The Gramophone Pick-up

Radio Record, Volume III, Issue 3, 2 August 1929, Page 37

The Gramophone Pick-up

How it May be Used

(By

MEGOHM

ROVIDED that a good amplief fier is in use on the radio receiver, yeproduction of. gramophone records upon the loudspeaker is now an easy matter, and involves practically no expense beyond the purchase of a pickup to vattagh to the gramophone. Those who do not already possess a gramophone may, through the medium of a pick-up and the audio amplifier, provide themselves with: a satisfactory substitute at comparatively low cost. Any turn-table that is regulated by a (oe rnor may be used in connection actt a pick-up, and may be quite capable of giving the best reproduction that may be obtained from the amplifier, as it is upon. this latter that the quality of the music depends, provided, of course, that the pick-up is one of recornised merit, °

Some pick-ups now on the market ‘may be obtained with or without tonearm, the latter in many ¢dses. heing accompanied by 4 volume éontrol, which js a variable high resistance suitably connected. If the tone-arm type is purchased, the arm may be fastehed in position on the gramophone, leaving the original tone-arm in place, to be turned back out of the way. The correct position for the tone-arm must be properly determined, or else the "wear of records may be excessive, and \ reproduction may not be. the best possible, The method of placing .he tone-arm is to lay a straight-edge upon the turntable, with one edge touching the central spindle. Orie of the largest records -to be used ‘must be upon the turn table, and the beginning and finishing positions of the: music noted, as at A and B.- Half-way between these two points (©), a line is drawn at right angles to the straight-edge, and carried ont in the direction’ of D. A néedle is now. placed in the pick-up, and the tone-arm pillar moved algng the line © D until the needle rests upon the central line at a point C on the straightedge, that allows it to swing in an are from A to B, the extreme points of the recording grooves, *s shown in the diagram. The tone-arm is then secured in position with screws.‘ When finding position the needle must be kept at its correct angle, and the tone-arm must Y gorratey if necessary, with packing, that it is the correct height to allow of the néedle running at the proper angle. If the pick-up. unit alone is purchased, it will fit into the head of tonearm instead of the regular reproducer. Some units are provided with two removeable sockets, tach a different size to fit standaftd tone-arms. Any ordinary gramophone needle is used, but somé users may prefer to avoid constant’ changing by employing one of the semi-permanent type which play about two dozen records.

Constructors who are fixing up their turn-table movement may in some cases mike a. special tone-arm, Failing one of metal, this may be of wood, but should not be less than one inch square, otherwise rigidity will be lost. Such a tone-arm may be rather longer than the usual gramoéphoné arm--say, 12 inches from needle to point of rotation.

Pick-ups generally weigh about five ounces, and this weight is usually allowed to rest upon the record, but where it is considered that the weight is more than is advisable for the life of the record, some kind of ‘spring or counter-weight may be employed: to take a little of the weight off the record. This, however, must not he carried too far, or the deep notes will not be reproduced correctly, and if the pressure is much too light, general quality suffers badly,

When a pick-up is attached to the ordi --y gramophone tone-arm, it has a deceptive appearance with regard to finding the position for their attachment to the top of the cabinet. A straight line joining the needle to the centre of "the tone-arm pivot must be considered as the tone-arm, ‘ignoring any curves or turns constituting its actual shape. . In ‘constructing a tonearm the pick-up niust be attached at such an angle. that fixes the cise at right-angles to a line drawn from the needle to the pivot of arm. Connections and Volume Control. HERE are two ways. of connecting a pick-up to a broadcast receiver. The usual method is to. connect the two leads either direct to the primary of the first udio transformer, or in an indirect way by means of | an adapter, sometiiees- supplied, which plugs into the detector: socket instéati of the valve. The alternative method is to .connect the pick-up unit to the grid of the detector valve, conveniently by a flat adapter . which ~connects to the grid and filament negative prongs, after which the valve js inserted in its socket. . This method makes ase of the detector valve as an amplifier, and so will giye added volume, but on the other hand, the low frequencies (deep notes) will not, be reproduced s) well by this artangement, as the presence of the vilve causes the pickup to give more output at the high frequencies, and thus amplification is rendered uneven. Needless to say, if reproduction is equal or beats that of a good gramophone, thé amplifier must bé a good

one; it must give fairly even amplifieation throughout the musical scale, and must be capable of reproducing well the lower frequencies. The same remarks will apply to the loudspeaker, which should be a good cone or dynamic. — ot If a volume control is-already provided on the amplifier, it may be used to regulate the gramophone music, un-. less it is desired to locate the gramophone away from the amplifier, perhaps in another room. In such case a volume control may be provided at the machine. When running pick-up leads for distant operation, care must be taken to avoid. running them near to the loudspeaker leads, otherwise a whistle or howl, caused by interaction, will be set up. Pick-up leads may be 26’s to 30’s d.e.c. . It is quite probable that a pick-up will zive highly satisfactory results without either compensating resistances or scratcli-filter, but. the methods of utilising these are given to enable experimenters and others to deal with éxtreme cases. Compensation Methods. Y7OLUMD control in direct connection with the pick-up may be effected in different ways, with particular regard to suiting the actual pick-up to be used. These methods are illustrated in diagrams. The first shows how amplification may be to some extent eyened up when the first .{ransformer has a high peak, that is, gives empha‘sis to the high audio frequencies. With a volume control of comparatively low resistance, say 10,000 .chms, the pick-up will not respond well to high frequencies, and the lower this resistance, the less will be the response. The peak of the transformer is then relied upon to restore the weakened high frequencies instead of over-empha-sising them.

