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BELL SYSTEM TECHNICAL JOURNAL 



the coupling amplifier be not correspondingly decreased, since, for 

 efficient operation, the high impedance of the microphone necessitates 

 a close spacial coupling between the two. 



Use of a special miniature type vacuum tube affords the possibility 

 of materially reducing the size of the coupling amplifier. It is but 

 slightly greater in diameter than the microphone just described, the 

 inter-electrode capacities are quite low, and the plate resistance is not 

 too high. This tube, with the necessary coupling resistors and a 

 small stoppage condenser are placed within a cylindrical metal tube 

 of about 0.8 inch diameter, to one end of which is attached the con- 

 denser microphone. From the other end of the cylinder extends a 

 shielded cable along the axis of which runs the plate lead from the 

 vacuum tube. This cable is constructed so that the capacity of the 

 lead to ground is small. Surrounding the lead are two filament 

 supply conductors and a conductor for supplying polarizing voltage 

 to the microphone. The impedance between these and ground being 

 very small, their capacity to ground may be as large as desired. 

 Of course, the longer the connecting cable, the lower must be the 

 capacity of the plate lead per unit length. The second stage, to 

 which the cable runs, may be large, and as distant from the miniature 

 first stage as is consistent with these lead capacity requirements. 



Fig. 4— Mew of miniature microphone and attached miniature* coupling aniphfier, 

 together with second amplifier stage. 



