When it comes to CABLING, it pays to have the right CONNECTIONS. WILLY T gives some practical advice on the matter.

All audio cable – other than those connecting the power amps to the speakers – should be ‘screened’ to reduce the amount of external noise getting into a PA system. The screen intercepts any external noise or interference and drains it away to earth before it can affect the signals passing along the inner wires.

Screened cable can be easily recognised by its co-axial construction where one or more inner conductors is enclosed in a wrapping or ‘screen’. The screen itself may be formed from woven copper braiding, metal foil wrapping, or it may even be made from conductive plastic. By far, the most common type of coaxial cable used for audio is ‘braided-screen’ cable as they offer excellent screening efficiency, combined with reasonable flexibility. Braided-screens have excellent screening properties making them an indispensable in situations where long cable lengths are needed, but where flexibility is essential – mic cabling, for example. Braided-screen cable is also used extensively in professional patch leads and instrument cables.

The major downside of braided-screens are that they are a dead-set pain in the arse and time-consuming to work with! To make a connection, you either have to unpick the last 15mm or so of the screen, or part the strands enough so that you can pull the inner conductors through the side of the screen. You then have to twist together the strands of screen so that they can be soldered to the appropriate terminal in the plug.

Shielded cable should not be used as Speaker Leads. Since the signals travelling through the cable are at relatively high levels – i.e. lots-o’-watts – they tend to ‘drown out’ external RF or stray signals, so there is no need for shielding. Also, having a strong current running through the shield that surrounds the internal conductor’s current can result in strange and audible electrical inductance problems.

BALANCED OR UNBALANCED

The simplest difference between Balanced and Unbalanced leads and circuits is the number of conductors or wires inside of them.

BALANCED LEADS

Balanced circuits and leads offer the best method of minimising external noise being introduced to the PA system. A balanced lead carries the signal on two conductors, plus the shield. The signals on each conductor are the same level but opposite in polarity – i.e. one is positive when the other is negative. 

A balanced circuit amplifies only the difference between the two signals and rejects noise that is identical on both wires. Any noise or hum that is picked up by a balanced – 2 conductor cable – tends to be identical in both conductors and is therefore rejected by the Balanced input, while the equal but opposite polarity original signals are amplified. [Technically, ‘balancing’ is known as common mode rejection or differential input circuit.] The screen intercepts any external RF interference and drains it away to earth before it can affect the audio signals passing along the inner wires.

UNBALANCED LEADS

Inside Unbalanced Leads are one conductor and a braided ‘Shield’. They have the Hot (+) signal running down the centre wire, and the Cold (-) and the Ground (Earth) running down the outside shield together. They are unable to reject any electrical noise or hum that has been picked up by the lead and amplify any signal present in the conductor.

Unbalanced leads are extremely susceptible to picking up noise and as such, they should only be used for short cable runs under 3 metres – e.g. FX patch leads to and from the mixing desk. A long length of unbalanced lead can act as a giant aerial, picking up all sorts of electrical noise and radio frequencies (RF), all of which will be amplified the same amount as the audio signal running down the line. Ever been to a gig when suddenly you hear Taxi radios blaring through the PA? This is usually the result of RF picked up by guitarist’s unbalanced pick-ups or leads. Fenders, with their single coil pick-ups, are notorious for picking up RF.

If an unbalanced lead is being made up from a ‘two-Core & Shield’ cable, one of the cores is not needed. Connect the spare core to the screen at both ends of the cable or just cut it off. It will still help as a passive barrier to RF and stray fields.

In for short cable runs and recording in general, a compromise is necessary between the noise reduction given by balanced lines and the extra cost of the extra circuitry they require.

COMBINING BALANCED & UNBALANCED CIRCUITS

Sometimes it is necessary to run from a Balanced 3-pin connector circuit to an Unbalanced 2-pin circuit, or vice-versa. For example, when running leads in to and out of effects units, or from a guitar jack output to a 3-pin XLR connector. [Technically we are ‘interfacing’ Balanced to Unbalanced!]

This is done easily by wiring the Hot lead to the centre pin of the guitar jack and twisting the Shield and Cold together and wiring them together to the other leg of the guitar jack. On an XLR that would be Pin 2 to the Centre Guitar; and twisted together Pin 1 and 3 to the other guitar jack leg. Too easy.

In general twisting the Shield and Cold solves the problem, but in certain cases the negative output should be left disconnected. If you are unsure what the wiring standard is, look up the manufacturers manual or on the spec plates on the back of the gear, they usually have a little diagram telling us ‘what’s hot and what’s not’.

WILLY T runs Giraffe Media. Providing Words, Pics and Live-Audio Recording Services to the entertainment industry. Email  for more information.

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