The mixing desk - or any other bits of gear in the signal path with adjustable gain - will amplify any 'noise' in the system by the same amount as the audio signal running down the line. As well as carrying the audio signals, the aim of any lead or circuitry is to reduce the amount of 'noise' in the system.

All cable used for carrying signals (other than those connecting the power amps to the speakers), should be screened. Screened cable can always be recognised by its coaxial construction where one or more inner conductors is enclosed in a 'screen'. The screen intercepts any interference - noise - and drain it away to earth before it can affect the signals passing along the inner wires.

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.

When it comes to cabling, it pays to have the right connections.

The major downside of braided-screens are that they are a pain in the arse and time-consuming to work with. To make a connection, you either have to unpick the last half inch or so of the screen, or part the strands enough so that you can pull the inner conductors through the side of the screen, about half an inch from the end. You then have to twist together the strands of screen so that they can be soldered to the appropriate terminal in the plug. It is due to their excellent screening properties that braided-screen cables are useful in situations where long cable lengths are needed, but where flexibility is essential. Eg: mic cabling. Braided-screen cable is also used extensively in professional patch leads and instrument cables.

Balanced or Unbalanced...and I don't mean psychotic!!

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

Balanced leads.

'Balanced' circuits and leads offer the best method of minimising external noise in a PA system.

A balanced lead carries the signal on two conductors + shield. The signals on each conductor are the same level but opposite in polarity, i.e. one is positive when the other is negative. (In technical terms, it is 180 degrees out of polarity, Pin 2 provides a positive voltage and pin 3 provides a negative voltage when the microphone diaphragm receives a positive pressure wave that pushes the diaphragm inward. Thus, pin 2 is "hot"; pin 3 is "cold".)

A balanced input amplifies only the difference between the two signals and rejects any part of the signal which is the same in both conductors. 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 a differential input.]

The screen is there to intercept any external RF interference and drain 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, eg. 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 guitarists unbalanced pick-ups or leads.

If an unbalanced lead is being made up from a 'two-core-n-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 off.

Combining Balanced and 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

Europe vs. USA/Japan - DANGER!!!.
On three-pin XLR connectors it is universally agreed that Pin 1 is Earth/Ground, but for a long time there was a dispute between Europe vs. USA/Japan about whether Pin 2 or Pin 3 is Hot !!

This causes mega problems when mixing Balanced with Unbalanced circuits. For example, if Pin 3 is being used as HOT, and #1 and #2 wires are twisted together to form an Unbalanced line, and this is plugged into a piece of gear that has Pin 2 HOT, the HOT signal will run down the Earth/Ground braided shield - picking up massive amounts of noise. NOT GOOD!!

Nowadays, the universal standard is Pin #2 'hot'. If you are using older and new gear or are unsure what the wiring standard is, look up the manufacturers manual or on the specification plates on the back of the gear, they usually have a little diagram telling us 'what's hot & what's not.'

And finally...

No matter how good your cable, an audio link is only as good as its connectors. Don't skimp on quality, you'll pay for it in the long run with increased noise and crackle and general unreliability. Make sure that any cable clamping system that may be fitted is used properly, especially if the cables are going to be constantly plugged and unplugged. Most of our hassles with leads happen as a result of stress on the soldered connections inside the plug due to inadequate or non-existent clamping. The clamp should always grip the outer insulation of any type of cable, to take the mechanical stress. (In particular watch the power plugs. In the event of gear failure, if the earth lead has come loose, you become the path for the current to take to ground!!!!) Regularly take the covers off your leads and inspect the solder joints for cracking or 'dull' appearance. If so, resolder, easy!!

When using "off-the-shelf" adapters to convert circuits and connectors, signal loss will occur at every converter site as a result of changes in resistance and capacitance from one connector to another. Use sparingly!! Preferably, get off your butt, go and buy the best quality connectors you can afford, and wire-up your own. This way you can make something up that's usually far more quiet, reliable and stronger/tougher than anything pre-made.

Last of all, it is in our own best long-term interests to avoid having our leads in a big pile of 'spaghetti.' No matter what type of cable and connectors you have, the less kinks 'n' twists and general plugging and unplugging the better. Keeping cables tidy, in the end, saves us time, $ and potential embarrassment at a gig!

Well that's about it for this link in the chain. Next time we will be looking at the polarity and phase related aspects of leads.

In the meantime, if you've got any audio questions give me a yell. Send me an email or post a message on ROADTALKZ. [I'm not the font of all knowledge, but if I don't know the answer to your questions, I know enough tech guru's who will!!]

Until then, have-a-good-one!!

Willy T.

willyt-downunderdesk@roadogz.com

This is extracted from the book:
"LIVE-AUDIO WORKSHOP: A Hands-on Guide to Rigging and Operating PA Systems."
ISBN 0 646 36408 1. (c) 1998 Giraffe Communications.
Available via email or at all good Music & Book shops.