Exhibitors Herald and Moving Picture World (Apr-Jun 1930)

58 EXHIBITORS HERALD-WORLD REPRODUCTION IN THE S. K. Wolf* April 12, 1930 THEATRE [Continued from last weekj 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 energy 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 energy 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 electrical energy obtained from the magnetic pickup for the disc, or from the photoelectric cell and PEC amplifier in the case of sound-on-film. The power amplifiers are used in addition to this gain amplifier to further increase the electrical energy so that sufficient power may be obtained from the last amplifier to properly operate the loudspeaker units. In a house of about 1,200 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 2,000 to 3,000 seats the gain and t\yo power amplifier are used. In houses such as Roxy’s with 6,000 seats and a million cubic feet power amplifiers are added in multiple. In the new municipal auditorium at Philadelphia there are about 24 of these amplifiers. MONITOR HORN 'C ONTROL HORN ..MONITOR HORN HORN HE' -INPUT CONTROL, ^FILAMENT CURRCNT 'meter METER' ■FILAMENT SWITCH* .PLATE CURRENT PUSH BUTTONS AMPLIFIER -FILAMENT HEY RN ELECTRIC FPE VACUUM (AMPLIFIER ) WESTERN ELECTRIC -239 TYPE VACUUM TU8E5( UNDER COVE WESTERN CLECTRK ‘20S TYPE VACUUM TUBES (RECTIFIER J METER ■STARTING SWITCH .PLATE CURRENT METER WESTERN ELECTRIC 211 TYPE VACUUM TUBES (RECTIFIER) :RN ELECTRICJ “E VACUUM ^ C AMPLIFIER ) AMPLIFIER ‘-STARTING SWITCH FIGURE 7 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 •Theatre Acoustics Engineer, Electrical Research Products, Inc. Ninth Paper of Technical Digest. Tl Pl each stage so that the voltage of the plate of each tube is kept at approximately 100 volts. In the circuit with these resistances are coils and condensers to smooth out the rectified voltage supplies for these sensitive first stages. Even the slightest knock or jar of the tubes is converted into electrical impulses which are transmitted through the system to the loud speakers where they appear as grating sounds. To prevent disturbances, the three vacuum tube sockets are mounted on a piece of sponge rubber which is fastened to a heavy steel plate and this plate is likewise suspended on a sponge rubber mounting. This suspension method is known as the inertia type and was the result of extensive experiment. The function of this sponge rubber is to prevent high frequency disturbances from reaching the output. The low frequency disturbances are filtered out because of the inertia of the heavy plate suspension. In order to control the gain or amplification of the system, a potentiometer is placed in the grid circuit of the first tube of the 41-A unit. This affords a gain control of 66 decibels in steps of three decibels each. A second means of controlling the gain of this amplifier is put in the grid circuit of the second tube. This gives an additional gain of 14 decibels. Gain controls could be put on the power amplifiers as well as on the 41-A unit, but this is not considered necessary. This gain control through the potentiometer should not be confused with the fader. The potentiometer is ordinarily set at the time of installation. Each of the power amplifiers consists of a single stage circuit known as a “push pull” circuit. This is shown in Fig. g. The power amplifiers operate entirely from alternating current. The 42-A and 43-A amplifiers shown in Fig. J consume approximately 80. watts and 300 watts respectively. The amplifiers each make use of our tubes, two as amplifying and two as rectifying tubes. The two rectifying tubes supply a plate voltage of 400 volts in the case of 42-A amplifier and 800 volts in the case of the 43-A amplifier. No means are provided for controlling the gain of these amplifiers. These amplifiers are operated by means of a three-position snao switch which controls the A. C. supply. In starting the switch is turned to the first position which lights the filaments only. After they have had time to become heated, the switch is turned to the next position which supplies the plate voltage, making the amplifier ready for operation. This procedure in starting reduces the strain on the vacuum tubes which would occur if a high voltage were applied while the filaments were still cold. The 42-A amplifier is capable of amplify ing the energy it receives 18 times or 25 decibels. The 43-A amplifier is capable of an amplification of six times or 15 decibels. The 41-A, 42-A and 43-A combination of amplifier are capable of an amplification of 10,000 times or 80 decibels. Where it is desirable to fill unusually large theatres, additional 43-A amplifiers may be added to increase the sound energy to any desired value. In a big theatre the entire circuit amplifies the little energy received from the photo FIGURE 0. FIGURE 9 electric cell literally millions of times before it is ready for transformation to sound energy again. LOUD SPEAKERS OR RECEIVERS After sound energy has been picked up from the sound record, transformed into electrical energy and amplified, it is then necessary to convert the electric energy into acoustic energy. For this purpose receivers or speakers are used. The types of receivers most used at the present time are the balanced armature, electrodynamic moving coil type and the electrostatic type. There are many other types of receivers such as the induction type, thermophone and piezo-crystal, all of which possess inherent limitations, making them impractical for sound picture reproduction. Most receivers make use of the fundamental electric principle which may be stated as follows : If a coil of wire is surrounded by a uniform external magnetic field and a current is per