Projection engineering (Sept 1929-Nov 1930)

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Page 28 Projection Engineering, April, 1930 saturation. Under such conditions, the susceptibility of the oscillating member to flux changes is greatly lessened, which fact shows itself in still further distortion. In the new audio tuned pickup, all this is overcome by varying the position of "A" between n and r2. until there is no flux traversing it. Fig. 2 shows how this may be accomplished very effectively in the laboratory. Assume that coil "B" is supplied with suitable audio-frequency current. The leads from coil "C" are connected to headphones or an amplifier. The position of the oscillating member is varied until no sound is heard in the headphones. When this condition is obtained there will be no flux traversing "A." This is the position in which the pickup will perform at its greatest efficiency, as to volume, range, sensitivity, in fact, in every way. Adjustment Of course, such a laboratory method is impossible in practice. However, it has been found that when "A" is brought into the magnetic center, the needle "E" when tapped on either side with the finger nail (see Fig. 3) will produce equal sounds, as to pitch and volume, — whereas, when off center, the two sounds will differ in pitch and Fig. 3. Simple method of magnetic adjustment. volume. This fact is utilized in the practical tuning of the herein described pickup. A small screw-driver is used to adjust the pickup. First the power is turned on, so as to obtain about one-quarter volume. The needle is then tapped gently on either side, as explained above. If the instrument is in tune, the sound on both sides will be equal in pitch and volume. If the right side sounds thinner, the tuner should be turned slightly to the right, — or vice versa, if the left side is thinner — until both sides are equal in pitch and volume. The tuner is equipped with a limiting stop, permitting it to move one-quarter turn to the right or left. In order to gain every advantage which the new tuning principle affords, it is essential that the pickup be correctly designed from the electrical engineering standpoint. When one stops to consider the difference between radio reproduction and sound-on-disc reproduction, this becomes all the more apparent. Preserving Desirable Harmonics In the case of the well-designed radio set, tone color is apt to be superior, due to the fact that the sounds impinging on the microphone are at once transformed to electrical energy and there is no further mechanical inertia to be overcome, until the reproducing loudspeaker is reached. Hence, the important third and fifth harmonics are more likely to be preserved, giving the reproduction a brilliance and color most pleasing to the ear. On the other hand, in the case of the disc record, it is necessary to reconvert the electrical energy into mechanical energy in order to actuate the mass of the knife which cuts the wax record. It can readily be seen that the higher harmonics will probably be lost, since the inertia of the cutter will damp them out. However, this loss in tone color is not important, provided the pickup possesses the necessary refinements to bring out the fundamentals and harmonics actually recorded. The correctly designed pickup can do this, and will reproduce a response characteristic which leaves little to be desired in the natural reproduction of speech and music. Some of the more important design features of the Audak pickup are given below. In this instrument, the entire length of the magnetic circuit is painstakingly balanced and made proof against leakage. Instead of an ordinary magnet, British cobalt steel is used. This is more important than at first would be supposed. Partly, the sensitivity depends on the quality of the magnet. With cheaper magnets, there is quite a heavy decrease in strength at first. Thereafter, as the demagnetizing force lessens, the remaining magnetic strength runs down less rapidly, but it runs down nevertheless, and in a comparatively short time. The pole shoes and oscillating member are cut and precision ground, from a solid piece of special highly sensitive (magnetically) alloy. The tough nature of this alloy makes this a most tedious and expensive job, consisting of a great number of operations. However, the remarkable results obtained, make this worth while. The stationary coil is wound and placed for maximum effect. The maximum free excursion of the oscillator easily exceeds the maximum amplitude recorded. This is of the utmost importance if the record groove is to be properly tracked. The extra play eliminates the quick destruction of the record grooves and other troubles in reproduction. This consideration alone, makes the use of the Audak pickup of extreme value in talking picture work, where a shut-down will produce disastrous results at the box office. SOUND AMPLIFICATION AS A BUSINESS {Concluded from page 20) worse the reproduction. A safe method is to always specify the exact voltage and current frequency (number of cycles or periods) and to accept no makeshift or approximate voltage changing devices. Remember a so-called step-down transformer does just that ; it changes the current and voltage but it cannot and does not change the current frequency. Also remember that a sound installation with power amplifiers is no better than its poorest component part, whether it be a pickup, motor or radio tube. Now the average layman has one idea of what constitutes music, while the real musician has entirely another idea on the subject. The writer has found that one country will positively reject dynamic reproducers or speakers with an accentuated bass while a neighboring market will deliberately feature this type of reproduction. A well-designed dynamic chassis is always important. Here again the engineering work is a factory problem. Any manufacturer of amplifiers will gladly make impartial recommendations as to the speaker to be employed on the output of his amplifier just as he will impartially recommend pickups for the input terminals of his amplifier. It is as important to recommend correctly designed components ate it is to make a correctly designed amplifier. The same reasoning follows in microphones. If a double-button carbon type "mike" is employed in a public-address circuit, it should be well-designed from an engineering standpoint. Its carbon granules can cause terrific "carbon hiss" which power amplifiers would only emphasize, and again the reproduction in the speakers would be more of a roar than reproduction of a human voice. But with components which are designed for use in conjunction with each other and which are correctly "engineered" in their manufacture, satisfaction is assured and service work ordinarily unnecessary. To the phonograph manufacturer, the theatre owner, the architect, the radio wholesaler, the radio enthusiast, the importer of electrical equipment and to advertisers, a new field of universal appeal is opening. What special training does it require? No more than the knowledge involved in installing a radio set. What equipment should the importer purchase? Only thoroughly tested and tried goods. By tried and tested is not meant what is commonly referred to as a demonstration. A trial with sound components means a test over hours, not minutes. The importer must be sure of the "backbone" and stamina of his power amplifier. With well "engineered" amplifiers, component parts and associated equipment, a real business exists wherever electricity is available.