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Electro-Music Engineer

Cables



Here's a comprehensive guide to mains, audio and speaker cables prepared by Ben Duncan

A well practised rigger tidying up after a major concert appears to take cables for granted when he neatly coils half a dozen leads with nothing more than a little wriggle in the wrist. Apart from such an apparent surface manifestation of attunement to cables, a good rigger also develops an instinctive awareness of cables which are damaged or suspect; an intuitive sense arising from prolonged - and often painful - practical experience. And for many musicians, 'painful practical experience' most aptly sums up the boringly iterative misbehaviour of leads and cables. Of course, badly wired, sub-standard connectors are frequently legitimate scapegoats in these circumstances. Nevertheless, the hassle-free application of cables in a live performance environment has its share of pitfalls too.

Covering materials



The cheapest and commonest mains cables make use of PVC or similar thermoplastic sheaths. Their prime shortcoming is a tendency to succumb to abrasion, deformation and outright damage from a host of live performance perils, including badly aimed soldering irons, well-aimed glass fragments, stampedes of steel-cloven foot-wear, antisocial door hinges, flightcases being dragged across the stage (sans wheels), and even the generous weight of an artic's rear axle!

Humour aside, it's sobering to recall that the casually abused PVC is often the sole material keeping sweaty palms away from Britain's uniquely lethal 240 volt juice. To recognise PVC's susceptibility to damage, and accordingly, to check PVC mains cables regularly is a sensible way to avoid abrupt power failures and fires, quite apart from electrocution. The potentially onerous task of inspection can conveniently become a part of the cable coiling routine, particularly if you run your fingers along the cable as you coil it up, feeling for serious lesions.

PVC's other antisocial habit is its misbehaviour in the face of temperature extremes. Below 0°C, it hardens and can become surprisingly brittle. Of course, audiences aren't renowned for attending gigs played in the arctic wastes of North Yorkshire, but apart from Icelandic concept albums, whenever cables are transported in freezing trucks, whether A1 northbound or on other cryogenic superslabs across the European and North American continents, frozen PVC is a distinct possibility.

In such extreme circumstances, allow warm air to restitute your leads before trying to unravel them.

At the opposite extreme, PVC becomes 'soggy' and easily melts outright when cables are thoughtlessly strung in front of heaters and stage lights. So for live performance rigging, it's wise to regard all vents, louvres and lamps as potential sources of surreptitious incineration, and steer cables well clear of them!

From a less spectacular viewpoint, PVC's predisposition to soften at high temperatures is a great aid to tidy and predictable coiling: by holding a PVC lead in front of a heater, or dunking it in hot water, it can be readily softened, coiled neatly into a convenient radius and then left to cool off, cold water being used to speed the process. The result is a 'trained' cable which will fall naturally into a coil without the need for bulky; expensive and impractical restraining devices such as drums.

Rubber is the principal up-market alternative sheathing material. It's relatively immune to temperature extremes and physical damage, so even if the expense of rubber forbids universal application, it's recommended for key mains extension leads. In comparison to PVC, rubber cables aren't so easily trained, the cable's natural radius being inherently determined by the length of time it's spent in a particular configuration. For new cables, this radius is frequently that of the manufacturer's drum, and the only foolproof means of training such a lead into a conveniently sized radius is to coil it into the desired form with brute force, using hefty tiewraps for restraint! The corsetted cable should then be left in a warm atmosphere for as long as possible. Of course, this situation is one of swings and roundabouts, and whilst initially stroppy, rubber cables rarely 'forget' once trained, unlike PVC, which requires frequent retraining.

Safety



As regards safety, cables complying with BS6500 meet the U.K.'s extremely rigorous electrical safety standards, and in practice, the vast majority of cables available from reputable suppliers in the U.K. are BS6500 approved. West European mains cables are mercifully standardised (or harmonised, in the unwitting advertising jargon of the very un-musical standards authorities!), thus nominal equivalents are intrinsically safe. However, USA mains practice is definitely not practicable in Britain, and American mains leads are best regarded as being potentially lethal. The same attitude should prevail in connection with oriental equipment, although to be fair, Japanese (Cf. Taiwan, etc) standards can be very good. Even if American or Oriental equipment is powered from an isolated 110 volt supply, a significant fire hazard can still exist. Replacing unsheathed skimpy 2 core leads with decently sheathed 3 core cables, together with competent line fusing and earthing of the equipment chassis is recommended in no uncertain terms.

