The motion picture projectionist (Nov 1931-Jan 1933)

December, 1931 Motion Picture Projectionist 29 Th e or y and Fundamentals By W. \Y. Jones ^= Mr. W. W. Jones, whose Department is a monthly feature of this magazine, has long been actively associated with the Motion Picture Industry. At the present time Mr. Jones is a member of the Engineering Department of RCA Photophone and has been closely identified with the educational activities of that organization since the time of its inception. He is a graduate of the Milwaukee College of Engineering and was at one time Instructor of Mathematics and Electrical Design at that institution. — The Editor. Matching the Loudspeaker and the Amplifier THIS is a continuation of the discussion, contained in the November issue of Motion Picture Projectionist, wherein there were outlined the principles of impedance matching for maximum power and for maxi Rp.lSOOn |4.| TO I RATIO LOUDSPEAKER I VOICE COIL Rp =150013 20 TO I RATIO TAP Fig. 4 mum undistorted power output, as well as the principles governing the selection of a transformer having the proper turns ratio for a given tube plate impedance and a given voice coil impedance. Sample calculations were included in the discussion. In the following will be found a discussion of the principles with sample calculations, of the selection of the proper turns ratio transformer for power tubes connected in a pushpull circuit; of multiple stage loudspeaker grouping; of parallel stage loudspeaker connections; of series stage loudspeaker connections; and of series — parallel stage loudspeaker connections. Push-Pull Tube Circuits By referring to Figure 4 will be found two power tubes in a push-pull circuit connected to a primary center tapped output transformer, the secondary of which is connected to th*: voice coil of a loudspeaker. Each tube has a plate impedance of 1500 ohms, and the voice coil of the loudspeaker has an impedance of 30 ohms. Let it be required to find the proper turns ratio for the output transformer for the connections shown in Figure 4. Two tubes connected in push-pull have a combined plate impedance equal to the sum of the plate im Rp.l500fl 10 TO I RATIO /J— =H|l|lM-3 lIC—; -> VOICE COIL 30ft £_^ ri 14 TO I RATIO TAP -'Zs-SOJI Fig. 5 pedance of the two tubes. In this case, the combined plate impedance is 1500 ohms plus 1500 ohms or 3000 ohms. From previous discussion it has been pointed out that for maximum undistorted power output a tube must operate into a load impedance equal to two times its plate impedance. For a push-pull circuit the tubes must operate into a load impedance equal to two times the combined impedance, or in this case the load impedance equals two times 3000 ohms or 6000 ohms. Since the voice coil of the loudspeaker has an impedance (Zs) of 30 ohms, it is necessary to select an output transformer having the proper turns ratio which will cause the 30 ohm voice coil to present an equivalent load impedance (Zp) of 6000 ohms. Calculating the proper turns ratio */200 14.1 The secondary of the transformer in Figure 4 is tapped so that two speakers may be connected in parallel and then operated from terminals 2 and 3. Reference to the use of this tap will be made later in this discussion. Multiple Speaker Grouping Often in sound motion picture work it is found that a single stage loudspeaker does not produce adequate sound in all portions of the theatre or auditorium. If the auditorium is wide and the stage loudspeaker is directed down its center of volume, the front right and left hand group of seats in the orchestra and/or balcony will in all probability have inadequate volume. Likewise if Rp-isoon the auditorium is high the rear balcony seats and/or the front orchestra seats will suffer in volume. Inadequate coverage with a single stage loudspeaker is occasioned by the common use of the loudspeaker horn, or directional loudspeaker baffle, which horn or directional baffle directs the sound beam in the direction in which it is pointed. In order to effect adequate coverage in a relatively wide auditorium it is necessary and common practice to install two stage loudspeakers arranged in a horizontal plane, and directed so as to distribute the sound uniformly and yet secure adequate sound coverage for the front, right and left hand group of seats referred to above. In the case of an auditorium which is relatively high two stage loudspeakers may be used and arranged one above the other, and directed for uniform distribution and proper coverage of the rear balcony and front orchestra seats referred to above. However, in the case of a large auditorium where it is both too wide and too high for a single loudspeaker it becomes necessary to use four loudspeakers arranged in a t Zs ='30fl -t Fig. 6 Fig. 7 bank. The bank of four speakers usually is arranged and so directed that the lower group of two speakers covers the orchestra seats, and the upper group of two speakers covers the balcony seats. When either the lower group or upper group is adjusted, the speakers are directed so as to provide adequate sound for the side seats in both the balcony and the auditorium. Then the upper and lower groups are adjusted as a group so as to secure uniform distribution throughout the auditorium and adequate coverage of the rear balcony seats and the front orchestra seats. Parallel Loudspeaker Connections From the foregoing discussion on multiple speaker grouping it can be seen that it is often desirable to provide means of operating from one amplifier two or more loudspeakers. This may be accomplished by providing an extra tap on the secondary of the output transformer. The location of the tap in the winding will, of course, be determined by the circuit used in connecting the loud(Continued on page 34)