Projection engineering (Sept 1929-Nov 1930)

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Page 22 Projection Engineering, May, 1930 NOTE.— GROUND MAY BE CONNECTED TO EITHER + 6 VOLTS OR-6 VOLTS Fig. 33. Condenser transmitter. Two stages of amplification. design, we may analyze the accoustical principles involved to some extent. To begin with, the diaphragm assumes under applied potential a paraboloid contour. Sound waves impinging upon the surface of this diaphragm cause it to vary between the outer limit of this paraboloidal curve and its uncharged normal position. If a plain electrode were used and the chamber between the diaphragm and electrode sealed to Fig. 34. Assembled microphone. prevent moisture from short circuiting the transmitter a considerable difficulty would be experienced from two effects; namely, the barometric change in air pressure would not be compensated for, and therefore the microphone instead of being allowed to assume this paraboloidal contour at all times, would be entirely dependent upon the barometric pressure. Also, the damping applied to the diaphragm by the small space chamber located between the back electrode and diaphragm would not result in either a uniform lineal response curve or a flat frequency response. It, therefore, is evident with these limitations in mind that the condenser transmitter in order to be practical must possess the following requisites : A. The space between the diaphragm and back electrode must be as small as practicable in order to insure a high factor of variable capacitance. B. The introduction of non-active capacity due to bulky electrode and frame construction should be as small as possible. C. An extremely high leakage resistance is of great importance in order to secure quietness of operation. D. A sealed and moistureless chamber is required in order to eliminate danger of the microphone sweating or forming moisture between the diaphragm and back plate under variant atmospheric conditions. As a rule, gas is introduced in the chamber instead of air, to eliminate this effect. E. A flexible back to the sound chamber must be integrally built in the transmitter in order to equalize the pressure on the front and rear of the diaphragm. F. The back electrode should be so constructed that the air damping compensates for the damping characteristics of the diaphragm and in this manner assure a flat frequency response curve. These conditions are fulfilled very well in the microphones displayed in Figs. 32 and 34. Condenser Transmitter Amplifier Construction The amplifier of any condenser transmitter should be considered as an integral part of the transmitter, as a considerable amount of compensation for the transmitter response can be introduced by correct design of the amplifier which is built in the case. The amplifier circuit shown in Fig. 31 is used in both microphones displayed in Figs. 32 and 34. By the constants of the condenser Ci and the resistances R, and R2, a wide variance in amplifier frequency characteristics can be obtained. This effect assists in obtaining excellent high-frequency response. An amplifier is constructed for the microphone shown in Fig. 32, of two stages of amplification. This amplifier allows the operation of the microphone at considerable distances from the mixer panel and is used quite extensively in motion-picture work. The circuit for this amplifier is shown in Fig. 33. The output impedance of the microphone amplifier generally is so arranged as to feed either into the mixer control circuit or into the 200-ohm facilities installed for carbon microphones. Considerable advantage is obtained by eliminating the repeat transformer generally used for carbon microphones and feeding the microphone Fig. 35. Microphone used in talking picture work. Fig. 36. Condenser amplifier. output directly into the mixer control. This practice eliminates the phase distortion caused by the mixer transformer and therefore is the more preferable system. Physical Construction of Microphone Cases The photograph of a microphone shown in Fig. 35 is probably best known to readers since this is the type widely used in the more modern broadcasting stations and in talking picture work. This particular type of condenser transmitter mounting is most practical for motion-picture work, as the suspension allows the microphone to be adjusted just outside the focus of the cameras without any great amount of difficulty. The condenser amplifier shown in Fig. 36 affords an idea of a very excellent type of amplifier mounting as the amplifier is completely swung on sponge rubber, in this manner eliminating any microphonic noise which might be caused by tube vibration. (To he continued)