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COMPONENT PARTS OF A SOUND SYSTEM 445
struction and operation. In many of the earlier sound systems some, or perhaps all, of the power conversion is performed outside the amplifiers, which in such case have nothing to do except amplify. If the reader will always and invariably make this distinction in his mind he will find his study of amplifier circuits immensely simplified. He will then avoid any possible confusion between the amplifying valves and their attendant apparatus, which perform one job, and the power conversion apparatus, inside or outside the amplifier casing, which does another and entirely different piece of work. Later on, when we come to analyze amplifier drawings, this distinction will be made, since it will immediately simplify some of the complication of modern amplifier blueprints by dividing them in half, each half to be analyzed separately. (18) In working on modern amplifiers (trouble-shooting or repair) the reader will find it helpful to distinguish clearly between the speech circuits and the power circuits ; doing so will take most of the difficulty out of his work.
Drives
(f) (19) The above classifications include all the purely electrical portions of a sound system, but leave* the mechanical parts to be considered. Sound is a sequence of events that can be made to create a mechanical or photographic record. This record can be made to produce, or "reproduce," a very close duplication of the original sound, provided it is fed into the reproducing mechanism bit by bit, exactly as it was made and at exactly the same speed.
A phonograph needle takes sound from a record bit by bit, but the original sound will not be duplicated accurately unless the record is played at precisely the same speed at which the recording was made. The film that excites the photoelectric cell must likewise be run through the projector at exactly the same speed at which the recording negative was made.
When the sound system operates from a microphone it is reproducing original sound, hence no driving apparatus of any kind is required. But when it reproduces a