Leads can be divided into two main types. Signal Leads and Speaker Leads.

Speaker leads

have to carry more current than signal leads and have thicker conductors. They also have high voltages on them. The voltage carried by the speaker cables in high power sound systems may be enough to kill. This one of of the reasons for the new Neutrik Speakon (trade mark) connectors that do not allow hand access to speaker voltages. Because these cables carry large voltages and are post amplifications they are immune from noise.

Signal Leads

These leads can be subdivided into two further groups. Balanced and Unbalanced

Unbalanced (2 wire)

These are guitar leads, record deck and domestic equipment. They use Phono plugs and two pin jacks. These carry low level signal; levels from between mVs (.001 V) and 2 Volts. The lower the voltage the more susceptible they are to Electromagnetic interference.

Electromagnetic interference

All electrical equipment give off noise in the form of electromagnetic waves. Motors and light switches are a common example. This radiation causes a voltage to be generated in any conductor. Your leads are long conductors and thus they can pick up noise. Ask any guitarist who does not use humbuckers.

Screened Cable

This is used to try to reduce the noise. The cable consists of an inner conductor surrounded by an insulating plastic. Enclosing this outer plastic there is a surrounding metal shield. Made from with a copper braid or a foil strip. This is called The shield. As this surrounds the signal wire in the centre, it shields the signal wire from interference. This interference signal now appears in the outer shield. However if we earth this shield the noise is grounded and disappears.

Signal Leads Balanced (3 wire)

In the amplifier there is a special amplifier type that has 2 signal inputs, one called hot + and the other called cold. This circuit amplifies the difference between the two signal inputs, so if the voltage is the same on both inputs there is no output from the amplifier. In a balanced cable there are three wires hot, cold and earth. Any noise interference in the wires will be equal (as they lie alongside each other). As there is an equal signal in both hot and cold there will be no noise amplified by the circuit. Transformers were used in balanced lines however electronic circuitry has replaced them in most amplifiers. The transformer is still used in balanced microphones.

A balanced input requires a balanced signal to be fed to it for noise reduction.