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450 RICHARDSON'S BLUEBOOK OF PROJECTION
Speakers: d. c. always. From 7 to 110 volts or higher: seldom more than 5 or less than 1/100 ampere.
The above list represents the actual, or primary requirements, of average sound systems. Since power lines do not furnish electricity in such variety means of adapting the power supply to sound purposes must be installed in the projection room. (2) Power converters are often built into speakers, amplifiers, etc., as integral parts of those units, in which case a speaker, for example, may seem to be a. c. operated.
Dry Cells
(3) Dry cells are no longer used to supply any of the voltage or current requirements of sound systems, except in a very few amplifiers in which a special, acorn-shaped form of dry cell is used to supply "C" bias to the grids of amplifying tubes. This bias consists merely of voltage applied to a permanently open circuit. No current flows. Therefore the drain on the dry cell is zero. An ordinary dry cell of the flashlight type deteriorates even on the shelf because of slight impurities in its internal materials; but exceptionally pure materials are used in these special "C" bias cells and they maintain their voltage indefinitely.
Line D. C.
(4) Direct current from a commercial power line cannot be used for all d. c. power purposes. It is, for example, useful as a source of supply for speaker fields, but useless for photo-cell supply. The distinction is very simple. The photo-cell has amplification behind it; the speakers have not. Any sudden voltage changes, ripple or other unsteadiness in the power line would be amplified if that line were used to bias the photo-cell, and commercial d. c, being commutated a. c, always has some ripple. The same irregularity in the loud speaker supply does no harm since it does not undergo amplification. At worst, a condenser filter may be necessary to smooth the ripple