In which Ben Duncan takes a break from mixing discs and instead, mixes electronics, philosophy and pragmatic advice to show that discotheque systems can be much more than a pile of indifferent gear plus mandatory Rosko-esque geezer at the mic.

At the Controls

The control desk is the nerve centre of the DJ's operations, and regardless of technical merits, the overriding requirement is to be on intimate and understanding terms with your console. Like a 48 channel concert desk, a Gibson or a Bonneville, it's an extension of your body and operating it must become instinctive. Another point that is rarely appreciated is that the desk is just another link (kink??) in the chain; later on we'll look at the discotheque as a whole, seek out the weak links in the system and look at ergonomics, but first a survey of the constituent parts.

Gramophonology

The ideal discotheque turntables are American broadcast models from Rusco, Gates and QRK, or the UK broadcast standard - the Japanese Technics SP10. Regrettably, a pair of these with matching arms will cost over £1,000 and most of us will have to make do with something a little less expensive. A good discotheque turntable is, of course, a single player, with two speeds (though 78 rpm can have its abuses!) and a cueing arm. Less obvious is the need for mechanical simplicity (which usually implies robustness and reliability), readily available spares and an arm that will comfortably track at 3-5 grams. Only a handful of turntables meet these criteria, and perhaps the best known and best loved is that exceptional budget Hi-Fi model - the Garrard SP25 in its Mk. III and Mk. IV guises. Although obsolete, second hand ones will be around for some years after stocks of spares have been exhausted. The only essential modification is to remove the superfluous bits that make up the autochanger mechanism. Rather than attempt to describe this operation, it's much easier for you to simply examine the deck and remove all the cogs and levers that appear inessential. Other useful modifications apply equally to other turntables:

(1) A piece of foam glued below the stylus will prevent damage should you drop the arm as it swings from the rubber mat to the arm rest.

(2) Wiring the mains and audio feeds via reliable connectors will make life much easier if you have to remove the deck in a hurry for repairs.

The SP25 III/IV has only a few faults. Sometimes the 45 rpm setting provides a very embarrassing 36½ rpm! The problem could be a badly worn pinch wheel (these should be checked monthly), grease or oil on the pinch wheel, the motor drive shaft or the turntable rim, or less often a motor drive shaft that has slipped down slightly, causing the pinch wheel to straddle the 33 and 45 rpm steps simultaneously. Another heartache is the ease with which the headshell snaps off, usually five minutes before the show! For this eventuality, a supply of cocktail sticks and PVC tape will bandage the damage until you have five minutes to replace the arm. Superglue could be used, but with the mind boggling possibility of being literally glued to your turntable for the remainder of the gig! Finally, the contacts under the headshell can make bad contact with the cartridge carrier, resulting in the sudden loss of one channel. Cleaning and re-springing the contacts may help, but often the best course is to replace the cartridge carrier.

Other low cost turntables for your evaluation include the old BSR models, the ubiquitous Technics models and the now obsolete Connoisseur BD2, although the latter turntable is unsuited to switch starting.

Many DJ's like to use slip mats, and whilst the SP25 can be used for slip-cueing, a far better turntable with a high torque motor is another obsolete Garrard model - the 401 (the fact that all of Britain's best disco turntables are obsolete reinforces the feeling that British industry is intent on self destruction). For many years, the 401, and its predecessor, the 301, were the basis of the standard BBC turntable, and their tank-like construction makes them perfect for discotheque applications. And because the 401 is out of fashion, it's readily available second hand at low cost - witness the classified advertisements in hi-fi journals. Since the 401 is just a turntable, you will have to find a suitably robust 12" arm. Most ex-Hi-Fi 401's will have SME arms; ideally, these should be exchanged for a broadcast arm (e g. QRK).

Mobile DJ's will often meet floors which are slightly uneven, and tilting the turntables only a few degrees can have amazingly nasty effects on sound quality. A spirit level allows you to check the tilt of the floor and beer-mats/cigarette packets can be used to cancel out any unevenness. Mobile work also involves the turntables in a lot of shaking around, and it's always a good idea to check the tracking weight is set correctly immediately after setting up and if the stage is wooden, a goose stepping colleague can seek out any potential record bounce problems at the same time.

Cartridges are much simpler. Ceramic cartridges are robust, cheap to buy and cheap to keep in stylii, and whilst the best ceramics are capable of reasonable reproduction, the equalisation required can be horrendously difficult to design. Contrary to popular belief, getting the best out of a ceramic cartridge requires much more than a high input impedance stage; rather, that is an excellent way of killing all the treble at the expense of thudding bass. In any case, a medium cost magnetic cartridge will always give better top end and the only drawback is the cost of replacing worn stylii. Domestic magnetic cartridges, especially the more esoteric types are exceedingly fragile, and will obediently self destruct (so decreasing profit margins) when subjected to back-cueing and excessive tracking weight, let alone being dropped on a record! Broadcast models are a must - these heavy duty cartridges are built to be thrown around, to withstand the rigours of back-cueing and the 3-5 grams of tracking weight necessary for bounce free discotheque work. In addition, the stylii are designed for good visibility (invaluable when you have 5 seconds to cue up track 7 - or is it track8?? - on a K-Tel album!). The almost legendary broadcast/disco cartridge is the Stanton 500AL, but the Stanton 680EL, the Shure SC35 and SQ39ED are also used. All of these cartridges come as near to audio perfection as is necessary in a discotheque, but they each have a characteristic 'sound' or colouration, and apart from cost, the choice is basically which sounds best to you.

