Chapter 4: Wiring; Low Level And Line Level Wiring - Toa A-503A Manual

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 profes-
sionals 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 envi-
ronment 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.
Low level and Line Level Wiring: Twisting, Shielding, Balancing and Isolating
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 com-
monly 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 differ-
ence between the two AC mains grounding points. This circuit is called a ground loop. The main
symptom of a ground loop will be a 60 Hz hum in the sound system, often with harmonics above this
at multiples of 60 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 (see page 16).
3) Use a "floating" balanced line for the audio signal, so that neither leg of the signal is tied to ground
(see page 16). Often, methods 2 and 3 are combined with the use of transformer-balanced inputs
and outputs.

Chapter 4: Wiring

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TOA Electronics Amplifier Guide