International projectionist (Jan 1943-Dec 1944)

VOLUME XVIII MAY 1943 NUMBER 5 A Modern Inverse Feedbnck Amplifier A SCHEMATIC of an AC operated, two-stage, inverse feedback, resistance coupled, power amplifier is shown in Figure 1. You will note combinations of resistances and capacitors in the feedback circuit, which is commonly called a warping circuit. This warping circuit is adjustable and is employed to vary the frequency response of the amplifier and therefore that of the sound system in which the amplifier is used. Further examination of this schematic will disclose that the amplifier also includes a single stage monitor amplifier, together with the associated monitor speaker. Choke Input Filter In the lower right hand corner is the full wave rectifier circuit. Three taps are provided on the primary of the power transformer so that the amplifier will operate satisfactorily on line voltages from 105 to 125 by proper selection of the tap. This transformer has a secondary heater winding for the tubes in the amplifier and the associated voltage amplifier. Following the output of the rectifier from the filament of the rectifier tube we pass through the reactor Li before we come to the other elements of the plate supply circuit. Thus we find that this amplifier employs a choke input filter. From the choke we go direct to the plate of the monitor By LEROY CHADBOURNE amplifier tube, VT6, the screens of VTs and VTi and the plates of these same tubes, through the primary of the output transformer, Ti. The only filtering in these circuits is the slight smoothing out action of the choke. But let us continue to the left on the line just above Li and soon we come to the capacitor, &3. This is the first of the main filter capacitors and reduces the ripples in the plate supply to the screen, suppressor and plate of tube, VT2. This circuit is quickly traced through Rio and R.->. Still continuing to the left on the same line we come to the second main filter condenser, C11, which means, therefore, that the plate supply to the first tube, VTi, has the least ripple. This circuit is through the resistor, R.j. Screen potential is obtained through Rio. This completes the plate supply to the several tubes in the amplifier. In considering the filtering in a circuit it must always be borne in mind that resistors also reduce the ripple. Let us consider that ripple is an AC wave superimposed on a direct current. When this combined voltage passes through a resistor the voltage of the direct current and the AC current are both reduced. It is obvious that the supply on the output of the resistor will have a lower AC ripple than on the input. This factor is one of the considerations in the design of a filtering circuit. The sound signal circuit is traced through VTi from the input in the regular way. But when we arrive at the grid of VT2 we note that the grid resistor, R?, returns to ground through Re. Thus there is a voltage on the grid of this tube equal to the drop across Rc. But, since Rs is in series with Ra in the cathode circuit, the grid is always negative with respect to the cathode by the drop across Rs. Note also that, in addition to the drop through Rn due to the combined plate and screen currents, there is also the effect of the modulation of the plate current by the signal current. That is we have a DC drop and an AC drop through this resistor. This AC will act to increase the voltage applied to the grid and since it is in phase with the signal it aids in improving the operation of the amplifier. Push-Pull Obtained by Phase Inversion A glance at the output transformer shows it to be of the push-pull type, but we do not find the push-pull tubes VT3 and VTi coupled to VT2 by a transformer. The coupling is by phase inversion. The direct sound circuit passes through VTa and the coupling capacitor, Ct, to the grid of VT:i. The grid of VT^ is coupled MAY 1943