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REPRODUCTION IN THE THEATRE 389
used interchangeably between film and disc pickup. As shown in Fig. 1 this change is facilitated by means of a transfer switch.
AMPLIFIERS
The energy produced by the pickup apparatus is not of sufficient magnitude to fill large spaces if it were transformed into acoustical energy. For that reason it is necessary to amplify the electric power of the pickup apparatus. The apparatus required for this amplification is a very important part of the equipment and must be carefully designed in order to insure against distortion of the original power obtained from the sound records.
Amplifiers have been designed in different sized units so that a selection of units may be obtained for the proper volume of sound for the widely varying size of theatres. In Fig 7 are shown three of the amplifiers used in the Western Electric sound projection system. These amplifiers may be classified as Gain Amplifiers and Power Amplifiers.
The gain amplifier is used for the purpose of amplifying the small electric power obtained from the magnetic pick-up or from the photo-electric cell amplifier to a level suitable for operating the power amplifiers, which amplifiers are to drive directly the loud speakers.
In a house of about 1200 seats, that is about 175,000 cubic feet, it is only necessary to use the gain amplifier and one power amplifier. In houses up to 500,000 cubic feet or from 2000 to 3000 seats the gain and two power amplifiers are used. In houses such as Roxy's with 6000 seats and a milion cubic feet power amplifiers are added in multiple. In the new municipal auditorium at Philadelphia there are about 24 of these amplifiers.
The first or gain amplifier in the wall panel is identified as the 41-A unit in the Western Electric System. Fig. 8 is a schematic diagram of the 41-A unit, a three-stage resistance coupled amplifier. The filaments in the tubes of this amplifier are connected in series and energized from a twelve volt battery supply drawing normally one-fourth of an ampere. The voltage supply for the plate circuit of this amplifier is obtained from the amplifier following, which has its own rectifier. This plate potential is obtained at 390 volts and is reduced by resistances placed in the plate circuit of each stage so that the voltage of the plate