Aside from leads falling out of shoddily wired mains connectors, experience suggests that the most common and certainly the most obnoxious olfactory faux pas of DIY stage power distribution is the aroma of the 'ignited drum'. This dramatic event arises from a misapprehension of cable current ratings. All cables exhibit resistance, and albeit small, it's sufficient to cause a slight temperature rise at the rated current. Indeed, we can regard any mains cable as a flexible electric element, with a tendency to dissipate between 1 and 50 watts. Generally, cables are rated for operation in 'free air', which implies an ability to dissipate this heat freely. But the danger of a catastrophic increase in temperature comes not so much by merely restricting air circulation (e.g., by taping cables onto a stage), but rather by bunching a number of cables - or lengths of cable - each radiating a small amount of heat, into a confined space. In this case, the rated current (viz: the amount of current a cable can handle before the insulation or conductors become hot enough to cause mutual damage or give rise to a fire hazard) falls dramatically, typically by an order of magnitude or more. Thus attempting to pass 13 amps - or even 5 amps - through the tightly coiled 13 amp cable on a drum can all too easily lead to unscheduled visits by fire appliances! The golden rule, them is to unravel every inch of mains extension cable, regardless of the length you require or the degree to which the cable's current capacity exceeds the power consumption of your gear. Then spread out the surplus length; to leave it in a tightly packed coil is only marginally less dangerous than leaving it on the drum.

Although the discussion above isn't encouraging, the wealth of 'not quite long enough' mains leads suggests a somewhat contradictory rule, at least as regards equipment cables. All mains leads should be made 50% longer than you think they need to be. Certainly, the 1½ metre cables supplied as standard on many items of equipment are frustratingly short in real life (though they do save the manufacturer 30p), and a much more practical standard length is 3 metres. Of course, the frustrations of improper length can be largely avoided if you make extensive use of pluggable mains leads (e.g., with Euroconnectors) and maintain a stock of varying lengths.

Audio Cables



As with mains cables, the basic choice of sheathing is between PVC and rubber, the latter being much the best choice for the rigours of microphone and stage connections, whilst PVC is fine for more sedentary occupations behind speaker stacks, between PA amplifiers and in 'outboard' equipment looms. However, whilst the ruggedness of mains leads in terms of the basic 'meatiness' of the cable cross-section is governed primarily by the current rating (i.e., a 20 amp cable is tougher than one rated at 2 amps, PVC or rubber sheath regardless) for audio cables, 'meatiness' and the integrity of the screening obviously have no such governing factor. Instead, these cables can be loosely classified as 'external' and 'equipment' types. External cables can be identified by their thick sheathing, often with supporting cords, numerous conductor strands and braided (i.e., woven) or foil screening, (see Table 1). Needless to say, construction of this calibre is the sine qua non for all external connections in sound equipment. Lamentably, run-of-the-mill music shops frequently offer only dainty equipment style cable, which whilst okay in its own right, is intended solely to lead an abuse-free life inside an equipment enclosure. Small wonder so many DIY microphone extensions and patch leads come to grief!

Table 1. Typical external and equipment cable specifications.


Audio cables broadly feature 3 styles of screening. Lapped screens (i.e., parallel wires wrapped around the central conductor(s) in a spiral) are prone to physical damage and degradation of their shielding abilities through flexing, hence they are usually restricted to equipment cables. The poor shielding properties of the lap screen is readily demonstrated by laying a lap screened guitar lead close to a mains cable. Hum! The braided screen is altogether a virtuous beast, being better suited to the perils of severe flexing and crushing, whilst its excellent shielding properties are reflected by the universal application of braided screens in radio equipment.

