Radio Broadcast (Nov 1926-Apr 1927)

DECEMBER, 1926 AUDIO HOOK-UP FOR THE BROWN ING-DRAKE 165 dency to block due to the fact that grid leaks of too high a resistance are employed. This, of course, is done away with in the case of double impedance, at the expense to the set builder of employing three more choke coils instead of three resistors. In comparing the types of audio amplifiers, nothing has so far been said concerning the B battery voltage or the amount of current consumed by each. A power tube should be used in the output of the receiver, so that ample power may be delivered to the loud speaker without overloading the last tube. As ux-171 tubes require between 135 and 180 plate volts, that much B battery is usually employed. These tubes draw a heavy plate current, too.so the slight variation of plate current drawn by the preceding audio stages, when different types of audio coupling is used, may be neglected when considering the total drain on the B batteries. HH 0.001 Mfd. iff UX-201-A UX112orUX171 I A.F.T. I O B+ 22V15 6 'C +0 4H Volts B+ 135 ^for UX-112 °B+180 for UX-171 -OA-.B 6 'C +0 9V.for UX" 40.5 V. for UX-171 FIG. 2 By joining together terminal D on this diagram with terminal D of Fig. I we have a complete Browning-Drake receiver employing two stages of audio frequency amplification of the transformer-coupled form 0.1 Mfd. 0.1 Mfd. 0.1 Mfd UX201-A II UX-171 FIG. I Here is shown the basic circuit of the BrowningDrake receiver. So-called developments of this circuit are, generally speaking, misleading, for they usually refer to the audio amplifier and not to the fundamental Browning-Drake circuit In the case of impedance, resistance, and double impedance amplification, there is one feature which has usually been overlooked by designers; it is the necessity of keeping the radio frequency currents out of the audio amplifier. This is why a radio frequency choke has been shown in the schematic diagrams given. When radio frequency currents enter the audio amplifier, they usually have a very serious effect on the operation of the amplifier. The r.f. choke does away with the above tendencies, leaving the audio tubes free to amplify audio signals exclusively. The construction of this choke for the broadcast band is not difficult. In fact, practically any coil which has a natural period of about 600 meters will do the trick. Fig. 5 shows an output circuit which is coming into common use to keep the direct current in the plate circuit of the last tube out of the loud speaker. In some cases this materially helps the quality of received signals, though with some types of loud speakers, there seems to be little difference. In the case of the Western Electric 540 AW loud speaker, the filter system shown in Fig. 6 is usually worth while when a semi-power tube, as the ux-171, is employed in the last audio stage. The tone filter then becomes essential as a protective device to the speaker. In Fig. 1 is shown the circuit of the Browning-Drake receiver without any audio stages added. The various amplifiers, indicated in Figs. 2, 3, 4, and 6, may be combined with the circuit shown in Fig. 1 merely by connecting terminal D of each unit together. Thus, if the reader desires a BrowningDrake receiver with resistance-coupled amplification, the circuits of Figs. 1 and 6 are combined by connecting the D terminal of each unit. Likewise may the output device, Fig. 5, be placed in the plate circuit of either amplifier output tube, by making common the posts lettered X and Y. It will be noticed that two rheostats are shown in Fig. 1 . However, when the audio amplifier is added, one rheostat is sufficient for both the detector and audio amplifier. It should then be a 2-, 4 or6-ohm affair. 0.1 Mfd. 0.1 Mfd. UX-201-A II UX-201-A ll UX 171 B+22!*-45 FIG. 3 Very excellent quality is obtainable by adding a three-stage impedance-coupled audio amplifier to the basic two-tube Browning-Drake circuit shown in Fig. I. I 4.0 Mfd lH-Aii Volts FIG. 4 Dual impedance amplification is an expensive proposition but may give better output than any other form of audio amplification. Connections of such an amplifier to the basic Browning-Drake circuit (Fig. 1) are similar to those of any other amplifier described on this page. Audio Frequency Choke 30 100 Henries Loud Speaker ? OA-.B B + 67V90 trtT FIG. 6 FIG. 5 The direct current in the output of the last tube is kept from the loud speaker by means of a choke and large capacity condenser wired as shown here. This output device is connected to any of the amplifiers on this page by joining X — Y on both units together A resistance-coupled amplifier is not unlike one of the impedance-coupled form. Merely are resistances of the correct size substituted for the impedances. This resistance-coupled amplifier may be coupled to the two-tube BrowningDrake receiver (Fig. i) by joining together the points designated by D. Either an output device or loud speaker connects to the points X — Y.