT 5 wiring is simply a set of twisted pairs).

In sound systems, twisted pairs are often used for speaker wiring, especially over longer distance runs. For other sound system applications, twisted pair wiring is seldom used, except in conjunction with shielding and balancing (see Balanced and Unbalanced Lines, below). So, while CAT 5 may be the cat’s meow in data networking, you don’t want to use it for your microphone wiring, or you risk serious noise problems.

Shielded cables are the most common, and a more effective, line of defence against noise pickup in audio applications. They protect the signal path from noise pickup by surrounding one or more of the cable’s conductors with another conductor (the shield) that is tied to ground at one or both ends of the line. Shielded cables should always be used for microphone wiring. They should also be used for all unbalanced line level wiring, such as the outputs of CD players, tape decks, or many other common music sources. Standard stereo RCA patch cords are a common example of shielded wiring for unbalanced sources.

The most effective defence against the pickup of induced noise through the wiring is to use a "balanced" circuit for the connection between equipment. This method involves not only using the right cable, but also having a certain type of input and output circuit. In sound systems, balanced circuits, or balanced lines, are typically run using three conductors a twisted pair of inner conductors surrounded by a shield conductor. Running a balanced line requires the use of balancing output and input circuits, which work by splitting the signal into two paths, then inverting the polarity of one path, so that each conductor carries a signal that is the exact electrical opposite of the signal on the other conductor.

While the signal is carried by the two conductors in opposite polarity, the noises that accumulate on the line will have the same polarity on both conductors. When the polarity of the reversed "low side" conductor is reversed again at the receiving end, any noise picked up by the line will be cancelled out. The combination of this balancing action with the use of shielded cable, and the twisted inner pair makes this arrangement the best for protecting audio signals from noise pickup.