Projection engineering (Jan 1932-Mar 1933)

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Page 12 PROJECTION ENGINEERING V. P. 1NTGK5TA&E TRANS. -njucn — i i — oil — rLTk □ PLATE , , &RID 1MPEDANCE5 IMPEDANCESr FIG. 9. With the above considerations in mind there have been developed output transformers which isolate the loudspeaker from both the direct current and voltage of the tube, and allow a proper relation of impedances in order that the most satisfactory results may be obtained along the lines indicated in the preceding discussion. The connections of these transformers, as substituted for an output impedance, are shown in Fig. 3. Push-pull amplification has been advocated for many years. The results obtained from this form of amplification have justified its introduction many times over. In addition to the consideration of greater allowable input voltage, with consequently increased power output, . push-pull amplification has other distinct advantages over other circuits. Chief among these advantages is the reduction of hum when alternating current devices are employed as a power supply. Since the tubes in a push-pull stage are so connected that there will be no output across the plates unless dissimilar conditions exist in the tubes, any disturbance arising simultaneously in both will cancel out. This allows the use of raw alternating current on the filaments, and also permits the plate and grid supply to have a considerable ripple without the introduction of undesirable hum. An output impedance is commonly employed to couple the plate circuits of push-pull tubes to the loudspeaker, as illustrated in Fig. 4. It is satisfactory, provided the impedances of the tubes and of the loudspeaker are properly related, though here again there is the objection that, in many cases, no adequate provision can be made for the maintenance of a proper impedance ratio. Fig. 4 also illustrates a method of Fig. 9. Coupling two push-pull stages. Fig. 10. Impedance coupling between stages. coupling that removes all danger of a direct current shock if a person should come into contact with the leads to the loudspeaker, which has been another valid objection to the use of an output impedance with push-pull tubes. The dotted lines illustrate a method by which all but the alternating voltage can be kept from the loudspeaker. The use of an output transformer removes the disadvantages of an output impedance as mentioned above, assuming, always, that the unit is carefully designed and constructed. There are several types, intended for use with tube impedances of two, five and ten thousand ohms. The transformers fall into two general classes, one class working with all standard loudspeakers, and the other class working directly into the moving coil of a dynamic loudspeaker with no intermediate transformer required. All the units allow the most favorable relation between tube and loudspeaker impedances, with maximum efficiency at the lower frequencies. Fig. 5 shows the proper transformer to use under various conditions. Figs. 6 and 7 show the connections to these transformers when substituted for output impedances. Fig. 7 illustrates a method whereby too high a value of direct current may be kept out of the transformer windings. In addition it is well to note the fact that the high plate impedance of the 226, 227, and 120 types of tubes makes any but parallel operation undesirable, in which case the impedance may be calculated by dividing the impedance of a single tube by the number of tubes in parallel. It is understood that these remarks apply to the output stage only. It has been assumed that the reader knows of the methods of obtaining an input to a push-pull amplifier. For -'Uli.CD TJIj.O this purpose there is a transformer known as the Type Y, which may be used to couple a single tube to a pushpull stage, or to couple two push-pull stages. Figs. 8 and 9 illustrate these connections. Fig. 10 shows a similar circuit with impedance coupling between stages. Double push-pull amplification is new and many seem to find its use of benefit. Before attempting to employ this type of amplification, however, it is well to consider when and where its application is beneficial or detrimental. A list of advantages versus disadvantages might be of help. Advantages 1. Minimization of disturbances introduced by power supply. 2. Greater voltage available for operation of last stage with no increase of supply voltages. 3. Reduction of saturation effects of primary current. 4. Cancellation of even harmonic components of distortion introduced in first push-pull stage. 5. Reduced tendency to "motor-boat" when operated from high impedance power sources. Disadvantages 1. Lowered overall quality due to increased impedance of primary supply source unless low impedance tubes are used or an interstage transformer with a higher primary impedance is employed. Impedance coupling also obviates this difficulty. 2. Increased instability of system. This difficulty is more apparent with good apparatus than with poor, and is due to resonance effects plus an in Fig. 11. The — 250 tube is on the right. Fig. 12. Connection when considerable distance from amplifier to loudspeaker. TUBE TO LINE TUBE TO LINE b+I c-4 FIG. 11.