Abstract
Although well screened against electric fields, the ribbon microphone is susceptible to interference from magnetic fields on account of the relatively large loop in the low impedance circuit formed by the ribbon and its connecting leads. The existing Type βAβ microphones design gives a maximum interference output of about 3 mV (open circuit on 300 ohms) for a 50-cycle magnetic field of 1 line per sq. cm. and this figure appears to be adequate in the majority of cases. However in locations exposed to the field from rectifier equipment heavy current circuits containing commutator ripple and from other sources including components at the higher audio-frequencies, the interference may be troublesome.
Experiments have been carried out with a view to reducing this magnetic pick-up. To this end it has been found necessary completely to re-run the internal wiring from the ribbon to the line transformer, but when this is done in the manner to be described and a fine adjustment carried out in an artificial interfering field an improvement of some 30 dB, on the original arrangement is attained.
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