Radio Broadcast (May 1928-Apr 1929)

208 RADIO BROADCAST AUGUST, 1928 FIG. 3. THE AMPLION REVELAPHONE that now remains is to amplify this sufficiently to be heard well from the modern loud speaker. Here is where the radio receiver comes into the picture. So far, we have only used the turn-table, motor, and record of the old photograph. The electrical pick-up unit has taken the vibrations off the record and has transformed them into Pick up Wires Pole Piece -Actuating Arm Fixed Pivot -Needle FIG. 4. A CROSS-SECTIONAL VIEW OF AN ELECTRIC PICK-UP UNIT electrical currents; now they are to be amplified by the radio set. ONLY THE AUDIO IS USED THESE currents are not radio currents at all. They are merely the amplified music and voice from the record. Hence, only the audio amplifying end of the radio set is utilized, and the pick-up unit must be so designed as to be readily attached to this audio amplifying section of the radio receiver. The currents are then amplified through the audio amplifier and are reproduced through the loud speaker, just as the radio programs are amplified and reproduced after they are detected by the detector tube. One would guess from this that the pick-up units are attached in some way to the detector tube, at the beginning of the audio amplifier. All of the electrical pick-up units tested in my laboratory operated on this principle. Most of them were arranged to operate in the plate circuit of the detector, although one was arranged to work into the grid of the detector tube and thus gain the amplification of the detector tube. A careful study of the constructional details of quite a number of different designs shows the necessity of some sort of damping on the vibrating piece of thin iron. Naturally, everything has a natural or inherent period of vibration. Just as a tightly stretched piano string will vibrate at a certain pitch when plucked with the finger, so will the thin iron reed in front of the pole pieces of the magnet tend to vibrate at some particular pitch whenever it is set in motion. If this were not damped or stopped in some way the unit would rattle on certain notes and blast and distort the music. This damping is accomplished by mounting pieces of soft rubber tightly between the iron reed or diaphragm and the pole pieces. The photograph in Fig. 5 shows how this is done in the pick-up unit made by the StrombergCarlson Company. In the Amplion Revelaphone the entire vibrating iron reed is pivoted in sponge rubber. The reed is thus left free to vibrate between the pole pieces, but is damped by the rubber mounting at its pivot. The Bosch Recreator also operates on the damping principle of a rubber pivot rather than rubber between the pole pieces. The Baldwin Needlephone has a rubber damped pivot as well as damping rubber between the pole pieces. MOUNTING AND VOLUME CONTROL THERE are two main methods of mounting these electrical pick-up units on the phonograph turn-table. Most of them are built with their own mounting arm which holds the magnet and the needle on the record. The mounting arm is swiveled on a supporting base which screws on to the top board of the phonograph, adjacent to the revolving table. This is shown in Fig. 3, a picture of the Amplion Revelaphone. The Bosch arm is swiveled on a leaded base which is heavy enough to hold the arm in the correct position without screwing the base down to your phonograph. The Stromberg-Carlson unit has a leaded or weighted base, but has provision for screwing it down also, if desired. The second principle of holding the electrical magnet and needle on the record is that employed by the Baldwin Needlephone and the Pacent Phonovox. Both of these units are arranged for attachment on the present phonograph arm which holds the regular sound-box. The regulation of volume of tone has brought forth almost as many ideas as there are different electrical pick-up units. In general, these volume control boxes are variable resistances which shunt out some of the electrical currents generated in the pick-up unit. An adjustable knob enables the volume to be reduced to an almost inaudible whisper. In the Pacent Phonovox and the Amplion Revelaphone these volume regulators are quite small and look more or less like an electric switch such as is often used in lamp cords. The Stromberg-Carlson control box is medium and is arranged for screwing on the baseboard of the phonograph, if desired. The control box for the Bosch is the largest of all, it being the largest piece of apparatus in the pick-up ensemble. Then, again, we have the other extreme in the Baldwin Needlephone, which has no volume control at all, but relies on the volume control in the radio receiver. The question of weight on a record is indeed a serious problem. Records as well as needles wear out, and we are quite familiar with the fact that needles should be changed rather frequently. PICK UP WIRES RUBBER PADDING BETWEEN REED and POLE PIECES MAGNET PICK UP COIL THIN IRON REED FIG. 5. THE COMPONENTS OF A TYPICAL PICK-UP UNIT However, one is not accustomed to think in terms of "record wearing." Nevertheless, this is quite an important factor, if the electrical pick-up unit is made at all heavy. It is this factor that has controlled the size of the magnet in the pick-up unit and accounts for the fact that all of them are extremely small. Of course, it would be possible to build a pick-up unit which would deliver considerable volume without amplification, but, the weight of the magnets would soon ruin any phonograph record. In this connection, it is FIG. 5. THE STROMBERG-CARLSON PICK-UP UNIT