International projectionist (Jan-Dec 1936)

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November 1936 INTERNATIONAL PROJECTIONIST 21 left through C-5, down to the center tap of V-3 filament secondary, and thence to the filament of the tube. The terminals "GR. WH" and "GR" of the primary of T-2 are not used in this circuit. The center-tapped secondary winding is connected, through the three upper terminals of J-3, to the frequency dividing network of the speaker circuit. Reverse Feedback Circuit A second load upon the output of V-3, in parallel to the primary of the output transformer, may be traced as follows: From the plate of V-3 up, left through R-29 and R-17, through C-3, and left and down to the screen grid of V-l. From the cathode of V-l to the negative bus through C-l, as previously traced, and from the negative bus, down and left through C-5 (16 mfd.), as previously traced, to the filament secondary of V-3. This is the reverse feedback circuit. Inspection of this circuit merely with reference to the parts included in it indicates that the speech power is fed back to the screen grid of V-l in phase with the control grid speech a.c. of that tube; but in fact those two grids are 180 degrees out of phase by virtue of the values chosen for C-7, C-3 (0.04 mfd.) ; C-3 (2 mfd.), and the resistors associated with them. If the two grids were actually in phase, this circuit would produce common or regenerative feedback instead of reverse or degenerative feedback. negative, offsetting the increase, not altogether, but to the extent determined by the impedance of R-17, R-29 and C-3 (2 mfd.) . When the control grid of V-l swings more negative, decreasing plate current, the screen or accelerator grid becomes more positive and tends to counteract the effect. One overall result is that the total volume output of the ?mplifier is considerably lowered. Another is that harmonic distortion is drastically reduced. Harmonic Distortion In the ordinary action of an amplifying tube, any tube, when the control grid voltage is changed by an input signal, the plate current changes proportionately. However, the proportion of plate current change to grid voltage change is not constant, but only nearly constant. Suppose, (to take round numbers) the control grid charge changes by one volt, and the plate current increases one milliampere in response. Then if the control grid swing continues until it reaches two volts, the plate swing will not be two mils, but possibly 1.9 or something of that order. In other words, the "straight line" portion of the characteristic curve of a tube is not entirely straight; there is a little curvature in it. The pattern of the output speech a.c. does not duplicate the pattern of the input speech a.c. exactly. Some degree of distortion is introduced Ri7 are associated with wires carrying such current, a real reversal in the direction of current flow could be obtained in the plates of such condensers and in the wires leading to them. In the same way, the slightly distorted pattern of speech a.c. produced by any amplifying tube, as just described, may be regarded as if the distorted pattern consisted of the original sound wave (amplified of course) plus a number of other frequencies associated with it. This holds true practically as well as theoretically. Just as in the case of the rectifier filter, in which condensers can draw a.c. from a line that contains current moving in one direction only, so in the case of this type of distortion, suitable filters can draw from the distorted output supplementary frequencies that were never present in the original input. These frequencies are called harmonics, and the type of distortion here referred to is called harmonic distortion. It is measured in per cent. An amplifier is said to have so-and-so many per cent harmonic distortion, and this is one method of indicating its quality. The harmonic distortion, or harmonic content of an amplifier is measured, in practice, by using suitable circuits to separate the original or fundamental frequency from all the others (just as in the case of the rectifier the filter sepa INPUT The former type of feedback is familiar to projectionists with radio experience, also to all who have used a microphone and placed it too near a loud speaker. The feedback of Fig. 1 is novel in that it is "reverse," 180 degrees out of step with the input voltage. When the control grid of V-l swings positive, increasing plate current, the screen grid of the same tube swings by the action and the nature of the tube. Now, in the case of the more familiar rectifier and filter, it was noted that although the fluctuating current drawn fiom the rectifier tube could not possibly flow in more than one direction, it might be regarded as containing a.c. as well as d.c. — not only theoretically but for every practical purpose. For example, it was seen that when condensers •A) mr w' V4 w ,fW^ J3 T3 ■^m> D2 l05V-1ZSV;SOt60~AC POWER SUPPLY