Balanced cables and connections are standard in all professional audio systems. On the other hand, instrument amplication systems (eg guitar, bass, keyboards) and semi professional and domestic hifi systems use unbalanced connections.
A typical balanced connection requires 3 conductors and consists of a cable that contains two identical wires (individually insulated) which are usualy twisted together and then wrapped with a third conductor (foil or braid) that acts as an outer shield. Professional microphone cable is a perfect example.
The two wires carry the audio signal; one wire is usually referred to as “positive” (abbreviated +ve) or “hot”, the other the “negative” (-ve) or “cold”. The actual audio signal is simply the difference in voltage between the “hot” and the “cold” wires. [Note that the “hot” wire is also referred to as the one carrying the “in-phase” signal]. The hot and cold connections are often shown as In+ and In− (“in plus” and “in minus”) on circuit diagrams.
Connectors used with balanced cables therefore require 3 contacts (2 for the audio signal and 1 for the shield or ground/earth). Most commonly these are XLR-3-pin and TRS (Tip, Ring, Sleeve) Jack plugs.
The term “balanced” comes from the fact that both wires carrying the audio signal are identical and, in professional audio systems, are connected to audio equipment that is matched at both ends (ie they should have similar impedances, commonly 600 ohms). Any electromagnetic interference (eg from lighting, power lines, radio transmitters etc) will therefore induce an equal noise voltage in each identical signal wire. Since any audio signal travelling along the cable is the difference in voltage between the two signal wires, any noise that is identical on both wires will not be detected (ie there is no difference in voltage caused by the noise).
The outer “shield” in the cable is usually connected to a ground or earth connection at each end and thereby helps to further reduce the external electromagnetic interference entering the two signal wires. Balanced connections should also prevent hum from “earth or ground loops” by separating the equipment ground connection from the audio signal connection.
Balanced cables are particularly important when running audio over longer distances and/or near lighting and power cabling.
Unbalanced cables and connections, as used with instruments (guitar, bass, keyboard, electronic drums etc) and consumer electronics have 2 conductors only.
A single cable is used for the +ve audio signal and the outer shield is used for both the -ve audio and ground/earth connection. In this case any electromagnetic interference will induce different voltages in the signal cable and the outer shield (because of their different physical and electrical properties) and this will combine with the audio signal. The longer the cable the greater the potential for interference, particularly from stage lighting systems, and this will become quite audible as hum and buzzing.
Hum caused by "earth loops"
Because unbalanced connections use a common ground or earth connection for the audio signal and the equipment ground there is a high likelihood of also producing hum from what is known as an “earth loop”. This is a common problem with unbalanced audio systems when you have two or more components connected that are also connected to mains AC power (ie a power point).
The hum is caused by very small voltage differences that exist between equipment that is connected to mains AC power (with an earth or ground connection) as well as one or more additional ground connections via audio and other cables connected to the other equipment (eg USB cables between a computer and an audio interface).
Eliminating earth loops can be quite a challenge, but basically you must remove one or more ground/earth connections or electrically isolate the audio signal connections. The best solution is to add a DI (Direct Injection) box with a ground lift switch or insert a hum destroyer into the audio connection. Hum destroyers generally use a small audio transformer that electrically isolates the audio connection.
IMPORTANT SAFETY NOTE
Never remove the earth connection that is used by your equipment’s power cable. It may solve your earth loop hum, but can be very dangerous. Equipment that is correctly supplied without an earth connection must meet strict Australian and New Zealand electrical standards and be identified as “double insulated”.