Radio Broadcast (May 1928-Apr 1929)

JANUARY, 1929 NEW APPARATUS AND ITS APPLICATIONS 195 New Dual PushPull PublicAddress Amplifier Provides 15 Watts of Power GENERAL VIEW OF PUBLIC ADDRESS AMPLIFIER The amplifier pictured above employs three transformer-coupled stages and has an undistorted output of i 5 watts. 7 be unit, which is completely a.c. operated, was designed for use in large theatres THE item of new apparatus described in this article is a three-stage light-socket-operated audio amplifier possessing ample amplification to boost the output of a radio set's detector tube, a microphone, or a magnetic phonograph pickup unit up to a volume level sufficient for a large theatre or an outdoor crowd. The gain-frequency characteristics of the amplifier are such that an effect of naturalness for human voice or music will be conveyed to every listener. This amplifier has little application in the average home radio outfit. Its real appeal, rather, is to those experimenters and professional setbuilders who have found that there is much real demand for public-address amplification that cannot be adequately met by ordinary equipment designed primarily for home use. To such individuals, this amplifier offers the possibility of sale or rental to moving-picture theatres, skating rinks, schools, race tracks, and conventions, not to mention other uses. The fine possibilities of such sales can be grasped when one considers that a skating rink or theatre can avoid the considerable expense of even a small orchestra with a pair of phonograph turn-tables and record pick-up units, a supply of good records, one to four loud speakers, and this amplifier. Whereas the smaller theatre or rink could only afford a a small mediocre orchestra at most, the amplifier installation brings out music played by the orchestras of Paul Whiteman, Vincent Lopez, the New York Philharmonic, the Boston Symphony, etc., with all of its original color, tone and volume, yet the total cost need not be more than three to five-hundred dollars! To the movie exhibitor, the radio fan, and the wide-awake professional set-builder, no more need be said. POWER REQUIRED IN DESIGNING this amplifier, much experi1 mental work was done to determine the approximate power needed for various classes of coverage. In 1000 to 2000-seat theatres, for instance, five to seven watts, taken from one UX250 tube, was found sufficient in most cases to give realistic reproduction. As the desire of many exhibitors was to produce greater than natural volume, . more power was found necessary for such "volume hounds." Conclusions reached experimentally indicated that for such conditions an undistorted power of fifteen watts would give coverate of theatres seating up to 2000 or 3000 people, under conditions of maximum absorption and with all seats occupied. Outdoor tests indicated that this same power would give natural understandable speech and music at volume sufficient for crowds of 10,000 to 15,000 people. From a gain-frequency standpoint, it was found that an accentuation of bass frequencies was desirable, particularly as phonograph records and radio programs are generally lacking in the lower bass registers. Practical experience indicates that if the amplifier accentuated frequencies between 60 and 200 cycles (lower notes being used infrequently in music and speech), the most pleasing effect would be obtained. With this information at hand, a power out put stage was first developed, after which suitable input stages were designed to insure the operation of the output stage at its full capacity with the lowest input voltage to be anticipated in practise. Adjustment of the transformer characteristics was made to obtain the desired gainfrequency curve. The power-handling capacity of the input stages was made so great that no overloading would occur in them, even though the output stage were operated at well over its maximum capacity. The whole amplifier was then adapted for full a.c. operation. The requirement that 15 watts of undistorted power be available from the output stage automatically eliminated the possibility of using tubes smaller than the 250 type, and since the maximum output of one tube is 4.65 watts, a push-pull circuit with two 250-type tubes seemed to be a good starting point. From tests it was determined that, with a load impedance equal to four times the Rp of one tube, the requirements set down could be satisfied with a plate potential of 450 volts, and a grid voltage of 80, maximum safe values for selected 250-type tubes. An undistorted output of 15. 75 watts was obtained from the two 250-type tubes in push-pull with a specially developed output coupling impedance. The results of some gain measurements are shown in Fig. 1. It will be noticed that 10 watts may be developed without any appreciable decrease in gain (which would indicate distortion). The curve of Fig. 1 varies only 2 ~ru between 1 and 15.75 watts output and, as 2 tu is the FIG. I. OUTPUT CHARACTERISTICS FIG. 2 FREQUENCY CHARACTERISTICS