Die Vinyl Solution

Considering the cost of replacing stylii, and that of replacing 2,000 ruined records, knowing when to replace the stylus is a natural headache. A good test is to compare the sound quality of a suspect stylus with a spare (virgin) stylus by crossfading quickly between two identical recordings - preferably good 12" pressings which are kept purely for test purposes. However, the virgin stylus should initially 'run in' on a well worn disc fora few minutes to polish the tip. Alternatively, a small self adhesive label, bearing the date of the last replacement on the side of the cartridge can be invaluable if you don't have time to check your stylii regularly and can only be bothered to change stylii at, say, quarterly intervals. Lastly, broadcast cartridges are invariably supplied in exotic boxes with hinges lids, reminiscent of jewel cases. These can be glued onto the space between the turntables and used to store record centres, stylus cleaning brushes and stylii covers.

Disc Amplification

Two conflicting requirements arise here - good headroom and low noise. They conflict because to keep the noise low, it's necessary to bring the disc signal up to line level as soon as possible. Once at line level (775 mV) we can start to process the signal without undue worry about adding hum and extra noise to it. However, low noise also demands boosted high frequencies when records are cut (and simultaneously bass frequencies are attenuated to achieve a sensible playing time). The resulting constant amplitude recording characteristic results in a very inconstant input to the disc amplifier, the treble being up to 20 dB higher in level than the midrange and the lowest bass being 20 dB down. Clearly, if we raise the midrange to line level in one jump to achieve low noise, an additional 20 dB of head-room or input overload margin is necessary to handle the high treble. In addition, 8dB's of extra headroom are required to handle heavily modulated, well pressed 12" singles with toppy synthesiser riffs, and another 10dB's or so are required as leeway, because many amplifier stages show significant distortion just before the onset of overloading. Thus we need at least 38 dB of headroom and if for the sake of low noise, we decide to amplify to line level in one jump, the output of the disc input amplifier will require a 110 volt peak output capability! A more practical alternative is to send the disc signal through a unity gain amplifier with treble cut - once the high amplitude treble is tamed, the headroom requirement falls to a more reasonable 20 dB and the next stage can happily amplify to line level. Unfortunately, this can be a noisy solution and a good compromise is to provide the input stage with a mild (15-20 dB) gain and treble cut, followed by a preset attenuator to allow for variations in the output levels of different species of cartridge, and finally another stage to bring the disc signal up to line level; this stage can also handle tape inputs.

Bass boost also has to be applied to one of the stages, and the choice is essentially a question of which stage will handle the burden of an additional feedback network without incurring excessive distortion. With this arrangement, an overload margin of 40dB can be readily achieved and the signal to noise ratio is only marginally (2 or 3 dB) lower than the theoretical maximum for bipolardisc input stages (around -70 dB, depending on the reference level). Fortunately, this degree of residual hiss is just adequate in the disco, for whilst ambient sound levels of 110-120 dB make the residual noise rather louder than it might be at home, hiss ridden moments between vocals and music will invariably be masked by your audience.

The need for good headroom is much more important and the commonly encountered 20-25 dB of headroom is inadequate unless you roll off all the treble above about 8 kHz; otherwise the sound will be 'gritty', distorted and very obnoxious at high levels. Finer aspects of disc input stages are discussed at length in the serious hi-fi and electronic journals, and remembering that your discotheque system is but a chain with links, expensive improvements, to (for example) the speakers might well go hand in hand with an improved disc input stage, because these are two particularly fussy links in any record playing system.

Tape Sources

Apart from discs, some DJ's use taped material in their shows, and to allow almost any tape machine, from Revoxes to nondescript cassettes of dubious oriental origin to be patched in successfully, regardless of output level or impedance, a high sensitivity (typically 50 mV), high impedance (50-100 K) stage is required, together with a fader to match the level to that of the disc output. A headroom of 20 dB will normally be quite adequate, provided the fader/attenuator lies before any amplification stages; often the tape input stage is designed for unity gain and is merely a buffer.

Patching in external mains powered tape machines can bring hum as well as Sister Sledge, and whilst ripping out mains earth leads may solve the problem at home, it's simply too risky in a public place rife with inquisitive fingers and spilt Guinness; Disco-electrocution may be spectacular, but the court damages can be somewhat sobering. Later, we'll look at safe ways of avoiding niggling buzzes.

In the next issue - console structure, crossfading and voice-overs.