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236 IX. SOUND-RECORDING EQUIPMENT AND ARRANGEMENT
ordinarily be made for them. Thus a microphone equalizer is intended to compensate primarily for differences in types of microphones rather than for individual differences between different microphones of the same make and model.
In addition to the microphone equalizer, a dialogue equalizer is usually associated with a microphone together with a high-pass filter to limit the amount of low-frequency noise transmitted, and with a low-pass filter to limit the amount of high-frequency noise transmitted. For convenience and for reducing the cost, all these equalizers may be combined and made up as a single network. As the output of a microphone is quite low in level, the introduction of an equalizer directly into the microphone output circuit would risk the introduction of consider
200-n.
Fig. 50. Correction equalizer for Western Electric RA 1142 microphone. This equalizer is also suitable for the Western Electric 639A and 639B microphones, and for the RCA 77B, 77C, and 77D microphones.
able hum and noise and would require magnetic shielding that is very expensive and probably inadequate despite its high cost. For this reason such equalizers are customarily designed to operate in the output circuit of the preamplifier following the microphone where the signal level is higher by 30 db or more than it is directly at the microphone output. At this higher signal level, the reduction in signal-to-noise ratio resulting from the use of the equalizer should not be serious for a well-designed and well-shielded unit; the increase in distortion and noise should be almost inaudible on even the very best monitor speaker system.
Figure 50 shows a correction equalizer designed for the Western Electric RA-1142 microphone. This equalizer maj^ be used with the Western Electric 639 types or with the RCA 77B, 77C, or 77D. As the re