At the same time, the inherent efficiency of braiding has led less than scrupulous manufacturers to skimp the density of braiding hoping that nobody notices the difference in performance! The moral here is to resolutely inspect braiding prior to purchase and to demand cables with thickly woven screens whenever efficient shielding and physical strength are called for: a satisfactory braid will completely obscure the inner conductors.

Figure 1. Sample of Belden equipment cable with foil screen.

An altogether different animal is the foil-screened cable, epitomised by the name 'Belden'. Here, the centre conductors are wrapped in an all-embracing shield of aluminium foil, an idea which makes other methods of screening look rather silly; that is, until we laconically consider the difficulties of reliably terminating the laughably delicate and hard-to-solder foil. This minor shortcoming of the foil concept is circumvented by sandwiching a bare wire, known in American terminology as the 'drain', between the foil and the insulation of the inner cores. Although it's not soldered to the foil, the drain wire makes a good connection by virtue of being pressed against it throughout the length of the cable. And at each end, the drain wire provides a means of termination that's not only workable, but indeed, easier than the task of combing and twisting a braid screen.

Although it's not as hardy as braided cable, particularly under sufferance of severe kinking, the superlative shielding of the foil screen renders it the ideal choice for long cables carrying low level signals, where the potential for severe RFI (Radio Frequency Interference) is especially great. In a word, multicores! Foil screened cables of the equipment variety are also widely used en masse in racks for patching purposes, essentially because they're quick and and easy to wire up and they exhibit a natural springiness which can give rise to well behaved looms. They are also a natural choice in studios where the avoidance of spurious interference from Radio Moscow or Kid Jensen is rather more important than ultimate durability!

Similar in concept are cables with conductive plastic screens, e.g., Filotex or Turnkey's 6527. These are considerably more rugged in terms of damage through flexure, but they are still essentially for equipment rather than external cables, so for stage use, they're best reserved for situations where their excellent screening properties will be most appreciated, microphone cables being a good candidate.

Whether braided or foil screened, sloppily terminated screens frequently become self-defeating by enthusiastically shorting out live conductors. Serving or booting the exposed shielding with an expandable rubber sleeve (or binding sleeve) is thus a great aid to reliable termination, and the special sleeve expansion tool, colloquially known as 'honeymoon pliers' is an invaluable aid to craftsmanship when making up leads.

Being available in a variety of exotic colours, binding sleeves placed just above the connector are also an excellent aid to lead identification, being much more durable than taped mnemonics. Another garnishing which aids discrimination is to use a variety of sheathing colours (although this scheme is obviously less applicable if rubber cables are in use), typically black for mains, grey for mic. and line level leads, and orange for speaker connections. Going to even more zealous lengths, screened cables are available in a gamut of colours, and clearly this can make complex multimic or patching setups much easier to trace and check. As a means of cable identification, it's prone to errors arising from colour blindness, however. Thus it's inadvisable to adopt such a scheme where mistaken colours might have serious consequences.

Speaker leads



The general rule here is 'stout and short', and if you use male and female XLR plugs at each end of your leads, then it's easy to lengthen one that's too short. Thus generous lengths serve no purpose. Placing an amplifier rack equidistant from the speakers of a large stack implies cable lengths of 6 to 15 feet, and in this case, conductor cross-sections in the range of 1.25mm2 to 2.5mm2 (13 to 20 amps) are suitable per driver. Two core sheathed cables of these dimensions aren't readily available, but of course, 3 core mains cables can be used, the earth conductor being either chopped off or wired in parallel with one of the other cores. Alternatively, where Power Bridged and single ended (conventional) amplifiers are in simultaneous use, the third core can serve as a foolproof connection for the bridged outputs only, in conjunction with 3 pin XLR's, whilst maintaining speaker leads which are universally applicable.

For treble drivers, the concept of electrical damping has little significance and cables with 0.75mm2 (6 amp) conductors are ideal for lone units. However, treble horns are frequently mounted in arrays, up to 500 watts, their total impedance being circa 2 to 4 ohms, and in this case, peak currents of around 20 amps will be attained, calling for 2.5mm2 cable. In the bass and midrange, peak currents in kilowatt arrays quickly become astronomic (e.g., 112 amps for 5kW driving 4 ohm units), and without exception, drivers should be wired to amplifiers individually, or at worst, in pairs.

