International projectionist (Jan-Dec 1935)

July 1935 INTERNATIONAL PROJECTIONIST 11 equally well have been drawn horizontally at the left-hand end of the Red series, since it is wired in series with all the Red horizontal resistors. The Red projector output therefore "looks into": 223.5 7.18 80.6 5.10 57.3 3.61 40.4 2.56 28.7 3.08 20.3 2.32 14.3 .584 10.17 .196 TOTAL: 499.900 Ohms. The output impedance of the projector is 500 ohms. Fader Output Impedance Now, to consider the output impedance of this circuit, assume the arrowhead contact were moved to Red volume control Terminal 15. Then all the red horizontal resistors, in series, would look toward the fader output, and, consequently, the system amplifier input. The output impedance would therefore be the same as the total shown above, except that the No. 5164 vertical resistor (223.5 ohms) at the extreme left is no part of the output circuit and must be subtracted from that total, leaving an output impedance of 276.40. On the other hand, with the arrowhead at point 5, as shown, the output impedance would be 190 ohms for the vertical resistor in series with point 5, plus all the red horizontal resistors between point 5 and point 0, or: Ohms 190. 2.56 3.08 2.32 .584 .196 198.740 With the arrowhead on volume control point 1, the output impedance is, as the drawing shows, 190.196 ohms. The output impedance of this fader therefore remains approximately constant (between 190.196 and 276.40 ohms), at all ranges of volume setting. The input impedance of the system amplifier is 200 ohms and "looks into" the output impedance just given. Maintaining this approximately constant impedance is the function of the vertical resistors that extend downward from the horizontal resistor line. The drawing shows that from points 1 to 7 (the horizontal resistance being relatively small) all the vertical resistors have a value of 190 ohms. On the other hand, the vertical resistor in series with point 14 (the horizontal resistance being large) is only 42.4 ohms. If the vertical resistors were omitted from this circuit the output impedance at point 1, for example, would be only .196 ohms working into 200 ohms, which would be a very serious mis-match. The drawing also shows that such small mis-match as occurs in this fader is found chiefly at the higher settings. Impedance mis-match causes loss of volume, which is more tolerable at the high settings than at the very low ones, since it helps to prevent any possible amplifier overloading. This arrangement constitutes a T-pad. The 223.5, 5164 horizontal resistor, plus horizontal resistors between point 15 and arrowhead, constitutes one half of the top of the T. The other half of the top consists of whichever of the vertical resistors below the horizontal series is at the moment in contact with the arrowhead. The horizontal resistor series from the arrowhead to 0 is the upright limb of the T. With the arrowhead on point 15, however, the arrangement is an L-pad, since there is no lower vertical resistance in series with point 15. L-pads and T-pads are very commonly used in speech transmission lines to maintain approximately constant impedance while varying volume. Why the Volume Varies To understand the variation in volume obtained with this constant-impedance arrangement, suppose the arrowhead to be set at point 1. Then the relatively very small voltage drop developed between point 1 and point 0 is wired across the system amplifier input through 190 ohms series resistance. On the other hand, with the arrowhead on point 14, very nearly all the voltage drop developed by the input sound current across the long line of horizontal resistors is connected across the system amplifier input, through the much smaller series resistance of 42.4 ohms. White Side of the Fader The White circuits of this fader duplicate the Red circuits, but operate on the right-hand half of the horizontal resistance line. Consequently, by swinging arrowhead through zero point to the white side, the projectionist changes-over from one projector to the other, the circuits of Fig. 1 performing the work of a switch. Having traced the Red circuits backward from the fader output (as more convenient while the drawing was relatively unfamiliar) we can now trace the white circuits forward from the input. The left-hand, or No. 4, White input terminal is wired to the input jumper, and may be traced left to the vertical line running downward from input terminal No. 6 to the frame ground, to Point 0 and thence to Output Terminal No. 16. The right-hand white input terminal, No. 3, is wired to a line running down, right and up to the left-hand prong of the right-hand three-prong assembly of the 479 K key. Thence to the center prong of that assembly and right to the center prong of the lower three-prong assembly of the 479 GS key. To the left-hand prong of that assembly and down, right and down to the righthand, or White, 223.5 ohm resistor, and thence to the extreme right-hand end of the horizontal resistor series. The White input therefore looks directly at the righthand half of that series. The output White circuit consists of one wire which runs from Point 0 to the grounded, or No. 16, Output Terminal, and of the arrowhead (when thrown to the White side) which runs through switches to No. 15 Output ter 479-GS KEY G REP W R 8 Q6 05 Q4 Q3 pa Ql 4 79 ft KFY FIGURE 1