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The Microphone Input stage

Thankfully, vocals amplification for discotheque applications is relatively free from the 'catch 22' syndrome inherent in the design of microphone amplification for recording or amplifying live music. To begin with, Agent Provacateur No. 1 — long cables — is mercifully absent. Additionally, the use and abuse suffered by the microphone at the hands of the average DJ is relatively predictable. Indeed, we can specify order of magnitude SPLs (Table 1) and thereby arrive at approximate figures as to the voltages arriving from the microphone; both sets of figures being based on close miking (at 1") and therefore giving worst-case voltages, in one extreme at least!

Providing you're happy to be restricted to the region of your console when making announcements, there's nothing especially wicked in the use of high impedance microphones, for with lead lengths of 6 feet or less, the usual lack of treble needn't be manifest. But, of course, the inevitably ragged response curve and poorly governed directional properties of low cost microphones are likely to give rise to howlround problems, the picture being complicated by two conflicting factors. Firstly: a DJ's microphone is usually situated particularly close to the speakers, and secondly; in mitigation, voice-over (attenuation of the music to make the vocals audible) is a classic and thus acceptable cop-out. Assuming, then, that you can achieve satisfactory results from a high impedance mic.the reward is a delightfully simple input circuit (Figure 1). R1 defines the input impedance, which isn't critical, though higher values will tend to improve the top end response at the expense of extra hiss. The gain of the op-amp is arranged so that the output voltage from a typical high impedance microphone corresponding to the SPLs generated by exuberant DJ's at distances of 1" to 3" (circa 110 to 125dBA) don't cause overload. R4 drains away any residual DC voltages arising from the next stage, so avoiding bangs and clicks when the microphone on/off switch is operated. The latter can either be panel mounted, or in the form of a footswitch, which is helpful if you break your arm or like to gesticulate. Note also the switched lamp, which illuminates a bold legend warning that the mic is 'live' and prone to blatantly broadcast potentially embarrassing comments; alternatively, it saves the humiliation of making announcements over a dead microphone! The lifespan of the lamp is extended by marginal underrunning, this being achieved simply by dropping the voltage to about 90% of the nominal voltage (i.e. 11V) with R1. R2 has the same aim: improving the lamp's reliability by reducing the turn on surge — its job is to maintain a 'tick over' current.

Figure 1. Input stage for high impedance, capacitor and electret mics.
(Click image for higher resolution version)

When the shortcomings of high impedance microphones are unacceptable, electret and certain capacitor microphones (notably the Calrec 600 series types) can make use of the same simple input circuitry without significant degradation of their excellent characteristics, and indeed, alternative component values suited to the Calrec microphones are depicted in brackets. Note here the revised input circuit wiring, wherein a non-standard 4-pin socket (e.g. 4-pin XLR) ensures that mics other than Calrec's don't accidentally receive the polarising voltage (45V), so avoiding potential damage. Aside from capacitor microphones, a substantially flat response and well-behaved directional properties usually spell a low impedance dynamic mic, and without the problems of long cables and the necessity to design for the ultimate in low-noise performance, we can again 'get by' with a relatively simple input stage (Figure 2). In this circuit, a transformer provides much of the voltage gain, but more important, it allows both the microphone and op-amp to 'see' load and source impedances respective to their needs, these being commensurate with low noise and good treble response. It is possible to achieve good results from a transformerless input stage, but the expense of a transformer is exchanged for an equally expensive op-amp having paralleled input devices, not to mention the extra circuit complexity. Returning to Figure 2, low impedance mics have, as a rule, balanced terminations, but the beauty of the transformer input is the ease with which unbalanced mics can be accommodated, simply by shorting half the transformer winding. Sometimes it may be necessary to use long microphone cables, and if this is likely, R1, R2 and C1 can be added to maintain stability under such conditions; their values should be determined empirically with varying cable lengths. Finally, the specified low-noise op-amp can be substituted by members of the cheaper BI-FET species (viz: TL071/LF351N) if a marginal degree of extra hiss can be tolerated. The answer to this question depends largely upon the noise generated by your audience versus the SPL, and hence the degree of amplification of your vocals.

Figure 2.
(Click image for higher resolution version)

Microphone Husbandry

Having purchased an expensive microphone, you should find that the gain you can achieve just prior to feedback will provide ample vocals intensity at a speaking distance of 3" to 6"; with greater proximity, the sound quality will be severely muddied unless the mic has 'vocals compensation' (bass rolloff) and an effective pop shield (see 'Sound on Stage', E&MM December 81). Susceptibility to howlround in a discotheque sound system frequently calls for fastidious mic positioning, largely as a result of the proximity of the speakers. Here, a boom stand (e.g. P+N 139) is an invaluable aid to finding — and maintaining — a trouble free mic position, as well as keeping both hands free whilst making an announcement.

Another advantage of boom-mounted mics is the ease with which the directional properties of your voice may be exploited, notably different vocal sounds are available by positioning the mic either above or below the lips.

Perhaps the most irritating aspect of using a microphone stand is the ease with which run-of-the-mill plastic mic clips are destroyed — usually with the aid of clumsy footwork. These are not cheap to replace, and yet a solid-metal spring clamp with rubber pads to provide a secure grip (e.g. Keith Monks 'MC1') will cost only twice the sum and probably last ten years. To end, bear in mind that whilst high-quality dynamic and capacitor microphones are rugged creatures, their unscheduled failure can have unpleasant repercussions; if your mic doesn't have a foam-lined carrying case, an old padded bag is an excellent, if shortlived, means of providing protection in transit.

Table 1. Peak SPLs and output voltages.

Typical peak SPLs with close miking
Normal conversational talking 106dBA
Exuberant talking 125dBA
Very loud laugh or shout 130dBA

These measurements taken with the lips 1" from the microphone mesh.

Typical microphone output voltages
Microphone Model Type Sensitivity Nominal Output Voltage
(per uBar) @110dBA @125dBA
Shure 515SA High Imp. moving coil 1.1mV 66mV 390mV
Calrec 654 Capacitor, unbalanced 1.5mV 90mV 532mV
Calrec 1051 Capacitor, Phantom powered 0.8mV 48mV 284mV
ElectroVoice PL80 Low Imp. Moving coil 250uV 15mV 89mV

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


Electronics & Music Maker - Feb 1983

Feature by Ben Duncan

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