US2005154A - Phonograph pick-up - Google Patents

Description

June 18; 1935 s. MCCLATCHIE 2,005,154

PHONOG'RAPH PICK-UP Filed June 25, 1951 [NVEN T 01?.

A T T ORNE Y5.

Patented June 18, 1935 UNITED STATES PATENT OFFICE Application June 25, 1931, Serial No. 546,705 In Germany June 28, 1930 10 Claims.

This invention relates to improvements in electrical phonograph pickups, more particularly in electromagnetic pickups. The chief objects of the invention are to provide improvement in the quality of reproduction from phonograph records,

to reduce the wear on the record and on the reproducing needles, and to simplify the construc-. tion of pickups.

My invention is illustrated in the accompanying drawing in three different forms. One form is shown in Figures 1 to 4. Fig. 1 is a front elevation, with the coil 8 shown in section. Fig. 2 is a top elevation with the magnet I and one of the pole-pieces 2 removed. Fig. 3 is a plan view of the armature system as seen from the side, one set of pole-pieces and one of the armature supports 1 being removed. Fig. 4 is a bottom plan view of the armature 6, with portions of the associated pole-pieces. In Figs. 2 to 4. the coil is eliminated.

A second structure embodying my invention is shown in Figs. 5 to 8. These figures correspond exactly to the foregoing.

A third structure is shown in Figs. 9 and 10. Fig. 9 is a median sectional view of the device as a whole. Fig. 10 is a bottom plan of the armature with portions of the associatedpole-pieces.

Referring to Fig. 1, I indicates thepoles of a permanent magnet, the upper portion of which is not shown. The soft iron pieces 2, 3 and 4 form a four-pole system of the standard type. They are secured together by means of the screws 5.

Within the four-pole system is the usual pickup coil 8, preferably form-wound and held together with wax, in order to save the space which would otherwise be occupied by a supporting spool. The structure thus far described is wellknown in the art. My invention consists more particularly in the structure of the armature. In the embodiment of Figs. 1 to 4, the armature 6 consists of two sheet-metal parts of iron or other magnetic material, formed as shown and fastened together, preferably by spot-Welding. This armature is supported by the feet 1. which are aiiixed to the pole-pieces 4, preferably also by spolt-welding. These feet are also best joined to the armature by welding.

The two stampings which together form the armature are each provided with a trough. These two troughs match in such Way as to form a pocket So for receiving the stylus or needle. This pocket is of tapered cross-section, as shown in Fig. 4. The needle wedges in the pocket and is firmly secured. without the use of a holdingscrew, through the weight of the pickup head pressing the needle angularly against the record.

This method of securing the needle is known in the phonograph art. It is however not generally known that the method is entirely efficacious; with proper dimensioning no chatter of the needle in its holder occurs at any audible frequency. On the other hand, there has not heretofore been found any simple means for retaining the needle in its holder while the pickup is being lowered onto the record. Magnetic holding of the needle has indeed been suggested, but no simpleand efficacious means for applying magnetism to the needle has previously been known. With the usual massive type of armature commonly employed in electrical pickups, not enough magnetic field acts upon the needle to secure the same against dropping out while the pickup-head is being lowered.

The thin sheet-metal structure of my armature allows a magnetic field to act upon the needle which is amply sufiicient to secure the same. In fact, With a field magnet of normal size, it is nearly impossible to dislodge the needle by shak ing or jarring with the hand.

The elimination of the usual needle set-screw greatly facilitates the changing of needles. But I achieve with my invention another and even more important object. In the first place, the elimination of the screw greatly lightens the entire armature structure, and makes practicable the use of thin sheet metal, instead of forgings as has heretofore been customary. In the second place, with the screw out of the way, my invention provides for pivoting the armature at its normal center of mass, which in most practical cases lies somewhere within the pickup coil. I am 3 aware that pivoting in the center of mass has heretofore been suggested, as has also pivoting within the coil. But neither of these measures has previously found practical fulfillment, and this for the following reasons: In order to keep low the moment of inertia, a needle set-screw must always be placed in the axis of rotation of the armature. If the axis falls within'the coil, the latter must be divided in, order to allow the screw to protrude; by this method however too much winding space is lost, and in the end nothing is gained. If the axis is fixed outside the coil, and with it the set-screw, the center of mass will never in any practical pickup coincide with the axis. There is, so far as I am aware, no pickup on the market in this country in which the center system and to keep as high as possible its natural resonance point. The height of the resonance point is practically a measure of the excellence of a pickup, as no eiiective means of entirely eliminating resonance has been applied.

My invention enables the pivoting of the armature in the center of mass of the moving system (which includes of course the needle), and provides extremely simple pivoting means. The supporting feet 7, which are spot-welded to the armature are so formed that lateral and longitudinal vibration is practically excluded, yet a slight torsional movement about an axis coinciding closely with a line drawn through the points where the feet join the armature is admitted. These feet serve a double purpose; they pivot and at the same time they center the armature, the latter by virtue of the resilient force residing in them. With my invention no further means are required to maintain the armature in its central position in the air-gaps, such as rubber buffers or cushions. This is of great importance to the life and effectiveness of the device.

