Radio Broadcast (May 1929-Apr 1930)

KADIO BROADCAST -X" CeCoTypeJ-71-A — A 5-volt ^ amp. tube for use in the output stage of audio amplifier. Handles 12 times the undistorted volume of the usual type A tube. Lhe tremendous and constantly growing demand for CeCo J-71 and J-71-A Power Tubes is due to two things — first, their capacity for handling greater undistorted volume, and second, making possible an unusually excellent tone quality under full load, clear to the end of their long life. Inquire today about the interesting possibilities afforded by these well known CeCo tubes. gZ^ifTZ Radio Listen in on the CeCo Couriers — on the air every Monday evening at 8:30 Eastern time (7:30 Central time) over the Columbia Broadcasting chain. CeCo MFG. CO., INC., Providence, R. I. No. 280 Radio Broadcast Laboratory Information Sheet Characteristics of the Ear May, 1929 'T'lIU ear is undoubtedly the most commonly used of acoustical devices and the curves on "Laboratory Sheet" No. 281 illustrate a very important and interesting characteristic of the ear. These curves are known as curves of "equal loudness," for each curve shows the pressure required at different frequencies, to produce sounds of equal loudness. The lowest curve marked "threshold curve" is sometimes called the curve of minimum audibility and it indicates the pressures which will produce sounds just audible to the average ear. This curve shows that at minimum audibility the ear varies greatly in sensitivity, at different frequencies. The upper curve, which indicates how the sensitivity of the ear varies with loud sounds, shows the ear to be almost equally sensitive throughout the entire range of frequencies. These curves have a definite relation to the reproduction of radio programs and indicate why we seem to lose the bass when the volume is cut down very low and why a loud speakenseems to boom (too much bass) when the volume is increased greatly. Sounds must be reproduced at a normal volume level, i.e., that level at which we are accustomed to hear music, if the reproduction is to sound natural. Even though the intensity of all the tones in the music are amplified equally well the curves-indicate that a relatively increased effect on the ear will come from the bass portion if the sounds are too loud and a relatively increased effect from the treble if the sounds are too low in intensity. The data from which these curves were plotted was obtained from experiments made in the Bell Telephone Laboratories. The subjects listened to pure tones from a telephone receiver driven by current from an audio oscillator. The listeners compared different tones, two at a time, as to loudness and adjusted the intensity of the two tones so that they were equally loud. All of the apparatus was calibrated carefully and the engineers were able to determine from the setting of the adjustment the sound pressure in dynes per square centimeter. Some additional data on this same subject will be found in "Laboratory Sheet" No. 109 in the July 1927, issue. No. 281 Radio Broadcast Laboratory Information Sheet Characteristics of the Ear May, 1929 THESE curves show the sound pressures which, acting on the ear, give sensations of equal loudness. They were prepared from data obtained from experiments made in the Bell Telephone Laboratories. CO > 0.01 0.001 150 200 340 440 700 FREQUENCY 1000 1500 2000 3200 4000 No. 282 Radio Broadcast Laboratory Information Sheet Amplifier Input Circuits May, 1929 POWER amplifiers such as are coming into prominent use in auditoriums, theatres, dance halls, etc. may be employed to produce entertainment by connecting the input of the amplifier to a regular receiver, by making connections to a phonograph pick-up so that phonograph records may be played, or, in other cases, by connecting a microphone to the input. The amplifier may be used in any of these ways with practically no change in the circuit — the only change necessary is at the input to adapt the circuit to the source from which [the signals are to be obtained. These amplifiers are arranged normally so that they may be used with either a radio receiver or a phonograph pick-up. Appropriate terminals for these two devices form an integral part of the amplifier. In case the amplifier is designed for use only with a radio receiver, a phonograph amplifier may be used readily by connecting the two terminals of the pick-up across the primary of the first a.f. transformer of the amplifier. However, when it is desired to use a microphone at the input of such an amplifier a change is necessary since the microphone requires direct current for its operation and also the microphone is a low-impedance affair whereas the phonograph pick-up and radio receiver have high output impedances. With a microphone it is necessary that a special microphone transformer bo used to adapt the impedance of the microphone to the input impedance of a tube. The primary of the microphone transformer should be connected across the microphone and in series with a few drycell batteries or a storage battery, the latter being preferable because of its greater capacity. The secondary of the microphone transformer is connected to the grid of the first a.f. amplifier tube. Microphone transformers are made by most of the well-known transformer manufacturers and complete instructions regarding their use can be obtained easily. It should be realized that the above notes do not apply to all power amplifiers since some of them are equipped with three sets of input terminals so that either pick-up, radio, or microphone may be used. Obviously, the thing to do when purchasing such an amplifier is to decide what is to be used to supply the input signals and to then make certain that the amplifier under consideration is arranged with the proper input connections. This sheet is the result of several letters received at the Laboratory from readers who have been under the impression that it is possible to obtain good results by simply connecting microphones across the input of an amplifier originally designed for use with a phonograph pick-up unit or radio receiver. • may, 1<>2<) page 54 •