Radio Broadcast (May 1928-Apr 1929)

New Trends AT THE present writing (SepZ_\ tember) several d i s t i n c t <* A» trends in the progress of radio may be observed by any one who gets about even a little bit. These trends are often first evidenced in home-made receivers and in the kit sets put out by wideawake manufacturers and are later seen in the sets built by the large complete-set people. Sometimes, of course, the set manufacturer is in advance of the others, but in general the tendency has been for this branch of the industry to follow the leader. Whether this is due to lack of engineering initiative, or to hesitancy to adopt something new, or to patent arrangements, is difficult to say. It is true, however, that among the licensees of the R.C.A. there have been few major new ideas that have seen the light of the dealers' shelves. These people are still making t.r.f. sets or neutrodynes. Small improvements have been made, but they are in the nature of refinements of existing circuits and apparatus. What are these new trends, and what do they mean to the future of the radio business^say the 1929-30 season? THE DYNAMIC SPEAKER WE HAVE already discussed in this magazine the moving-coil or dynamic speaker. After considerable effort to find an unbiased source of loud speaker information, we discovered Joseph Morgan of the International Resistance Company, who wrote the article "All About Loud Speakers'' in the August Radio Broadcast. There is no doubt that the present season is going to be a dynamic speaker season. Nearly every set manufacturer of note will have a model or two equipped with this newest milestone on the way toward more perfect reproduction. There are many such speakers now on the market; some of them are the Magnavox, Jensen, Peerless, Rola, Newcombe-Hawley, Marco, Radiola 105, Farrand, etc. Why is the moving coil speaker superior to our present cone and horn types-" Briefly, because it presents to the final power tube a nearly pure resistance rather than a complicated combination of inductance, capacity, and resistance whose frequency characteristic is anything but a straight line. The dynamic speaker has a moving system that can move through large distances, up to a quarter inch, which means that There are plenty of low-frequency power can form here. be put into it with the certainty by another for 1929-30 that sound energy will come out, and that there will be no clatter of armature against pole pieces. No other type of speaker has been tested in the Laboratory which reproduced fundamental tones below 100 cycles. They emit sound, to be sure, but it is not like the original. Will 1929 See These New Developments in Everyday Use? 1. THE DYNAMIC SPEAKER 2 THE BAND SELECTOR 3. THE ONE-STAGE AUDIO 4. THE FIVE-ELEMENT TUBE The dynamic speaker, then, is here, and in 1929 will take its place in the best commercial sets and in the homes of the most critical home set builders and engineers. It entails several hardships on the constructor. The filter in his power supply must be better than is necesaary with other speakers that do not reproduce tones Shield FIG. I no tubes in the Jones band selector which is shown in schematic All the selecting is done by one set of apparatus, the amplifying The tuned circuits are coupled together by the small inductance, Li below 100 cycles. He must put the speaker in the middle of a rather large and awkward and solid board not less than three feet squaie for best results. His amplifier, to utilize the advantages of this kind of speaker, must be very good. Because the speaker is more efficient — at least some of them are — he can get along with less power, but at present it is not safe to economize at this point. The constructor had better plan to use 171's or better in his final stage. The day of the 199 tube loud speaker is not yet here. On the other hand, we do not believe it necessary for the home listener to go to 250-type power output tubes, although the reserve power possible with such an amplifier as was described in the July Radio Broadcast (page 141), is something to strive for as the ultimate. In the laboratory of Dr. John P. Minton, the well-known acoustical engineer, we heard a 10-inch Peerless dynamic speaker which was mounted in a three-foot baffle of not too solid construction. The speaker was operated from an amplifier employing a single 171-type tube in the output stage, and the results were very satisfactory. The signals were made louder than could be tolerated comfortably in a small apartment, and they were considerably "up" from the output of a W.E. 560 AW. Persons who invariably talk louder than the radio when the latter is turned on would have considerable trouble in preventing one enjoying a symphony concert from a local station with such a speaker con-' nected in the output circuit. At the present time the Laboratory staff is busy measuring the characteristics of a number of the newer speakers of this type and when the datum is available in its final form it will be published in Radio Broadcast. the band-pass r.f. ANOTHER very distinct trend is the band selector business. In December, 1927, Dr. F. K. Vreeland read a paper before the I. R. E. on his ideas of what a good radio-frequency amplifier should be. He had two suggestions. One was to stagger the three tuning condensers of a t.r.f. set slightly, that is, tune two of the condensers slightly above and below the exact resonance point. This, according to Dr. Vreeland, would tend to broaden the top of the response curve and to steepen its sides. This would make it possible to receive the high audio frequencies that are