The Principle of Gramophone Pick-ups

Radio Record, Volume II, Issue 11, 28 September 1928, Page 26

The Principle of Gramophone Pick-ups

TLECTRICAL pick-up units first started to appear in radio circles early in 1926 states an excellent article in "Radio Broadcast." 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 existence, and, of course, knew much less of its workines. 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 connecting your old type phonograph with your radio receiver. This has required a few circuit kinks, which 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 agaiust this thin piece of iron, it would vibrate and oe SS ae Oe

create currents in the windings of the magnet similar to his voice vibration. This was the first electric telephone. EW people realise that the telephone receiver to-day is practically unchanged from Bell's original conception of the compicte 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. Now, Thomas Edison brought out the phonograph a couple of years after Bell’s telephone. This was a device that took the minute vibrations of the thin iron dise, and, instead of changing them into electrical impulses, recorded them on a Wax e¢ylinder which was rotated when recordings were made. This was done by attaching a sharp needle on to the centre of the thin dise at the end of 2. horn, and the needle

was meade te travel over the impression in the wax cylinder as the (ise moved to and fro under the influnee of the person’s voice. Then, when it was desired to hear the record, the thin dise was placed at the end of a horn and the needle was made to travel over the same waving path which it had previously cut. The wax groove forced the needle to and fro, which in turn actuated the diaphragm. AN electrical pick-up 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 electro-magnet. 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. Of course, these curventS are extremely weak, although very clear and excellent in tone quality. Yf we ssould place a pair of headphones across the output of this electric pick-up unit, we would hear some very fine music. The only problem that now remains is to amplify this sufficiently to be heard

well from the modern loustaspeaker. 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 phonograph. The electrical pick-up unit has taken the vibrations off the record, and has transformed them into electrical currents; now they are to be amplified by the radio set, Only the Audio is used. YHESE currents are not radio cur rents at all. They are merely the cunplified music and voice from the res cord. Hence. only the audio amplify} ing end of the radio set is utilised, and the pick-up unit must be so designed as to be readily attached to this audio amplifying section of thé radio receiver. The currents are then. amplified through the audio amplifier and are reproduced though the loud speaker, just as the radio programmes 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. Most of them are arranged to operate in the plate circuit of the detector, although one is 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 2 natural or inherent period of vibra-_ tion. Just as a tightly stretched 4 Yano y string will vibrate at a certain pitch when pluc ked 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. In the Amplion Revelauphone the entire vibrating iron reed is pivoted in sponge rubber. The reed is thus left free to vibrate between the pole piecesy. but it is damped by the rubber mount ing at its pivot. The Bosch Recreator also operates on the damping principle of a rubber pivot, rather than rubber between the pole pieces. The Needlephone has a rubber dampe pivot as well as damping rubber bee tween the pole pieces. Volume Control. ‘Pub 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. The question of weight on recora is indeed a serious problem, P Records as well as needles, wear out, ard we are quite familiar with the fact that needles should be changed. rathet.fre-. quently. However, one is not accustimed 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 accouyts for the fact that all of tem are extremely small. How the Units are Connected. "THE methods of connecting the pickup unit to the detector tube vary considerably in each case. The Bosch Recreator is attached by means of a special plug which fits into the detector socket in place of the detector tube. A the top of the plug are two tip jacks into which the cord tips are inserted. The Amplion Revelaphone is built with a combination plug and socket. The detector tube is removed, the plug inserted in the detector socket, and the detector tube replaced in the ipciat socket at the top of plug. The Stromberg-Carlson electrical pick-up unit is built with a standard plug, which is made specially to fit into the proper jack for the purpose in their various models of radio receivers. The Pacent Phonovox Unit is designed with a very unique method of attachment which utilises the amplification of the detector tube and so permits the tube to be reinserted in its socket. A thin bakelite strip with che proper holes is pushed on the pins at che base of the tube. The tube is then inserted into the socket. By means of eyelets in two of the holes (the grid and filament) eontacts are made with two pin jacks at the outside edge of the bakelite strip. The electrical pick-up unit is eonnected by cord tips in these two pin jacks. The electrical coils wound on the pick-up magnet must be very small, mainly because of the smallness of the pick-up magnets themselves. As previously discussed the magnets must be a&& 8mall and as light as is consistent

with ‘ood results, in order to reduce the wear on the photograph records. In order to make a -ompact, small, — electrical coil, it is necessary to use very small wire. This wire is hardly larger than a hair, being in the neighbourhood of No. 40 B. and 8. gauge. Obviously, such a coil would quickly burn out if any battery currents were allowed to flow through it. Of course, any unit which is attached to the grid of the detector tube does not have to have any special precautions for elimjnating the battery currents from the electric winding, as no battery current exists in this part of the circuit. All of the pick-up units that are designed to operate on the plate circuit of the detector or the primary of the figst audio transformer, must be arranged with a fairly large condenser in series. This condenser prevents the fiow of the detector battery currents through the pick-up unit, but allows the audio current generated in the unit to pass through into the primary of the first audio transformer. How to Get Best Results. Alt of the electrical pick-up devices | which we have tried operated very satisfactorily. Some were particularly low pitched while others covered the higher ranges. It would be entirely a matter of taste which would be corWMered the best from the point of view of tone quality. In this connection several important things shoulc be mentioned in order to do jus_tice ,fo all types of electrical pick-up units. In the first place, it is most desirable to use the tatest type of electrically recorded, phonograph records.

eee On the Victor records this can be easily ascertained by the letters "VEX" at the top of the name-plate at the centre of the record. This means "Victor Electrie." Electrically cut records of other makes are also plainly marked on. the signature of the disc. The old-fashioned records were not made with the bass notes because there was no way é@f reproducing "hese bass notes. You wil not obtain the best tone quality on any electric pick-up with ai. old record. In the second place, it is essential. for best results, to employ a power tube in the last audio stage of your radio set. Such a tube should be operated on at least 135 volts and should preferably be of the 171 type This has more effect on the reproduc tion of the bass notes than the audio transformers of your receiver. I have heard of a case where a man went to all the trouble of replacing his audio transformers in his radio set to give him the bass notes and then did not employ a power tube in the last audio stage. In the third place, a good cone speaker, an exponential horn or a dynamic speaker shoul be used on the set. The use of the old type horns or small cone speakers will take away much of the excellent tone quality which is noticeable in the modern Orthophonic and Panatrope phonographs. The most important of these suggestions is that you should have a power tube and good speaker if you are seriously interested in good quality. The reason for the excellent tone quality on the latest phonographs is not solely the electric sound-box. A loud-speaker and a good power tube, such as is used in the. Panatrope, are equally essential. x