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POWER SUPPLY FOR SOUND SYSTEMS 491
Speakers: d. c. always. From 7 to 110 volts or higher: seldom more than 5 or less than 1/10 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) One method of providing for some of these power needs is to use dry cells, which is often done wherever the amperage requirements are low enough to make it practical. Dry cells are quite commonly used to bias photo-electric cells; also occasionally for the "C" requirements of amplifiers. They may also be used for the "B" power of very small amplifiers.
Line D. C.
(4) Direct current from a commercial power line cannot be used for most 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 a trifle, and to take out the clicks caused by abrupt changes in voltage.
(5) In general, line d. c. can be used for sound power only in such connections as do not subject its inherent irregularities to amplification. Tn practice, this confines its use almost wholly to speaker field circuits, with or