US2494438A

US2494438A - Phonograph pickup

Info

Publication number: US2494438A
Application number: US673476A
Authority: US
Inventors: Everett A Gilbert
Original assignee: RADIO FREQUENCY LAB Inc
Current assignee: RADIO FREQUENCY LAB Inc; RADIO FREQUENCY LABORATORIES Inc
Priority date: 1946-05-31
Filing date: 1946-05-31
Publication date: 1950-01-10
Anticipated expiration: 1967-01-10

Description

8 u Q r E M wn A, n 2 a QM 1mm Jan. 10, 1950 Patented Jan. 10, 1950 UNITED STATES OFFICE 3-Clainis. (01. 179-10041) This" invention relates to a device for trans: lating selected mechanical vibrations into'corre-. spending electrical oscillations and more particularly to a novel phonograph pick-up for reproducing recorded sound waves with a minimum of scratch noises.

A. recorded sound. track comprises a spiral groove out into the surface of a suitable disc, the depth of the groove being maintained as constant. as is possible with the best available mechanical equipment. The edges of the groove consist of minute lateral undulations corresponding to the form, amplitude and frequency of the actual recorded sound waves. In reproducing the recorded sound it is essential that the pick-up device be capable of following those undulations which are substantially co-planar with the disc to the exclusion of all other vibrations: likely to produce spurious noises from the loud speaker. In particular, scratch noises result when the needle point of the pickeup vibrates axially, that is, in a plane substantially normal to the disc, such vibration being caused. by irregularities in the depth of the sound track, foreign particles imbedded in the disc, accumulated lint, etc.

.To'reduce the level of the'scratch noise, phone. ograph pick-ups of the type contemplated by this invention are designed to exert a minimum pres.. sure against the; disc or record, the generally accepted pressure being approximately three (3) ounces. Yet, the pressure must be suflicient. to compel the pick-up needle to follow each and everyundulation of the recorded wave else the sound produced is not a faithful duplicate: of the; original.

Much efiort has. been directed toward the provision of a high qualitypick-up having a minisusceptibility torspurious vibrations; Needies have been made of non-metal1icmaterial,and I special alloys having so called softf character isti s. Other needles ar pro ded. havin a special shape intended to damp out axial vibrations or the needle while at the same time maintaining a. maximum compliance to transverse vibrations. Furt er... to ri ty pi -up devi es hav been proposed to eliminate directcontact. between the pick-up and the record. I am not aware or: any'existing orproposed device that is recogni'gfied as an entirely satisfactory solution of the problem.

An object of this invention is. th provision of a simple, compact and inexpensive arrange-J merit. for translating mechanical vibrationsinto elec rica o c llations and nclud ng a me be having a maximum compliance to transverse vi- 2 brations and a minimum compliance to axial or longitudinal vibrations.

An object of this invention is the provision or,

a phonograph pick-up employing a'conventional needleepoint and including means interposed be! tween the needle and the electrical circuit where! by spurious. vibrations of the needle are prevented from affecting the reproduction of the desired sound waves.

An object of this invention is the provision of a phonograph pick-up comp-rising a needle; a metallic vane movable between spaced coils of an oscillator circuit, said needle secured to the vane by a resilient member pivoted for'rotation in only'one plane whereby the vane is free to follow transversevibrations of the needle and is unaffected by axial vibrations of the said needle.

An object ofthis invention is the provision of a phonograph pick-up comprising a base, two

' axially displaced coils supported in fixed position relative to thebase said coils being connected in an oscillator circuit, a resilient strip the inner end of which is disposed between the two. coils and including means on the outer end for at taching a needle thereto, means pivotally scour ingtheistrip. to. the base, and resilient members clamping the strip in the region of the pivot area whereby transverse vibrations of the needle produce corresponding vibrations of the inner end of the strip in a plane parallel to that of the coils to thereby alter the voltage transfer ratio between the'coils and theresilient members are efiective to absorb or damp out axial vibrations of. the needle.

These-and other objects and advantages will be apparent; from the following description of the invention when taken with the accompanying drawings The drawings are for purposes of illustration and. are not to be construed as define ing the scope or limits of the invention, reference being. bad for this purpose to the appendedclaims.

In thedrawings wherein like characters identify like parts in the several figures;

Figure 1 is a vertical, cross-sectional viewofa. phonograph pick-up made in accordance with my invention;

Figure 2 is a; bottom view of the pick-up showing the relationship of the vibratory vane to the oscillator coils; and

FigureB is a circuit diagram showing the mannerof; converting mechanical vibrations into elece trical oscillations.

Referring now to Figures 1 and, 2, the individual components of the pick-up are carried by Threaded into the base are screws 20 which serve to fasten the soldering terminals 2| to which the connection wires 22 may be soldered.

Lead wires from the ends of the coils l3, l4 may i also be soldered to the terminals by passing through holes 23 in the base.

A vibratory strip 24, preferably made of be-- ryllium-copper, is clamped between two rubber washers 25 or the like. The stud 26 is provided with a shoulder 21 and an axial bore 28; spinning over the end of the stud 26, the strip 24 and the washers 25 are clamped against the shoulder 27 resulting in a fairly rigid sub-assembly that is removably secured to the base It by the thumb screw 29. graph needle 30 is secured to the strip 24 by means of a thumb screw 3| that threads into the member 32 which is rigidly fastened to the strip by rivets 33 or in any other suitable manner.

The inner end of the strip 24 is disposed between the coils l3, l4 and preferably is so shaped that one edge of the strip lies in a plane bi-secting the coils when the strip is stationary i. e., not vibrating. The outer end of the strip is tapered and includes an offset section 34 that aids in damping out undesirable vibrations of the needle, as is explained in more detail hereinbelow.

