Radio Broadcast (May 1929-Apr 1930)

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tact point. The weakest pressure on the crystal gave the best detection — that is, the greatest variation in current on positive and negative half cycles of input voltage. Of course, a weak presure indicates an unstable one, and so if, as on shipboard, one wants stability a comparatively heavy contact is used. The experimenter can have considerable excitement by plotting a.c. voltage waves and the rectified current waves as various a.c. voltage waves are placed on such a diagram. The effect of sliding the average value of the input a.c. voltage wave up and down the curve will show how such a rectifier detects. sufficient. With a moving-coil speaker in a three-foot baffleboard, or larger, an output of a watt is probably necessary, and more power than this will make less remote the possibility of overloading. We have operated a single 250-type tube with about 250 volts on its plate, which indicates a power output of less than one watt, but on loud low-frequency notes it does overload. Boosting the plate voltage to 300 or 350 will give all the power that is needed for home reception. Movie Recorders Make a Discovery SOUND -Movie recorders have discovered that audiences are not very critical regarding the quality of the miscellaneous sounds that make up a picture. That is, if the villain shoots a revolver, it is not necessary to shoot a revolver in the studio, provided someone whacks something at the exact moment. In one test case there were several kinds of airplanes on the screen, and, while great efforts had been made to simulate the distinct sounds emitted by the individual planes, only a small percentage of the audience admitted they knew the difference. Half of this percentage were aviators. And so now, when you hear a talking moving picture, note how little difference it makes to you if a series of revolver shots sounds like someone throwing a bucket of coal down the cellar stairs — provided the sounds and sights are synchronized properly. Power Output Required MANY READERS take exception to our statemi. ment that a single 171-type tube provides sufficient power output for home reception. Many state that this is the maximum power output that can be used with the majority of cone-type loud speakers and several state that when they changed from a 171 to a 210-typetube (twice the power output) the difference on a conetype loud speaker was barely noticeable. The fact that many loud speakers rattle if you put 1.5 watts of audio power into them is irrelevant. Our contention was that the sound output from a good loud speaker when less than one watt of electric power went into it was sufficient for home reception. It is certain that, as the response to low frequencies is built up, either by using amplifiers with a hump at some low frequency, or by using a flat amplifier with a moving-coil loud speaker, the power output required increases. It is probable that an output of a full watt is desirable, and this may be obtained easily with the new power tube (the 245-type) that is now on the market. The difference between a single 171-type tube and a single 210-type tube is only 3 db, which is scarcely noticeable to the ear, and if a loud speaker or amplifier, or both, is used which does not reproduce the bass, the difference is not worth while. It is on the low notes, 150 cycles and less, that considerable power output is desirable — and there is no use in providing this power unless there are frequencies of this order to be reproduced, or if the loud speaker will not respond to the fundamental of such frequencies. This seems to indicate that with cone-type loud speakers which do not respond to low frequencies, and which do not show distortion in the amplifier until this distortion is excessive, a power output of less than a watt is The vacuum-tube test rack pictured above was designed by Herbert H. Chun, of the Arcturus Radio Tube Company. The rack is vibrated by an eccentric cam and at the same time the tubes are turned on and off by an automatic switching arrangement. Any defect in a tube becomes apparant within thirty minutes when treated in this manner. Now will the adherents of high-powered receivers come forward and tell us that we are still old-fashioned, and that two or three watts is absolutely necessary for modest home reception? We may, in time, be forced to admit even this! One reader, Fred D. Pinkham, of Topsham, Maine, points out that the problem for the listener far from stations is decidedly different from that of the listener within the shadow of a local station. We admit this but cannot agree with his contention that the receiver must have sufficient power output to handle the following case. Suppose the volume control is fixed so that when a station which fades badly, is at a minimum, it is delivering an audible signal. Then the power output must be sufficient so that when the station "fades in" to a maximum the amplifier will not overload. Let us suppose the minimum signal that is satisfactory is one milliwatt. A station can fade at least 60 db which represents a power ratio of one million. When the station fades in 60 db the power output from the set must be one kilowatt — which is considerably in advance of the most hardened user of power tubes. If the maximum power output is one watt, which is reasonable, the least signal that will be heard, if there is a 60db fade, will be one microwatt, which is pretty far down. It is our hunch that even with one watt output, there must be a time when a fading signal will be inaudible — or will overload when "fading-in." The following from R. J. kr> ter, Engineering Department, Prest-O-Lito Storage Battery Sales Corp., Indianapolis, gives some interesting data on this problem of power output necessary. It checks very closely the data taken by the Everyready engineers a year or so ago. This test determined that a 171-type tube was about all that anyone needed for home reception. It was before the days of moving-coil loud speakers, however. " Some time ago the writer conducted a series of tests in which the signal currents and voltages occuring in the loud speaker circuit were measured for various types of music and with various types of loud speakers. The music was supplied both by phonograph and radio and included concert orchestra, jazz orchestra, military band, various trios, violin and piano, solo piano, singing voices from bass to soprano, and speaking voice. The loud speakers included short-horn, orthophonichorn, magnetic-cone, and dynamiccone types. The input to the loud speakers was supplied by a highquality push-pull 210-type amplifier. The listening tests were made by persons of widely differing tastes and musical accomplishments. "The results of these tests were as follows: (1) " Low" volume was produced with an average signal voltage of 8 volts and an average current of 1.7 mA., corresponding to an output of 14 milli-voltamperes. (2) "Normal" volume was produced with 18 volts and 3.8 mA., or 68 milli-voltamperes. (3) "Loud" music was produced by 40 volts and 10 mA., or 400 rnillivoltamperes . (4) "Very loud" music was produced by 120 volts and 24 mA., or 2900 milli-voltamperes. (5) The extreme limits were: Minimum 0.4 volt and 0.5 mA.; Maximum 195 volts and 55 mA.: maximum power ratio, 53,000. It is of interest to note that both the minimum and maximum values occurred in concert orchestra music, although the maxima were closely approached by the piano. (6) The "average frequency" of music and speech as determined by correlating the average impedance calculated from the above figures with the impedance curves of the loud speaker units was 380 cycles. (7) The impedance of the various loud speakers averaged about 3000 ohms at 50 cycles, 7200 ohms at 400 cycles, 13,000 ohms at 1000 cycles, and 25,000 ohms at 3200 cycles, ranging all the way from 1500 to 60,000 ohms in the 50-5000-cycles band. "You will note that the power is given in milli-voltamperes rather than in milliwatts because the measurements were taken on actual loud speakers rather than on a fictitious resistance load. Calculations made by the writer, however, indicate that the distortion occurring in a vacuum tube feeding into a reactive load will be determined by the voltamperes rather than the actual watts expended in the output. "This data lends further weight to your conclusion as. to the adequacy of the 171-type power tube for average home use. At the same time it demonstrates in startling fashion the great increase in power necessary for a given increase in sound output. Also, it is to be noted that peak values were frequently twice and sometimes three times as great as the above average values. Therefore, if overloading is to be avoided on sustained bass passages or on sudden fortissimos, an output stage capable of supplying I to 2 watts is justified." — Keith Hknni:\ . • may. 1<)2<> page 14 •