The console's monitoring circuitry is first and foremost a means of cueing in discs and line sources by ear; nevertheless, it's helpful when signals go amiss to be able to extend the monitoring facility to other areas in the console, this scheme being akin to the metering facilities. There are three ways of interfacing the monitor amplifier, depending on the style of switching which suits you and the degree of flexibility you require.

In Figure 1, a single rotary switch is used. This is a simple and compact arrangement, but it isn't really suited to the rapid A-B comparison of sources at opposing ends of the switch. Additionally, if two or more sources are available simultaneously, it's necessary to carefully count the switch clicks or peer at the panel legend to be sure you're hearing the signal you require! The series circuit in Figure 2 solves this niggle by making use of discrete SPDT switches; note that the switches towards the bottom (on this diagram) have priority.

Mounting can be in a line — with enough space for large paws(!) — or the switches may be spread around the panel, being sited adjacent to their associated faders or EQ controls. In this case, the priority of the switches in relation to their function must be arranged thoughtfully; otherwise the desperate operator may be unable to hear 'disc A' because another, obscurely sited switch in the series with priority over 'disc A', has been inadvertently switched on. This underscores the expedience of choosing switches which boldly display their status, e.g. toggles, illuminated push buttons or lever switches.

In Figure 3, the problems of serial switching are overcome by enabling each source via an independent SPST switch, their outputs being summed. Thus any combination of sources may be monitored in rapid succession, A-B'd — or even heard mixed together.

Although they call for differing degrees of dexterity and attention from the DJ, the three switching techniques have but one aim — to enable the show to run smoothly, and the choice should accord with whichever method will enable you to do this best. At the same time, the switches used for cueing will be amongst the most used in the console, so be sure to invest in reliable switches with the right 'feel'; this is not the place for 'agricultural' switchgear!

Amplification

Figure 3 also shows the general arrangement for the remainder of the monitoring circuitry. If the only supply rails available are ±15V, then the TDA 2030 IC power amplifier is a simple and cheap general purpose choice for driving headphones. Attention should be paid to the powering arrangements, however, for whilst most 'cans' will usually be driven to 500mW at most, peak currents circa 200mA may be drawn from ±15V rails; apart from reviewing the power supply capacity, careful routing of the amplifier's supply cables — in particular the 0V rail — will be necessary to avoid oscillatory misbehaviour from the preceding small signal stages.

Referring again to Figure 3, and noting that small DC offsets considered innocuous to loudspeakers can be effective headphone assassins, C1 and C2 are wired back to back to protect the phones from DC offsets to either polarity, whilst R1 prevents a build up of charge, which would otherwise cause the headphones to 'thump' when plugged in. R2 limits the potential 10 watts or so of output power to a value roughly equal to the capacity of the 'cans'; although apparently messy, this technique is simpler and cheaper than designing and debugging a precisely tailored discrete stage. At the same time, ten watts can terminally damage both ear drums and headphone drivers, so be wary of being unduly generous when selecting R2's value!

Some consoles feature a single high current 12V auxiliary rail for powering switches, relays and the like. Being apart from the critical small signal audio rails, it's an ideal power source for a monitor amplifier. In this case, IC power amplifiers intended for single rail operation are most suitable, e.g. the TBA810S or better, the LM383/TDA2002. Being single ended, an output isolating capacitor will be required to partner these amplifiers as a matter of course, and since this component is invariably provided for on the PCB, C1 and C2 may be omitted.

The sound pressure battle

When in proximity to particularly high power speaker stacks busily displacing the air with heavy metal bedlam, the DJ is frequently unable to hear the more subtle aspects of the record he's intent on cueing up. Indeed, such sonic assault can render the intuitive approach to choosing the next record very dicey. Apart from investing in headphones featuring good insulation against ambient sound, signal processing may also be called in to play. As the ear is most sensitive in the midrange, a simple means of making the monitor output 'cut through' is to remove high and low frequencies with an equaliser, and then boost the resulting midrange-ridden signal.

