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INTERNATIONAL PROJECTIONIST
VOLUME V
NUMBER 4
JUNE 1933
TROUBLES ORIGINATING IN THE P.E.C. AMPLIFIER
A. C. Schroeder
MEMBER. LOCAL UNION 150, L A., LOS ANGELES, CALIF.
EXTRANEOUS noise is the bane of good sound reproduction and one of the most serious difficulties with which the projectionist has to contend. Sometimes such noise is only a faint ticking or rushing sound; sometimes it is so bad that the show must be interrupted. The chief difficulty lies not in correcting such trouble but in locating it.
Careful manipulation of the fader or the gain control sometimes is very helpful in determining where such noise originates. The effect in the theatre of such procedure is none too good, biit this method has one redeeming feature in that it facilitates discovery of the trouble. Raise the fader as far as you dare, then lower it as much as possible and yet be able to hear the sound. While doing this, notice whether the noise varies in volume the same as the desired signal does, or if the noise remains at the same level while the signal goes above the noise and then drops below it when the fader is turned down.
Obviously, if the noise stays at a
constant level it is not affected by the fader and it therefore originates at some point further on in the system. Should the noise vary as the fader is moved, it shows the trouble to lie ahead of the fader.
Common Trouble Sources
Most noises originate ahead of the fader. This is due to such things as vibration of the machine, affecting all parts of the photo-electric cell amplifier; the optical system and the exciting lamp, difficulties at the sound aperture, and the low level of the signal. Assuming that the signal applied to one of the lubes in the p. e. c. amplifier is .1 volt. It is quite likely that a loose connection in the amplifier would put an unwanted signal (noise), on this grid easily amounting to .1 volt and often much more. The signal and the noise would be amplified to the same degree, thus the sound in the horns will consist of as much or more noise than signal. The signal voltages on the grids of tubes in the power amplifier stages run much higher.
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Suppose the voltage on one of the grids is 20. If, through a fault in the amplifier or outside trouble, a noise is introduced causing a change of .1 volt on this grid, the noise produced in the horns will be so slight that it will be impossible to hear it.
This comparison shows the vast difference produced in the horns by impressing the same atnotmt of noise in the p. e. c. amplifier and in one of the power amplifiers. It must be remembered that the level of the noise introduced in the final stages will probably be higher than the level of the noise introduced in the p. e. c. amplifier, but it very seldom is so great that it will produce noise in the horns comparable to that originating earlier in the system.
If a change-over is about to be made, one can often tell in what part of the system the noise is by noting if it stops when the sound is switched over to the other machine. Should the noise stop it is quite certain that it originated in the outgoing machine. However, if it does not stop, it is not