By my invention the necessity of damping the armature by means of cushions or buffers is also eliminated. Damping is in other pickups made necessary by the fact that the resonance point of the armature system has heretofore always fallen within the musical band. My new type of armature system is on the other hand so extremely light and correctly balanced that resonance is shoved far above the musical range. By musical range I mean the scale o'ffundam ental musical notes; the upper limit of this scale lies in the neighborhood of the frequency 4500. This is also the practical upper limit of phonograph recording and reproduction. Now previously known pickups of the best quality resonate in the region 3000 to 4000 cycles. Such resonance is annoying to the car, even though it is reduced by rubber damping cushions in the usual manner. Through my invention the resonance point of the armature system is raised to the region of 6000 cycles; A resonance peak in this region is not in the least disturbing; indeed with the average amplifier and loudspeaker it cannot even be detected by the ear in any way whatsoever. v Mostimportant hereby is the fact that I attain this high resonance point without any sacrifice in the voltage output. Indeed, I have with my invention-achieved average potentials of as high asfour volts, using commercially standard pickup coils and magnets. So far as I am aware, four volts is an absolute maximum for commercial electromagnetic pickups, one and one-half volts being the value most commonly attained.

Moreover, I achieve this extremely high resonance point purely through lighteningand properly balancing the armature, and not through any stiffening of the moving system. Indeed, I prefer to reduce the stillness to the absolute minimum required for adequate centering of the armature in its air-gaps, in order to reduce record wear to a minimum. As a measure to keep as low as possible the requisite centering force, I prefer to make the armature and pole-pieces of material the same two thousandths of an inch (which is the maximum deflection ordinarily caused by the record groove). The best commercial pickups of the standard rubber-damped type require 40 or 50 grams pressure under the same conditions.

I believe that I have achieved a new result which was not attainable by the means heretofore known, in that I have in a pickup of maximum voltage output and minimum stifiness placed armature resonance above practical audibility, and eliminated all need of damping cushions.

In Figs. 5 to 8 is shown an alternative form embodying my invention, in which the centering means is divorced from the pivoting means and in which a special form of needle pocket is employed. In these figures I2, 13 and I4 refer to'the members forming the usual four-pole system, these members being held together by means of the screws I5. The armature I6 is provided with knife-edges 16b, and is pivoted in the saddle member H. A spiral spring member l8 draws the knife-edges against the saddle and at the same time centers the armature in the air-gaps. I8 is secured to the adjustable blocks I 9. The latter are fastened to the pole-pieces I2 by means of the screws I So.

The form of Figs. 5 to 8 is designed as a .horizontal type of pickup, whereas the preceding form was of the vertical type. Figs. 6 and '7, the magnet (only partly shown) lies in a horizontal position. Accordingly, the needle must lie at an angle to the vertical axis of the armature. To facilitate the removal of the needle I provide as part of my invention-aneedle-pocket of novel form, illustrated-in Fig. 7. In this view the protruding portion of a needle in normal position is shown in full line. By a simple rearward pressure against the needle point, the needle is automatically ejected. A midway position in process of ejection is indicatedin the figure in dotted lines. If the forward face of the needle end is tapped quickly with the finger-point, the needle will fly out and awayfrom the pickup without further ado. This novel effect is due principally to the curvature Ilia of theneedlepocket.

The structure just described functions electrically in substantially the same manner as the first structure. In either case, the armature may advantageously be made inch long, wide and 0.020" thick (ea-ch half of 0.010" thick sheet material).

Figs. 9 and 10 show a third embodiment of my invention. Here the armature is kept so exceedingly small and light that, although the axis lies below the coil, the axis neverthless coincides with the center of mass. The projecting portion of the needle here balances the armature plus the inserted needle portion. The armature 26. is practically nothing more than a hood or thimble over the upper needle end. This design has the structural advantage of extreme simplicity. The armature is formed, as previously described, of two matching stampings of thin sheet material, spot-welded together. In the present form how'- ever the armature is supported and pivoted by feet which are simply portions of the armature stampings themselves, bent outwardlyat right angles to the main armature body. These feet are spot-welded to the lower pole-pieces 24. The entrance to the needle-pocket is slightly flared, to allow easy insertion of the needle. About this flange a portion of the stamping is cutaway, as shown at 28a in. Fig. 10, to allow free movement of the armature. As in the design first described, the armature is centered by the resiliencyof the As shown in magnet 2| by screws 25.

joint Where the two feet come together. This joint forms also the axis.

The armature is surrounded by the coil 28. The upper pole-piece 22 is secured to a horizontal This pole-piece is hollowed to match exactly the rounded form of the upper end of the armature. The pole is formed of sheet material, and is so bent as to extend from the magnet over and into the coil. The lower pole-piece 24 is secured to the magnet by screws 25 and 29. This piece'is of horseshoe form, corresponding to that of the magnet. At either extremity itextends out across the coil to form the two lower pole-pieces.

The structure just described has the advantage of great simplicity, but the disadvantage of relatively low voltage output. g

However, with this type of construction the resonance point may be shoved into the region of 10,000 cycles. This may be desirable where ex tremely high-class records and reproducing equipment are involved.