This stratagem is somewhat similar to that adopted for stage monitoring about a decade ago, and the unpleasant and damaging results of listening to telephone quality music for several hours at levels of 110dBA+ are by now well known; ask Roger Daltrey or Pete Townshend! In short, unbalanced 'middy' sound rips the ears and the temporary threshold shift it causes is indicative of potential hearing damage. Thus telephone quality monitoring, whilst effective, should be restricted only to occasions where short bursts are all that's necessary to cue-up.

A more elaborate alternative, better suited to those who regard headphones as clothing is to compress the signal. Compression will bring the subtle and quiet sections of the record into prominence and permit higher average monitoring levels with less risk of driving both the monitor amplifier and your ears into prolonged clipping. At the same time, it pays to be aware of the ability of compressors to convert music into Musak, such that judging the impact of the energy and dynamics in an unfamiliar piece may be impossible.

Headphones

Although a cheap pair of headphones costing only £10 can provide acceptable sound quality, and discotheque monitoring doesn't call for high fidelity results, the right choice is more subtle, being governed by other parameters; principally physical robustness, reliability, the cable pattern, acoustic isolation properties and comfort.

Suitable disco headphones must withstand being thrown to the floor and then trampled on. Their cable anchoring must be superb, and the driver compartments must be readily accessible so that cable or driver failures and intermittent connections can be made good, or at least verified. A 'no tangle' cable pattern simply demands that the cable enters at one side of the headset only; the trendy 'Y' pattern cables can all too easily become a vexatious embroilment when you desire to tear the cans from your head.

Equally annoying is straight cable, the length of which alters in inverse proportion to your distance from the console: here, a number of strategically placed hooks along the underside of the console stand lip can sometimes be usefully employed to guide a long cable out of the entanglement zone. The elegant alternative — curly cable — is often feared because of the tendency for internal fractures to occur spuriously in the 'curlies' fitted to low cost oriental 'phones. This fear is strictly irrational, and you need only spare a moment's thought for the tens of millions of British Telecom curlies which survive the daily abuse meted out to them. Lamentably, standard British Telecom curlies are a trifle too short for our application, but reliable three metre curlies are readily available from most electronic component suppliers, without the need to explain to a GPO engineer why your telephone lacks a cable!

Acoustic isolation is at best a compromise. Unless you're prepared to part with several hundred pounds in exchange for a pair of helicopter pilot's cans that will squeeze like a mousetrap and distort the finer features of your visage accordingly, then the 20 to 25dB of isolation available from a well padded pair of headphones will have to suffice. Many manufacturers fail to specify the isolation parameter in their brochures, in which case it's advisable to insist on testing the goods; hunky padding doesn't necessarily indicate good isolation.

In return for greater comfort, lightweight 'phones offer greatly reduced isolation, typically 10dB in the midrange, and little if any at low frequencies; thus higher sound pressure levels (SPLs) will be called for to achieve equivalent audibility, and aching in the temples and ear lobes is simply exchanged for the psychological fatigue of high SPLs.

The options of headphone ownership are broadly twofold. A cheap oriental pair with an amenable balance between acoustic isolation and comfort will give good service provided they're treated with due respect. Moreover, you will probably be able to afford a spare pair as a backup, in which case the suspect reliability of low cost 'phones needn't be a deterrent. Hands up those of us who've arrived at a gig to discover that the cans are still back at home plugged into the Hi-Fi?(!). If you frequently indulge in this frustrating habit, the value of carrying a second pair in the 'disco kit' is self evident!

Alternatively one can invest in a pair of high quality yet robust cans which can be 'shared' with the domestic Hi-Fi or used for sundry other electro-musical pursuits. The Beyer DT 100, 202 and 480 are highly recommended here; indeed, these rugged models are virtual music industry standards in the UK. Beyer also produce a single earpiece version of the DT 100 — the DT 102 — especially for discotheque applications. Finally, a simple task that begs attention regardless of the headphones you've chosen is the ruthless pruning of the ubiquitous moulded/hard plastic plugs, these being replaced with a high quality metal-bodied connector.