Projection engineering (Sept 1929-Nov 1930)

Projection Engineering, April, 1930 Page 27 Notable Advance in Pickup Design Accomplished Through Scientific Research Electric Pickup Now a Perfected Instrument Through Application of New Principle By H. G. Cisin, M. E. IN this age of scientific wonders, marvelous technical developments are accepted as a matter of course by the general public. Electric lights are improved from short lived flickering toys to rugged enduring torches. Vacuum tubes are developed in the course of a few years from crude experimental devices of few uses, to perfected instruments, with hundreds of useful applications. In the field of sound projection, improvements follow one another with bewildering rapidity. The talking motion picture, which a few years ago was merely an impractical idea, is today a reality. The unprecedented popularity of the audible pictures has Fig. 1. Electrical equivalent of magnetic circuit of pickup. resulted in the concentration of a vast amount of research work on problems involving clearer and better tone quality. Sound-on-disc reproduction has been improved enormously through the utilization of finer electric recording and also through the use of better designed audio amplifiers and of superior dynamic speakers. Up to the present time, however, the electric pickup has been one of the weakest links in the chain of sound reproduction. It is true that the comparatively crude instruments of early vintage, were gradually refined through the use of more precise workmanship and of superior materials. Unfortunately, increased refinements, with proportionately greater sensitivity, rendered the pickup more susceptible to disturbances occasioned by handling and also by changes in weather conditions. The principal trouble experienced with every pickup seemed to lie in the fact that sooner or later the device would get out of "tune." When this happened, nothing could be done to remedy the fault, except to disassemble the pickup and recondition it. In most cases, it was almost as economical to install a new instrument, as to do this. For this reason, a new audio tuned pickup has been acclaimed by sound projection and radio engineers, as one of the outstanding developments of recent years. For the first time, ultrasensitivity in a pickup, is combined with practical utility. Weeks, or even months may go by without this pickup getting out of tune, but when it does, it can be brought back to its point of highest efficiency in a few seconds, and this can be done by anyone, since no skill is required. Incidentally, since the magnetic forces acting on the oscillating member are under control at all times in the new tuned pickup, the most common of pickup troubles — "freezing," is completely eliminated. Before explaining the principle of operation of the new tuned pickup, the action of pickups in general will be reviewed. The electric pickup, along with many other important electrical devices, was made possible through the research work of Michael Faraday in 1831. At that time, Faraday discovered the principle of electromagnetic induction, viz : That an electric current would flow when a closed conductor was cut by a magnetic field. In the electric pickup, the closed conductor consists of a stationary coil made up of hundreds of turns of fine enamelled wire, wound on a bobbin. The ends of the stationary coil are connected directly to the input of the audio amplifier. An oscillating member, constructed of a sensitive magnetic alloy, is free to move within the field of a powerful permanent magnet. The stationary coil is placed at the pole-pieces of the permanent magnet, in such a position that any movement of the oscillator tending to change the intensity of the magnetic field, will cause the changing field to cut across the coil, thus inducing a corresponding electric current in the coil. Needle Mounting The phonograph needle is fastened directly to the oscillating member. This needle, following the track inthe record, vibrates in accordance with the impressions. Hence, the oscillator vibrates similarly, causing the corresponding changes in the magnetic field. Through electromagnetic induction, a fluctuating electric current is set up in the stationary coil, which corresponds in every respect to the sound vibrations recorded on the disc. This current flows to the audio amplifier, where it is stepped up and made available for the operation of a suitable dynamic reproducer. In the electric pickup, it is of the utmost importance that no flux shall traverse the oscillating member when at rest. Before the advent of the audio tuned pickup, the oscillating member was set at the geometric center, thus approximating a point where no flux would pass through it, while in a static condition. As explained above the oscillator often became badly unbalanced or out of "tune," and there was no adequate remedy for this trouble. Fig. 1, shows the electrical equivalent of the magnetic circuit in the Audak electro-chromatic pickup. When the oscillator "A" is in the exact magnetic center (not necessarily the same as the geometric center), no flux will traverse it while at rest. This is the ideal condition. Assuming that Ri, R2, Rs. R<, represent the respective reluctances of the magnetic paths of the four pole-shoes and that ri. r?, r3. r4, represent the respective reluctances of the air gap paths, it is then evident that when Ri + ri R3 + r3 R2 + r2 R« + n no flux can traverse "A." When such a condition is brought about, the forces acting on the oscillator "A" are equal and therefore they balance in all directions. Any shifting of "A" to one side or another, will cause the magnetic flux to pass through it. At this time, the unbalanced pull on "A" will cause it to shift gradually more and more from the magnetic center. This brings about a change in the response characteristic of the pickup and in time, when it is finally very near the pole shoe, performance becomes greatly distorted. "A" is most susceptible to flux changes when the above equation holds good. Frequently the unbalance causes flux to traverse "A" to an extent near as Fig. 2. Laboratory method of biasing.