2 a « a. el The disadvantage of this scheme’ is that it weakens the low frequencies as well as the high. If the amplifier as a whole, or the pick-up, has too high a peak, this can be reduced by inserting at Roan extra resistance of 10,000 to 20,000 chms. oo. . The second scheme is for reducing the high frequencies by alternatively placing the extra resistance in the grid return of the transformer secondary. This resistance may be from 50,000 to 250,000 ohms to suit the pick-up and transformer. This method does not reduce the low frequencies, but should.

rent ‘not be used with detector valve input, or the high frequencies will be practi"eally lost. A diagram is given of the usual yolume control, also showing the position of a scratch filter should it be required. A variable resistance of not less than 50,000 ohms should be used for the control. In the other diagrams. a potentiometer control is shown, the resistance of which must be Bigh-500,000 ohms if the detector is used as amplifier. In cases ‘where the leads from the pick-up to amplifier are lengthy, an in-

put transformer should be provided at the amplifier. This is already provided for in a transformer-coupled amplifier. As a general rule, the best results with the lower frequencies will be obtained when the pick-up is connected to a transformer primary of high impedance, and this applies particularly to the ease where a dynamic speaker with baffle-board is employed.

* Serateh-Filters. [t is. not likely that with efficient _ apparatus there will be sufficient needle scratch noise to be objectionable. Such trouble is more likely to be encountered when the pick-up signals are fed to the grid of a detector valve, as this method is very much in favour ‘of high frequencies. © Should it be considered necessary to provide a scratch-filter, a honeycomb inductance and fixed condenser are connected as shown in the diagram. A 150-turn honeycomb coil will be suitable, and may have’a small amount of iron or stalloy placed in its centre in order to slightly broaden the bypassed waveband. With a fixed condenser of .008 mfd., the filter tunes.to about’ 5500 cycles, so that most of the seratch noise is cut out without affecting to any extent the quality of the music. Selecting a Pick-up. ANY different makes of pick-up are now to be obtained, and probably all may be classed as good, but at the same time there is a variation in their characteristics, chiefly with regard to volume, and as to whether the high or low frequencies are most emphasised. Low-note response should be sought, as the high frequencies are rarely subordinated. Prices are now so reasonable for a reliable article that no attempt at construction is advised. The factorybuilt pick-up is a collection of a vast amount of research and experience, enabling very perfect reproduction to be effected. The home-made variety is usually a "tinny" sounding production requiring liberal "damping" with rubber, a condition which puts severe wear upon the records, causing theiv rapid deterioration.

Electric Turntables. LTHOUGH a_ good, double-spring motor will do its work well, an electrically driven turntable is a great convenience, obviatiing the necessity for continual winding. Several good makes of turntable, governor and motor combined may be purchased. In some of these a smooth-running motor is geared silently to the- turntable, whilst another type, scarce at present in New Zealand, runs by an induction motor using alternating current. In this type there is no gearing of any kind, but on the spindle of the turntable 'there is a copper disc about five inches in diameter, running between electro-magnet poles, which impart to it the necessary rotary motion.

Here, too, the constructor is warned against attempting electric. driving with an unsuitable motor. There are upon the market some small motors selling at a pound or less. Some of these, running on six volts, have sufficient power to run 4 gramophone, but they are hot constructed for lasting service, and after a few days’ work the spring brushes wear through, necessitating replacing by ~carbon brushes if too frequent renewal and undue

sparking are to be avoided. The coms, mutator is another source of trouble! in this type of motor-it soon wears" out. -Another great objection is the noise created, and unless this can be properly subdued, quality of reproduction will suffer. . us The only satisfaction to be obtained by fitting a separate electric motor to an existing turntable is by purchasing a well-made, quiet-running motor and coupling it to the machine by. a truerunning rubber pulley bearing against either the governor pressure-dise or the underside of outer edge of turntable. The former method is much the better if the speed works out satisfactorily. A brake is hardly necessary with the electric drive, as only a few revolutions will be run after the current is switched .off. a, An interesting problem for ope =! menters is to devise a simple ¢fp,.4t switch to automatically stop the motor when the tone-arm reaches the.end of the record. This may take the form of a light arm’ with adjustable stop, resting on the tone-arm. When the latter touches the stop it pushes a lever which cuts off the current, a smail mereury-cup making a very suitable contact. he instalment of electric reproduction on your gramophone will give the instrument a new lease of life, particularly now. that radio claims the premier place in so many homes. The improve- | ment in reproduction over the old style will be a pleasant surprise to many, and records per medium of the loud-. speaker will become a very popular adjunct to radio.