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SOURCES OF SOUND CURRENT 529
"Crystal" microphones, similar in principle and construction to the crystal-type disc reproducer, are also used for theatre purposes.
Microphone pickup in a theatre may be amplified by the sound amplifier, but in some cases a separate amplifier is provided backstage for this purpose. For reinforcing vaudeville it is common practice to conceal a second set of speakers in organ grilles or above the proscenium arch, the sound speakers being "flown" or moved aside to leave stage room. The vaudeville speakers may be supplied with sound by the projection room amplifier, a changeover switch connecting them in place of the sound speakers. Use of a second amplifier located backstage is not a necessity.
At the present time microphones and disc reproducers are installed in only a small minority of theatres, and even in those are for the most part seldom used. Most theatre sound is drawn from a photo-cell.
Transmission Line: Pick-up to Amplifier
(34) There are almost as many methods of coupling sound pick-up to the system amplifier as there are types of sound equipment.
The simplest method is by use of a short cable from each photo-cell to the system amplifier. These cables run to a double-throw switch inside the amplifier casing. That switch serves as a fader, selecting sound from either projector, as desired. Volume is controlled by a potentiometer in one of the amplifier circuits. See Page
When the photo-cell coupling system of Figure 120 is used, the length of wire between the transformer shown in that diagram and the system amplifier is of no importance. A fader or changeover may be built into the amplifier for use with this method of coupling, or may be mounted in a separate cabinet, the output leads from which are then wired to the amplifier. The volume control is a potentiometer in one of the amplifier circuits.
When the sound system includes a small photo-cell amplifier mounted on or near the projector (see Figure 118) the output from that amplifier to the main amplifier