In small sound systems, boasting one lonely amplifier, running a long lead across the stage is naturally unavoidable, but it is possible to improve upon the habitual and stingy bell wire! If there's a lack of cash, the essential requirement here is something tough, with a rugged sheath, rather than to concentrate on thick and expensive copper, which governs power loss and definition in the bass. It's easy to show by calculation that X metres of nasty cable reduces the power available at the speaker by 30%, for instance, but in terms of perceived loudness, the loss in long cables is often insignificant, and it's arguably more important to seek a heavily sheathed cable, even if it has relatively thin conductors, rather than succumb to a sudden absence of music from one speaker.

Ominous Facets of the System



In broad terms, rock music and the reliable creation and transmission of that music are basic incompatibles, because musical creativity and the corresponding physical requirement - equipment versatility - imply diversifying tendencies and a corresponding lack of reliability. The ultimate sense of this paradigm is for connectors and cables to be either reliable, and useless, because they are inflexible, or versatile and flexible, yet unreliable, though in practice, the options are less clear cut. For instance, we can use a multipin connector to reduce the time taken to plug in dozens of leads, but flexibility vanishes and the reliability of all the connections is gambled on one expensive multipin connector, wherein any single failure may spell total failure. The alternative is to accept lots of leads which continually fall apart, but at least the arrangement is flexible and a spare lead can usually be found to take the place of a bad one. In comparison, with the multipin approach, total failure here is improbable, because it calls for multiple failures in temporal synchronisation; but partial failure is endemic. Discrete cables are also a less reliable means of connection purely in respect of confusing channels 6 with 9, ad nauseam... In other words, flexible, creative systems are particularly open to human error.

Clearly, neither approach is wholly satisfactory, though each has benefits, and wherever the cost can be justified, the dilemma can be waylaid by using multiple XLRs in parallel with a multipin connector, an approach exemplified on the rear panels of the best mixing desks. Such a system is also a workable way to frustrate the ubiquitous law of Murphy, unless you forget to bring your XLR patching leads, whereupon you can be sure the multicore connector will promptly disintegrate! From another viewpoint, providing it doesn't compromise flexibility or performance, reliability is enhanced by arranging your sound equipment to use the bare minimum of leads and connectors.

Table 2. Cables for stage use.
Order Code or Reference Notes (All cables PVC unless otherwise specified)
Maplin XR01B/2C Light duty mains for equipment and instrument connection, not rugged enough for stage use.
Maplin XR03D/4E/5F General purpose mains cable for equipment or instrument connection, also suitable for speaker connections.
Maplin XR09K/10L/11M Medium duty cable for mains power distribution, also as a speaker cable.
RS Components 377-906 Medium duty rubber cable for mains distribution.
RS Components 378-101 Two core 0.75mm2 PVC cable for general purpose speaker connections.
Maplin XR60Q High current speaker cable, sheath not rugged enough for stage use.
Maplin XR16S/18U/08J General purpose braid-screened cable for external leads.
Maplin XR15R/20W Light duty lap-screened cable for internal (equipment) wiring looms.
*(Various) RG 58/CU Military spec. RF coaxial braid-screened cable having low capacitance and being rugged - makes good guitar leads, where curly leads are disliked.
Future Film Developments/Belden/West Penn: "M01" 2 core foil screened cable for equipment racks and internal looms.
Future Film Developments "HDX" 2 core heavy-duty rubber-sheathed braid-screened cable for stage use.
Turnkey "6527" 2 core conductive-plastic screened cable for equipment racks and internal looms.

*This cable is universally available under this US MIL-SPEC reference, but beware of low grade copies arising from the CB Market.



Previous Article in this issue

Making Notes

Next article in this issue

Electric Drummer


Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.

 

Electronics & Music Maker - Apr 1982

Scanned by: Stewart Lawler

Feature by Ben Duncan

Previous article in this issue:

> Making Notes

Next article in this issue:

> Electric Drummer


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