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1118 HANDBOOK OF PROJECTION FOR
applied to the main amplifiers by the pick-up amplifiers.
That, gentlemen, fully explains the principle upon which the fader operates, though of course Fig. 406 does not illustrate the actual mechanical means by which the fader operates. That will be explained further on.
Some may wonder why a variable resistance in series between the pick-up amplifier and the main amplifier could not be used. In other words, a resistance connecting at B, but not at A. The answer is that the resistance would have to be enormously great in order to bring the power anywhere near zero, whereas by the arrangement shown in Fig. 406, all that is necessary is to bring C to A to stop all current flow in the main amplifier circuit. Moreover, it is a law in the design of electrical equipment of this kind, that as a general rule it works best when connected to a resistance or load similar to its own internal characteristics.
A, being a fixed resistance insofar as has to do with the pick-up amplifier, meets this requirement, which a variable resistance could not be made to do.
Examining Fig. 407 we see what the general set-up is. Tracing the heavy, black lines, which represent the circuits carrying sound current ( Important : Each heavy black line represents a 2-wire circuit. Remember that fact or you may become confused), you will observe that the fader is located between the motion picture projector film pick-up amplifier (the first stage of amplification) and the main amplifiers. At A are the switches used to change from film to disc, or vice versa, where combination pick-up equipment is used. These switches are located on the upper part of the fader panel.