Projection engineering (Sept 1929-Nov 1930)

Projection Engineering ; September, 1929 Page 11 Treatment of Audio-Frequency Transmission Lines Data on Impedance Adjusting and Equalizer Circuits to Meet Average Requirements of P-A and Talking Picture Installations WITH increased general interest in public-address systems and talking picture installations some question has arisen as to methods of treatment for audio-frequency transmission lines of varying length. Cases of coupling such lines into and out of other apparatus have been reduced to several standard instances and transformers for such service are now commercially available. While the design of impedanceadjusting transformers for this purpose is a matter of general knowledge a few simple rules may be helpful in adapting available transformers to special purposes and their repetition may not be amiss. For a transformer to work out of an impedance Zt into an impedance By Charles H. W. Nason * step-up windings or as ratios of unity to some fractional part thereof, as "1 to .55" for a step-down transformer. In the case of high ratio step-down transformers the power considerations are the same as for all other cases in transformer design and the necessity for guarding against excessive I2B losses in the windings themselves and in the transmission lines through the tentiometer across the voice coil. The exact value of this resistance would depend upon the speaker used. The output device is an auto-transformer tapped at ten secondary points for output to from one to ten 15-ohm speakers. Fig. 2 shows a slightly different arrangement— perhaps of not quite so satisfactory a nature — employing . : ■:, mum iiiwimnimiiiii Another impedance adjusting circuit, similar in form and adaptation to that of Fig. 1, but working through a 2000ohm line. lllllllllllillllllilllllllllllillllllllillllllllll Z:; z, 7a FERRANTI ,--0P-M-1C 2000 OHM LINE \*—o~t — i FIG. 2 (i) where "a" is the turns ratio. This holds good also for vacuum tube output transformers where the impedance of the load varies materially with the frequency — in moving coil speakers, however, the impedance/frequency characteristic is substantially horizontal and we may avail ourselves of the full portent of the fact that maximum use of insufficient copper becomes evident. This is highly important in the case of multiple connection of moving coils for dynamic reproducers and only exceedingly heavy and short connections should be employed. The metal shielded cable used in aircraft radio work is ideal for the purpose and will meet with the underwriters approval for theater use. 8+ ■* AMERTRAN 'TYPE 332 15 OHM MONITOR FIG.1 AMERTRAN TYPE 377 500 OHM LINE 10 STEPS .15 (6 OHMS iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiun Impedance adJusting circuit for working out of 250's in pushpull, into one or more dynamic speakers, through a 500ohm line. inn iiiiiimiiimiimiiimiiiiiiiiiiiiii undistorted power output is obtainable when the tube works into a load impedance of twice its Rp. Hence 2 Z, (or 2 Rp) = a (2) Z, Naturally in push-pull circuits the two Rp's are in series and the full turns ratio must be taken as twice the value for the single tube as calculated. Ratios for impedance coupling transformers may be written either as ratio of whole numbers, as "1 to 3" for •Chief Engineer, Sound Service. Impedance Adjusting Circuits An impedance adjusting circuit for working out of two 250 tabes into one. or more moving coil speakers, through a 500-ohm line is shown in Fig. 1. The transformers shown are of a special nature but are available on order from the manufacturer. Provision is made in the tube output circuit to incorporate a dynamic speaker as B monitor. As a digression from the main subject it is suggested that an ideal volume control for this monitor would be through a rheostat in I lie fleld excitation circuit or by a 200 ohm po transformers of a generally available character. Impedance adjusting transformers to work out of a microphone, line or phonograph pick-up into the grid of a tube are available and the existing transformers having variable ratios may often be adapted to special cases. For example, the OP-M^c, shown in the circuit in Fig. 2 might be readily adapted to any of the following cases : out of a tube into multiple magnetic speakers : out of a tube into moving coil speakers in series ; out of a tube into a transmission line ; out of two tubes in push-pull in either of the foregoing cases; out of a transmission line into the grid of a tube, out of a line into various combinations of magnetic or moving coil speakers. In fact a study of the transformers now on the market will yield many such cases of adaptability. In some cases no such simplicity occurs — working out of a detector tube into a remote amplifier or from a microphone into lino, mixer or tube presents a problem noi so easily solved with makeshift equipment. Happily the Gods have been good and there is no lack of special transformers for such cases. In Fig. 3 is shown a reproduction of a sheet issued by one of the better known manufacturers, demons! ral Ing in part the adaptability of certain standard items. Necessary Frequency Ranges Transmission lines, though consisting merely of a resistance lor normal