All the various embodiments of my invention here shown have in common the elimination of rubber as a damping and centering means. This is in itself a great advantagaas rubber deteriorates with age and furthermore at no time assures the exact centering demanded by small air-gaps. In recognition of this fact, it has of late years become the general commercial practice to make the air aps at least ten times as great as the maximum armature movement. This is obviously extremely wasteful. Very powerful and ex pensive cobalt magnets are required to make up for the loss in flux density at the excessive gap. According to my invention, on the other hand, the centering agency is purely metallic and substantially changeless. Therefore relatively small air-gaps may be safely used, thus greatly increasing the efficiency of the device. I desire to emphasize however, that my invention is not limited to the rubberless construction here shown and described. Rubber buffers and other adjuncts might be added without thereby evading the scope of my claims.

While I have particularly emphasized the possibility of attaining a high resonance point with my invention, this feature is not essential to the spirit thereof. The structure claimed might also be carried out in such way as to bring resonance within the musical band. The extreme simplicity of my device, its cheapness to manufacture and its facility of use are sufficient recommendations, aside from the electrical advantages. the electrical advantages which may be attained are of great importance. In practice the best of the now generally available rubber damped pickups have a resonance peak Within the musical band which rises about 100% higher than the level of the remainder of the scale. This is due to the ineffectiveness of rubber as a damping substance, and to the fact that the rubber must be applied very lightly in order to avoid undue stiffening of the armature system. This resonance peak distorts unpleasantly the reproduction of the higher notes. But there is a further grave disadvantage. The usual peak happens to be located in the band of frequencies which contains most of the scratch noises. Thus with the present standard commercial pickups the scratch is unnecessarily accentuated. Through my invention However,

In this connection attention iscalled to my v articles in Radio News for March and April,

steel. I have found that the loss due to magnetic leakage across this small bridge is negligible. The same applies to the bridging feet of. the armature 26. i c

In all cases the magnet and pole-pieces would in normal use be affixed to a base-plate attached to a tone-arm. These latter accessory parts are not shown in the drawing. The air-gaps would in practice be fixed, and the armature centered, by adjusting the two halves of the four-pole system relatively to one another before permanently aflixing the same to the base-plate.

The pole-pieces are built up of six parts in order to allow for spot-welding the armature supports to the lower pole-pieces. This in contradistinction to the usual practice of machining these of two U- is aifixed in a hole through the spring. The spring v is stretched between supports, with a considerable free portion which is subject to vibration. The distance from the central axis of the armature to the free portion of the spring is considerable.

This type of construction admits of both lateral and longitudinal movement of the armature, in

addition to the desired turning movement about the axis formed by the spring support. In fact every possible movement of the armature inthe eifective weight of the armature itself is materially increased. My invention is distinguished from this known type of construction in that my supporting feet are in themselves substantially rigid, movement being admitted only about the line of juncture of the feet with the armature. my invention that the armature is very thin at the point where the feet are affixed. Thus displace: ment of the axis of rotation in the course of the movement of the armature is reduced practically to zero. In practice no secondary resonances are detectable with my construction.

While the embodiments of my invention here shown and described are considered advantageous, many other means of application will suggest themselves. The scope of my invention is not limited to the particular embodiments here disclosed.

I claim:

1. A pickup comprising a magnet, pole-pieces, a pickup coil and an armature operatively associated therewith, said armature being formed of two members joined together, each member having a, recess so disposed that the two recesses match toform a needle-holding pocket.

It is hereby an important feature of 2. A pickup according to claim 1 in which the armature is formed of recessed stampings.

3. A pickup according to claim 1 in which the to secure the needle through, wedging. brought a coil and an armature provided with a needleholding pocket, the forward edge of said pocket being so'curved as to cause the ejection of a needle resting in the. pocket when rearward pressure is applied to the free needle end.

5,..A pickupcomprising a magnet, pole-pieces, a pickup coil and an armature provided with feet attached to the pole-pieces, the line of juncture of the. feet with thearmature coinciding approximately with the center of gravity of the armature with its associated needle.

6. ;A pickup according to claim 1 in which the armature is provided with feet which are integral portions of the two armature members bent out- Wardlyand affixed to the pole-pieces.

'7. A pickup comprising a magnet, pole-pieces, a pickup coil, a screwless armature pivotally mounted withinthe coil anda phonographneedle held therein, the pivotal axisooinciding'. approximately with the center of gravity of the armaature and needle, andsaid armature being so short and light that its natural resonance-peak falls outside the musical band.

8. A pickup according to claim 7 in which the, armature is centered by the pivoting means, without the use of additional centering or damping means. Y

9. A pickup comprising a magnet, pole-pieces, a pickup coil, an armature pivotally mounted at its own lower extremity, and a phonograph needle partially inserted therein, the moment of inertia of the armature being substantially equal to the moment of the protruding needle end.

10. A pickup comprising a magnet, pole-pieces, a pickup coil and an armature operatively associated therewith, said armature having a. fiat portion and abulging portion forming a needleholding pocket and being formed of sheet metal.

STANLEY MCCLATCHIE.

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