The Shure Model 730B Uniplex
unidirectional crystal microphone (1945)

General
The Shure Model 730B is a unidirectional wide-range diaphragm type crystal microphone. The true uni-directional cardioid characteristics of the Uniplex allows highly satisfactory operation under adverse conditions of background noise and reverberation where a conventional microphone would be practically useless.

The inner conductor or “hot” lead should be connected to the grid of the first tube in the amplifier across a load resistance of 5 megohms. Input resistances as low as 1 megohm may be used if necessary, but higher values are recommended because of the better low-frequency response obtained. The shield or ground should be connected to the chassis. See Figure A. If the Uniplex is to be used with considerably less than 25 feet of cable, a shunt condenser having 30 pf capacity for each foot of cable removed should be connected across the terminals at the amplifier. Otherwise, an increase in high-frequency response will result.