Radio Broadcast (May 1928-Apr 1929)

Pick-Up" Shall I Buy? Grimes Here is an unbiased description of the various manufactured pic\-up units now available for modernizing the old-style phonographs with the aid of electrical reproduction and amplification. With the data given here you can choose the unit which best suits your own needs, and provide the accessories which have been proved most valuable in electric phonograph reproduction. THE EDITOR. ELECTRICAL pick-up units first started to appear in radio circles early in 1926. Since then several dozen different types have been manufactured and as many articles have appeared covering their operation. Yet at a recent public demonstration of a particular make in one of the large New York stores, nine out of ten people were amazed that such a device was in existance and, of course, knew much less of its workings. For the benefit of the uninitiated, let it be here stated that an electrical pick-up unit is nothing more than an electrical sound box for your phonograph. The name "pick-up" unit has been given to that particular arrangement suited for connects ing your old type phonograph with your radio receiver. This has required a few circuit kinkwhich are new, but the fundamental principle employed goes back to the early stage of the telephone and the phonograph. Alexander Graham Bell about 1875 discovered that when a thin piece of magnetic metal was vibrated in front of an electro-magnet, currents were created in the windings of the magnet. These currents were exactly similar in their electrical vibration to the mechanical vibration of the magnetic metal directly in front of the pole pieces of the magnet. Thus, when he talked directly against this thin Coil Wires---. j Pole Piece Wire Coil — wwJ . Thin Iron Diaphram FIG. I. A CROSS-SECTIONAL VIEW OF AN ORDINARY TELEPHONE RECEIVER piece of iron it would vibrate and create currents in the windings of the magnet similar to his voice vibration. This was the first electric telephone. Reference is here made to Fig. 1, which shows a cross-sectional view of the modern telephone receiver with its electrical windings, magnet, and thin metal diaphragm located directly in front of the magnet. But what has a telephone receiver to do with this subject? Bell's first telephone used only one of these receivers at each end. The subscriber talked and listened through the same device, switching it to his ear when he wanted to listen and to his mouth when he wanted to talk. Few people realize that the telephone receiver to-day is practically unchanged from Bell's original conception of the complete electric telephone. As the art developed, other more sensitive principles were used for the telephone transmitter or mouthpiece, but nothing has been found better for the receiver. And even now the receiver, when used as a transmitter, produces better tone quality but less volume than the ordinary telephone transmitter. We gather from all this that an electro-magnet with a thin piece of iron in front of its poles will act either as a transmitter or as a receiver. You can prove this for yourself at any time by holding your hand over the telephone transmitter and talking to your party at the other end of the line by shouting into the receiver. You will have to talk rather loudly, as the efficiency of this circuit arrangement is quite low, but you will be heard very distinctly at the other end of the line. Now, Thomas Edison brought out the phonograph a couple of years after Bell's telephone. This was a device which took the minute vibrations of the thin iron disc and, instead of changing them into electrical impulses, recorded them on a wax cylinder which was rotated when recordings were made. This was done by attaching a sharp needle on to the center of the thin disc. This needle cut a waving impression in the wax cylinder as the disc moved to and fro under the influence of the person's voice. Then, when it was desired to hear the record, the thin disc was placed at the end of a horn and the needle was made to travel over the same waving path which it previously had cut. The wax groove forced the needle to and fro, which in turn actuated the diaphragm. Such a device is called a soundbox on the modern phonograph. A diagram of it appears in Fig. 2. An electrical pick-up unit is merely the combination of these two inventions. A phonograph needle must be attached to the pick-up device. This needle actuates a thin strip of iron mounted directly in front of the pole pieces of an electromagnet. As the needle is forced back and forth by the waving nature of the grooves of the phonograph record, the thin iron diaphragm is forced to vibrate in unison in front of the electro-magnet. This vibrating magnetic metal induces electrical currents in the windings of the magnet whose vibrations are similar in nature to those of the diaphragm, and in turn to the grooves in the record. See Fig. 4. Of course, these currents are extremely weak, although very clear and excellent in tone quality. If we should place a pair of headphones across the output of this electric pick-up unit we would hear some very fine music. The only problem ■Thin Diaphram Actuating Arm Fixed Pivot Needle Holder Needle FIG. 2. A CROSS-SECTIONAL VIEW OF A PHONOGRAPH SOUND-BOX 207