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Electro-Music Engineer

Audio Connectors


Whilst connectors are very literal 'links' in the belaboured musical communications chain, their simple role belies the fastidious attention they deserve - at least in respect of initial choice and installation. For connectors suffer almost ritualistic physical abuse, greater than that offered to any other stage or studio equipment with the exception of drumsticks, headphones and microphone stands! It's perhaps out of context, but nonetheless a sobering thought to remember that £100,000 of sound equipment frequently hangs on the end of a weary jack plug during a solo performance on stage.

Similarly, certain key connections - notably mixing desk multicore and power plugs impose a ransom on the whole sound system, demanding to be made as reliable as money and ingenuity will allow.

Countless sound system failures occur unnecessarily and many can be traced to misguided visions of prudent economy via low cost plugs or half-hearted wiring up, coupled with an ignorance of Murphy's law: "If anything can go wrong - it will, and at the worst possible moment." If your favourite, but utterly decrepit guitar lead has caused concern amongst your colleagues, but has 'never let me down', you can be sure it's still awaiting the worst possible moment!

Figure 2. Common Jack connector termination styles.


Jacks



Being a single pole connector, the standard ¼" Jack is extremely versatile; it's also easy to wire and cheap. These factors make it the mainstay connector in small sound systems, and provided you eschew false economy and use high quality soft plastic or metal bodied versions, viz Rendar (RS) or better still, solid brass bodied types, e.g. Switchcraft and Whirlwind, then the problems of ill-timed disintegration need not apply.

However, the high quality jacks tend to make use of screws in their assembly, and these have been known to slacken under duress of bass-induced vibration. Even so, potentially slack nuts are preferable to the loose and flimsy rivets so common in low grade jacks of oriental origin, and the problem can be deftly side-stepped with a drop of thread locking compound e.g. 'Loc-tite'.

In studios and extensive PA systems, there's a need for a multiplicity of stereo and/or send-return connections. These requirements can be handled with versatility by the standard jack, viz, inverted lefts and rights can be readily straightened out. By the same token the margin for errors is much greater, so provided the wiring is initially correct, the use of stereo or 2 pole jacks is preferable - and can also save a lot of panel space in large installations. Unfortunately, this convenience only applies to unbalanced lines - for the balanced variety, we would need a four pole jack or a pair of two pole jacks.

The beauty of jacks in applications of this nature (i.e. patching) is the ease with which they can be pushed in and pulled out, and their ability to perform switching operations in conjunction with the socket. Using standard ¼" jacks, we're restricted to something akin to a double throw switch - insertion of the jack re-directs the signal out via the plug. However, by using the 'GPO', 'Telephone' or 'B gauge' jacks - these feature a smaller tip - the versatility of the switching functions can be greatly augmented, jack insertion being able to switch auxiliary functions such as lamps, apart from re-routing the signal.

Additionally, by using the 'B gauge' pattern, the need to use two plugs for stereo, balanced line operation is overcome, because 4-pole (2-finger) plugs are available. Note however that the'B gauge' plugs are not compatible with standard jack sockets, and that attempts to ram a standard jack into an apparently normal socket of the surreptitious 'B -gauge' variety will damage the latter.

Apart from their limited latching abilities, the major shortcoming of the jack is that plugs are always cable mounted and it's all too easy for the inebriate to plug microphones into power amplifier outputs! In addition, during insertion and withdrawal, the live tip is prone to short on the chassis terminal of the socket. For this reason, these connectors are best restricted to mic and line level stages, where little harm can occur, and they're best avoided altogether for DC distribution and high power (>100 watts) audio connections.

Figure 3. Common XLR connector termination styles.


The XLR



The 3 pin XLR or 'Cannon', having tripartite advantages over high quality jacks, is the alternative up-market universal audio connector. Firstly, it features a latching facility which neatly obviates the tendency for plugs to fall out/be torn out by clumsy footwork at vital moments. At the same time, the latch is readily disenabled, which makes the XLR suitable for patching purposes (where the space and expense can be justified), or wherever cable tension causing disconnection of the plug is preferable to causing equipment to topple over, viz: microphone leads at the amplifier end. Also, male and female XLR plugs (or cable mounting connectors) are very definitely assigned to inputs and outputs respectively.

Apart from being a universal standard, the male to female XLR cable also lends itself to being plugged together, not only to make extensions, but to outwit the tendency for cables to turn into spaghetti en route! Finally, all XLRs feature excellent cable clamps, so once they're properly assembled, they will withstand months or years of severe abuse before the cable termination succumbs.

