Radio Broadcast (May 1928-Apr 1929)

AUGUST, 1928 ALL ABOUT LOUD SPEAKERS 191 parison with the output resistance of standard radio tubes, it is necessary to use a transformer designed particularly to bring these impedances into the proper relationship. In some speakers this transformer is included in the housing of the speaker. Separate transformers are now available, which have been designed to be used between the moving coils and the standard power tubes. When purchasing one of these transformers, the particular power tube, or tubes (if the last audio stage is push-pull) used in the set must be specified, as different transformation ratios are desirable for different tubes. Most of these transformers are so designed that the impedance looking from the tube into the primary of the transformer is twice the plate resistance of the tube. This condition gives maximum undistorted output. In the dynamic speaker, if this relationship is made right for a frequency of about 1000 cycles per second, it is approximately right for the entire audio range. It is very important to mention that while some types of speakers are theoretically inherently better than others, in practice, due to inadequate design, this is frequently not so. The writer has seen more than one make of cone speaker which was inferior in almost every way to the average conical horn speaker. A dynamic speaker recently came to the attention of the writer which was one of the poorest speakers he had ever examined. The type of unit determines its theoretical excellence, but careful design makes a good loud speaker. REQUISITES FOR A BROADCAST RECEIVER THE question of judging the quality of a loud speaker is, properly considered, a laboratory study. However, the relative merits of different speakers can be determined with a fair degree of practical accuracy if they are intelligently compared while listening to a good local broadcasting station through an adequate receiving set. Before describing the method of comparison it will be well to discuss in general terms the requisites for a good broadcast receiver from the point of view of audio quality. In the first place the set must not be more selective than is absolutely essential for the separation of stations. As a rough measure of this, the set should not be capable of separating two stations of equal strength and distance, separated by 20 kc. in wavelength. Selectivity, in general, is incompatible with quality, since the more selective a set is the more it sacrifices the higher audio frequencies. An overloaded detector is a frequent cause of distortion. There is no need to overload the detector provided the audio amplifier has suf 22 20 18 16 1 M 1 12 I 10 Exponential / — / 1 1 1 1 1 "Conicat 1 1 1 ND HORNS 1 / CONICAL A EXPONENTIAL \/ 500 1000 FREQUENCY, CYCLES PER SECOND 5000 10.000 + 25 z. +10 5 0 50 100 500 1000 FREQUENCY CYCLES PER SECOND FIG. 6 5000 10,000 FIG. 7 These curves were made for the natural response of a cone or an exponential horn, without driving units ficient gain. Such an amplifier should have a gain of at least 60 tu and preferably 70 tu. With such an amplifier sufficient volume is obtainable from the loud speaker with a small detector output and without the use of regeneration in the detector circuit. The audio-frequency amplifier is a source of almost every known type of distortion. The modern transformer-coupled amplifiers using the best makes of transformer and connected according to the manufacturer's directions, paying particular attention to obtaining the correct A, B, and C voltages on all tubes, are capable of excellent performance. Unfortunately, there is no reliable way of testing such amplifiers without laboratory equipment of a type which is not generally available. The design of the power stage of the audiofrequency amplifier is very important. It is safe to say, that in order to obtain, in an aver age room , reasonable volume without appreciable distortion it is necessary to have available at least one watt of undistorted output, if the loud speaker has a good frequency-response characteristic. This requires a last stage at least the equivalent of a type 210 tube. With the best types of speaker available at the present time it is desirable to use a push-pull circuit for the last stage, with type 171 or type 210 tubes. There is now available on the market a tube (type 250) with a still greater output. One of these tubes is sufficient to give a large undistorted output for a loud speaker used in quite a large room. Two such tubes used push-pull with proper associated apparatus will deliver sufficient power to fill a fairly large auditorium. The method of coupling the speaker to the power stage is a matter of much importance. Two schemes are in common use; the transformer, and the choke coil and condenser. If the tube and loud speaker have the proper impedance relationship the choke and condenser scheme is efficient, but if the impedance relationship is not correct, a transformer should be used which has been designed for that particular combination of tube and speaker. In any case the maximum undistorted power is delivered to the speaker when the impedance looking from the plate circuit of the tube into the primary of the coupling transformer is twice the plate resistance of the tube. Loud speakers should never be compared by listening to their reproduction when connected to a poor receiving set, since the better of the speakers will sound worse. This is due to the fact that the better the frequency-response curve of the speaker the more the defects of the receiver (especially overloading), will appear in the reproduction. If, however, two loud speakers are connected to a double-pole double-throw switch, the knife posts of which are connected to the output of a good set, which set is tuned to a good local broadcasting station, a useful comparison may be made. The relative intelligibility of speech is an indication of the presence of the higher audio frequencies. If "i," "s," "v," "b," "p" and "th" are clearly distinguishable, the loud speaker has a good high-frequency characteristic. If, when listening to the piano, the tones are deep and rich, the low-frequency characteristic is good; on the other hand, if the piano sounds thin and tinny the low-frequency characteristic of the speaker is deficient. If the voice is full and clear and intelligible, and yet has an unnatural metallic quality, there is at least one high peak in the middle or upper range of the frequencyresponse curve. As shown by the curves in Figs. 4 to 8 inclusive, any or all of the above defects may be found in loud speakers. If, however, the comparisons are made intelligently, noting the above-mentioned points, a practical choice of speakers can be made on a rational basis, but the observer must assure himself that the defects noted are speaker defects and not defects arising from inferior broadcasting or an inadequate receiving set. The development of the art and science of broadcasting has been very rapid, and remarkable results have been achieved in every branch. A large and reliable manufacturer announced only a few months ago the perfection of a loud speaker for use in public address and other commercial systems, which has an efficiency of 50 per cent, and a practically flat frequencyresponse curve from 20 to 8000 cycles, devoid of all sharp resonances! It is likely that little time will elapse before such an instrument will be available to the broadcast listener. Perfect radio reception is no longer a remote phantasy. 1-20 -15 BAI EX jANCED 1 PONENTI iRMATURE \ AL HORN 1 100 500 1000 FREQUENCY, CYCLES PER SECOND FIG. 8 5000