Radio Broadcast (May 1928-Apr 1929)

JANUARY, 1929 "OUR READERS SUGGEST — " 193 Coil E is wound on a 2j-inch tube with 48 turns of wire. Coils C and D slip inside of coils B and E, respectively, permitting any desired variation of coupling. The condensers tuning these coils may be ganged. H. T. Gallaher, Rock Island, 111. STAFF COMMENT The arrangement suggested by our correspondent functions in many respects as a band filter — a system of station discrimination that characterizes the best of modern receivers. The arrangement suggested in Fig. 5 can be applied to practically any receiving circuit, it particularly recommends itself for use with the receivers having only one stage of tuned radio-frequency amplification. Frequency Compensation on MovingCoil Speakers \ A OVING-COIL speakers, when used with some types of audio amplifiers, have a tendency to over-accentuate the bass notes. The deep rumbling tone is a characteristic to which many people take exception when hearing this type of speaker for the first time. The cause is apparently due to a slight peak on the low tones in most of the present-day audio transformers, originally intended to make up for the losses suffered in the usual magnetic speaker. I have found that a very simple filter can be inserted easily in the input stage to eliminate this effect. The secondary of an old audio transformer, with core removed, is connected across the primary of the first transformer through a variable high resistance of about 100,000 ohms. Any desired balance may be had by simply adjusting this resistance. George H. Miller, Buffalo, N. Y. STAFF COMMENT The apparent preponderance of low notes when first using a properly baffled moving-coil speaker, is often a psychological contrast with the deficiencies of other loud speakers. The genuine cases of over-emphasis of low frequencies with which this department has had experience, were resonant effects, several moving-coil speakers having decided resonant peaks in the neighborhood of fifty cycles. Over-reproduction of low notes, if the reader is convinced that such exists, can be corrected by moving the speaker slightly away from the baffleboard. By varying this distance, any degree of low-frequency response can be obtained. This department editor's experience with moving-coil speakers has been more or less confined to high-frequency emphasis when the loud speaker is operated from a pushpull amplifier, employing the proper output transformer in the amplifier rather than that in the loud speaker. This substitution eliminates the high-frequency compensation circuit included in many speakers to flatten out the hump above five-thousand cycles. These high frequencies are, therefore, over-reproduced, with an unpleasant fringe on certain types of broadcasting, noticeably on tenor and baritone soloists. This effect generally can be compensated by shunting a 0.00025-mfd. fixed condenser across the secondary of the first audio-frequency transformer. Ra{or-Blade Condenser THE fixed condenser described below is highly efficient and is constructed with rigid-metal air-spaced vanes. It costs but a few cents to .Terminal 10 0 02 03 X P rear rang a movins. Nuts; To Circuit Id Blades Nut M— --Washer "---Brass Rod - Coupling B+ B+ FIG. 5 This "link" circuit, when used with the R. B. Universal receiver, will add selectivity to the set. Its design is such that it functions somewhat as a band selector. -To Circuit FIG. 4 A home-made ra^or-blade condenser. This airdielectric condenser is excellent as an antenna series capacitor in short-wave transmitting circuits. make, as old safety-razor blades of the "Gillette" type are utilized. The materials needed for constructing the condenser will be found in the cellar or workshop of any radio enthusiast. The necessary parts include: two pieces of ebonite, each 3" by 2" by j"; two threaded rods, tV in diameter, and about 3J" long; eight nuts to fit the rods; two terminals; twelve "Gillette" razor blades; and twelve metal washers. Washers are placed between the blades to separate them properly. In assembling the condenser, the two rods are bolted to the piece forming the base, a washer FIG. 6 ing the power-supply circuit for the possible use of coil speaker with a high-voltage jield-excilation coil. being added to one of them. The blades and spacing washers are then assembled alternately on the rods to interleave without touching, adding a washer to the other rod to level the top when all the blades have been added. The two top nuts are then screwed down tight, and the top ebonite plate, with terminals mounted, is bolted in place. The connections are made as suggested in the diagram, Fig. 4. A test should be made to determine that the vanes are not "shorting" and the condenser is then ready for use. The condenser will have a capacity of about 0.0001 mfd., and it will prove as satisfactory as the best condenser of this type on the market. By increasing the number of blades, one may, of course, increase the capacity. Edward Piranian, Philadelphia, Pa. STAFF COMMENT This ingenious condenser recommends itself particularly as an antenna series capacitor in short-wave transmitting circuits. Novel Power-Supply Device D EING called upon very recently to design and build an amplifier and power supply for use with a power tube and an ordinary magnetic speaker, but also having in mind the possibility of later using the device with a moving-coil speaker requiring a field supply of 1 10 volts d.c, I designed the arrangement illustrated in Fig. 6. Each terminal of the choke except the last was brought out to a binding post. A flexible lead with a phone tip soldered to the end was attached to the last terminal. The three binding posts were mounted on a bakelite strip. When an ordinary magnetic speaker is to be used the flexible tip from the choke is connected to binding post 1 . This places the choke in operation and the speaker is attached in the usual way. When a moving-coil speaker is to be used, however, the flexible lead is disconnected from binding post 1 and the field winding of the moving-coil speaker is connected to binding posts 1 and 2. A jumper made of bus wire is connected between binding posts 2 to 3 ; this cuts out the first section of the choke while the second section is replaced by the winding of the moving-coil speaker. Karl F. Oerlein, Philadelphia, Pa. STAFF COMMENT Such an arrangement has much to commend it. It is generally unsatisfactory to connect the field-winding terminals of the moving-coil loud speaker in series with the filter system of the power supply, since the additional resistance of the field winding may cause a considerable decrease in output voltage.