International photographer (Jan-Dec 1934)

July, W34 T h INTERNATIONAL F H O T O G R A I ' 1 1 E R I 'went) ji: Motion Picture Sound Recording Chapter X (The preceding chapter served to introduce the discussion of the main amplifying equipment used in a sound recording utudio. This chapter continues that subject by describing the actual amplifying equipment and the usual manner of its arrangement in the amplifier room.) The Amplifier Room I HIS large room, as has been explained, is located near the center of the sound studio, and the connections to practically all of the equipment comprising the several recording channels of the studio are brought here to a common connecting point, known as a main distributing frame (or MDF). This arrangement makes it convenient for the engineer in charge of the recording room to locate trouble in any channel quickly and to make such semi-permanent changes in the line up of the channels as may be necessary. Running across the middle of the amplifier room are the bays supporting the main amplifiers and their associated speech equipment. A bay is a metal framework made of I-beams about six feet high and wide enough to hold panels nineteen-inches wide. These frameworks are placed in a row, seven bays being required for each two recording channels, and bolted to the floor. The connecting lines between the amplifiers and the MDF run down inside the I-beams to a metal gutter sunk in the cement floor of the room. Opposite the amplifier bays are the panels of the battery switchboards. These panels extend across the room for about the same distance as the amplifier bays of two recording channels, but they are not quite as high. It is sufficient to say here that these panels are equipped with a rather large number of knife switches and meters, the arrangement being such that any set of batteries (storage -^WWV""*' — ; ^WV^r— *• [nbu-t 500 ohms Outjsut 5oo ohms YiQUrc 1 Ttupe Attenuator .ai rijp switcL ^Volta -dividiti<| f«si'sT3>ite Input transf /volume Ind> meter -c To Vol i-nd'r extension ynoter ■O ormer Ficjure 2. The Volume Indicator batteries of several sizes being used for both filament and plate supply for the tubes in the amplifiers) may be connected to the generators for charging or used to supply current for any or all of the recording channels by closing the proper combination of switches. Rheostats in the fields of the charging generators and the ammeters provide the By Charles Felstead, Associate Editor engineer with means for regulating and checking the rate at which the batteries are charged. All battery circuits are fused at the switches. The Main Recording Amplifiers We will consider now only the three bays that compose the main amplifier system for a single recording channel; for there is normally no difference between the amplifiers of different channels. The center one of the seven bays that make up a two-channel amplifier installation carries only the jack field that was mentioned in the preceding chapter. These jacks will be described more fully later. The right-hand one of the three bays carries the actual recording amplifiers ; the center bay supports the volume indicator panel, a smaller jack field that provides input and output connections for the speech equipment in that channel, and auxiliary equipment ; while the left-hand bay holds only the monitoring amplifiers that supply the two large horns in the monitor room and smaller horns in the recording and amplifier rooms. All panels are of iron, painted black like the supporting frameworks, and fastened in place by machine screws. Spaces on the frames where there is no equipment are filled by blank panels. The back of each piece of equipment on the frames is covered by a metal can cover that serves as both an electrical shield and a dust cover. At the bottom of each bay are terminal blocks, like those on the MDF, to which are connected the lines for all the equipment on that bay. Mounted beside these terminal blocks is a row of "grasshopper" fuses, one fuse for each battery circuit in the bay. These fuses are constructed and arranged so that when one burns out it raises a little flag to permit it to be found easily, turns on an alarm bell, and lights a bulls-eye signal light on the top of the bay in which it is located. By replacing the fuse these alarms are all automatically reset. There are five recording amplifiers in the standard recording channel, all of them mounted in the right-hand bay. The main amplifier has an input impedance of 200 ohms to match the impedance of the transmission line from the monitor room. This is a three-stage affair, the first tube being of the voltage amplifier type and the other tubes being of the power amplifier type, although low grid and plate voltages on the middle tube keep it from operating as a true power amplifier. The three tubes are coupled by means of impedance coupling, but transformer coupling is used at the input and output of the amplifier. A volume control having twenty-two steps, each step being two decibels higher than the preceding one, is provided on the panel. The grid-bias, or C, batteries for this main recording amplifier, and for all other amplifiers in the channel that do not obtain their grid-bias voltages from their filament or plate batteries, are mounted in a small wooden box on the panel where they are handy for checking or replacement. These C batteries are flat batteries of the flash light type and make contact with springs in the bottom of the wooden box. Rheostats are provided for regulating the filament current of the tubes in this, as well as in all the other, ampli(Tum to Page 30) Please mention The International Photographer when corresponding with advertisers.