There are broadly four makes of XLR, each with foibles and of varying price, available in Britain; and compatibility isn't automatically assured, as successful mating is crucially dependent on fine tolerances. So it's wise to seek the conformity of one manufacturer from the outset. At the same time, the purchase of XLRs is a long term investment, involving a large outlay, so choosing the right XLR - one which will suit your needs best - is equally essential.

The ITT-Cannon XLR, also available under the RS brand name is the most expensive of the four, but it features rubber insulation on the female inserts, which makes it particularly durable and resilient to misalignment. As a result, Cannon plugs and sockets are frequently the exclusive female connector in systems using other varieties of male connectors. The main shortcoming of Cannon XLRs is the use of non-captive grub-screws involved in their assembly. These frequently go astray, to make wiring up back-stage very tedious, although once they're in place, a drop of threadlock compound will keep them captive for some time. If all else fails, kits of spare screws, bushes and clips are available from Cannon distributors such as PSP and Future Film Development.

The Switchcraft, Neutrik and Deltron types feature hard plastic insulators throughout, which makes them more prone to shatter, but they are all considerably cheaper and whereas they are not entirely devoid of small screws, assembly in awkward situations tends to be much easier.

In general, the PA business is joined together with Switchcraft and Cannon XLRs - mainly because these are the most rugged, and the two styles usually mate and latch without tolerance worries. Also, the Switchcraft plugs feature a chassis terminal; this makes an extra terminal available in DC power distribution and other idiosyncratic applications, quite apart from giving greater flexibility in groundlifted and quasi-balanced sound systems.

Deviations



At some time, we all have to interface items of equipment that sport sub-standard 'consumer' terminations of the DIN and Phono variety. If you own the offending equipment, tearing out the offending sockets and replacing them with jacks or XLRs will save no end of hassle in the long run. Frequently, however, this approach is a little too extreme, particularly as regards the re-sale value of the equipment in cosmetic terms. As a compromise, replace the sockets with an up-market equivalent, (viz: gold-plated phonos and latching or screwlocking DINs). Then, using corresponding up-market DIN and phono plugs, wire into a diecast box containing the appropriate jack or XLR sockets. The box can then be bolted to the equipment, or simply retained on the flying umbilical cable. The general idea is to avoid unplugging the domestic style plugs, for if treated as permanent connections, their lifespan will be greatly enhanced, the 'patch box' can also usefully bring the awkward-to-reach sockets out into an accessible position.

Of course, the above arrangement will also suit equipment you don't own, but a more flexible solution is simply to make up a number of leads to convert other standards to your own. As Murphy's Law ensures that DIN sockets invariably beg for connection to XLR systems when shops are closed and soldering irons have been left at home, the motto for on-the-road is to be prepared for all manner of ad-lib connections.

Figure 1. Jack connector wiring.


Wiring up



High quality plugs do not alone make for reliable and hassle free interconnection; a partnership with skilled wiring is paramount. The pre-requisites for the latter are good tools, good lighting and a relaxed atmosphere. As the musicians' environment habitually lacks these, it makes sense to wire and check your connectors at home or in the workshop, so that on-the-road repairs can be as infrequent as possible. Figure 1 depicts the elements of a well wired jack plug, the essential requirements being two-fold. Firstly, the uninsulated wire - including whiskers - should be cut flush to curtail the opportunity for short circuits. Secondly, due attention should be given to strain relief, both tensile (tearing the cable from the plug) and flexure (bending the cable causes metal fatigue) by testing the action of the cable clamping and bushing for each combination of plug and cable style. If you lack the confidence or the enthusiasm to become skilled at making your own cables, the services of specialist audio cable makers are worthwhile, particularly as regards the most vital leads. Alternatively, the heavy duty, ready-made guitar leads (e.g. Whirlwind) or XLR cables (e.g. Red Devils) are an excellent investment.

Finally, a plea as regards the poetry of terminology. Although a number of unprosaic people insist that a plug is any male connector, viz, something with pins, regardless of whether it's cable or chassis mounted, a more down to earth attitude is to use the term plug for anything hanging from the end of a cable, and likewise, a socket is a fixed, or chassis mounted connector. The arrangement is then made totally unambiguous by adding the terms male or female to specify pins or holes. Thus, a Switchcraft 'A3F' XLR, or in shoddy electronics industry parlance, a cable socket, is termed a female cable plug. A description of this kind can attain a uniquely unambiguous vitality through imagery. In other words, use the description to build up a mental picture, and you won't waste time and money buying or wiring up inappropriate connectors.



Previous Article in this issue

Making Notes

Next article in this issue

Understanding Electronics


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

 

Electronics & Music Maker - Jan 1982

Feature by Ben Duncan

Previous article in this issue:

> Making Notes

Next article in this issue:

> Understanding Electronics


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