While the drawing illustrates a unitary strip 24 of beryllium-copper it is apparent that the strip may be made of any material having suitable physical properties and variation of the mutual inductance between the coils l3 and I4 may be accomplished by attaching a vane or flag to the inner end of strip in any suitable manner. It is also pointed out that the shape of the vane, in the region of the coils, is not critical but in general it is good practice to shape the member to obtain the maximum variation in the magnetic lines of flux cutting the coils per unit displacement of the vibratory strip.

In actual use the point of the needle 30 follows the spiral groove of the record as the latter rotates. In so doing the needle vibrates in a complex manner in accordance with the minute undulations impressed into the record. Those vibrations corresponding to the recorded wave forms must be transmitted to the electrical circuit for faithful reproduction of the recorded sound. By reason of the manner in which the sound track is cut into the record the desired vibrations of the needle lie substantially in a plane parallel to that of the record. Axial of longitudinal vibrations of the needle are caused by variables such as minute imperfections in the material of which the record is made, foreign particles imbedded in the groove,- variations in the depth of the groove, etc. These undesirable A conventional phono 30 produce a rotational motion 01' the strip 24 as the resiliency of the strip in this plane is substantially zero and the rubber washers merely give or twist without retarding the rotation of the strip about the axis formed by the stud 26. Consequently, the inner end of the strip vibrates in a, plane parallel to that of the coils l3, [4 thereby altering the voltage transfer between the coils, as is well known in the art. However, axial or longitudinal vibrations of the needle are effectively damped out due to the resiliency of the strip 24 in this plane and the absorbing characteristics of the rubber washers 25. Consequently, such vibrations produce no motion of the inner end of the strip. Thus, it is apparent that 'a device made in accordance with my invention results in a high fidelity pick-up having a minimum susceptibility to spurious vibrations of the needle. It is recognized that the actual vibrations of the needle comprise components lying in all planes and while the functional explanation of my device has been described with respect to desired vibrations of a transverse or lateral nature and undesired vibrations of a longitudinal or axial nature certain vibrations oc-- curring in intermediate planes may be transmitted to the inner end of the strip 24 in varying degrees. Inasmuch as the cross section of the sound groove is of approximate V shape most of such intermediate vibrations may be consid-' ered as desirable. Therefore, the total number of spurious vibrations eifecting the transfer of voltage between the coils I3, 14 is only a very small percentage of the true, desired vibrations and the overall operation of my pick-up is of exceptionally high quality.

Figure 3 illustrates the electrical circuit employed in connection with my novel phonograph pick-up. Rotation of therecord 35 imparts rotational vibrations to the needle 30 and results in mono-planar vibrations of the inner end of the strip 24. The coils I3, 14 form elements of a vacuum tube oscillator circuit of the tuned grid class and which is arranged to oscillate at radio frequencies in the region of 2 megacycles by prop-- er design of the coils I4, l4 and the condenser 35.1 Coil l3 feeds energy from the plate 3! of the tube 38 to the tuned, grid coil I4. A condenser 39 by-passes' radio frequency currents from the plate. 31 to the coil l3 whereas the condenser and associated resistor 4! charge to a peak voltage across the condenser 36 to limit the magnitude of the oscillations in the'manner as pertains to tuned grid oscillators. A conventional load resistor 43 and a choke coil 42 are connected into the plate circuit. The by-pass condenser '44 has a reactance that is small compared to that of the resistor 43 at 10,000 cycles and, consequently, radio frequency currents are vibrations should be prevented from becoming effectively bypassed to ground. On the other hand, audio frequency currents pass through the condenser 45 to the next amplifier stage represented by the circuit elements enclosed within the dotted line. The number of amplifier stages inserted between the oscillator and the loud speaker are determinedby the sound level desired at the loud speaker.

The device operates as follows. Vibrations of the inner end of the strip 24 vary the mutual in-' ductance of the coils l3, l4 at audio frequencies thereby producing corresponding variations in the transfer of radio frequency voltage between the'said coils. Voltage variations induced in the coil l4 appear across the condenser 36 and are of a magnitude of approximately .01 volt when the constants of the circuit are chosen to produce a normal transfer of 1.0 volts in the coil 14. These audio frequency, voltage variations across the condenser 36 produce corresponding variations of the bias on grid 46 and result in corresponding fluctuations of the plate currentfiowing through the resistor 43. The voltage drop across resistor 43 provides the control voltage on the grid of next amplifier stage and ultimately passes to the loud speaker.

Having now described one form of my high fidelity phonograph pick-up certain variations and modifications will be apparent to those skilled in the art. The specific form and construction of the various components of the device are subject to considerable latitude in design as is also the manner of mounting the components to form a practical, encased pick-up unit. Such variations are intended to fall within the scope and spirit of the invention as set forth in the following claims.

I claim:

1. A phonograph pick-up comprising a housing carrying a base, a stud extending downwardly from the base and substantially normal thereto, means securing the stud to the base and effective to prevent relative motion between the said stud and base, a vibratory strip having an inner section and an outer section, means pivotally securing the central section of the strip to the stud, a pair of electrical coils co-axially disposed adjacent to and on opposite sides of the inner section of the strip, means securing the coils in fixed position relative to the base. a needle carried by the outer section of the vibratory strip, and vibration damping means engaging the central section of'said strip and efiective to absorb those vibrations of the strip corresponding to axial vibrations of the needle.

The invention as recited in claim 1, wherein the vibration damping means comprises a pair of resilient washers carried by the said stud, the individual Washers being in contact with opposite surfaces of the vibratory strip.

3. The invention as recited in claim 2, wherein the vibratory strip is made of beryllium copper.

EVERETT A. GILBERT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS