US2441651A - Phonograph recording head - Google Patents

Description

MayA 18, 1948. A L, THOMPSON 2,441,651

PHONOGRAPH RECORDING HEAD Filed Dec. '27, 1945 INVENTQR. wcm/v Ffm/PSOM ATTORNEY tom than the top gaps.

Patented May 18, 1948 UNITED STATES PATENT OFFICE PHoNocRAPH RECORDING HEAD Lneohi Thompson, New Haven,` Conn., assgnor to The Soundscriber Corporation, New Haven,

Conn., a corporation of Connecticut Application December 27, 1945, Serial No. 637,37

armature is pivotally mounted to vibrate with respect to a iixed'solenoid which is electrically energized in accordance with the sound vibrations to be recorded.

' The present invention is an improvement over that disclosed in applicants prior Patent No. 2,222,342, issued November 19, 1940, and entitled Electromagnetic sound recorder. Y

In the present invention the sound recording head has a pair of pole pieces which embrace a two-piece armature formed of two vertically aligned sections, the upper section being stationary and surrounded by a short energizing coil, and the lower section being movable, and carrying the recording stylus-said armature sections being spaced from each other by a small air gap.

One object of the present invention is to provide a recording head ofthe above nature in which the lower stylus-carrying section of the armature is provided with an integral horizontal rearwardly-extending bearing shaft which is resiliently secured to the head frame, the stylus being free t-o oscillate within a rubber-like block which dampens the vibrations thereof.

A further object is to provide a recording head of the above nature having side lgaps between the upper stationary armature section and the pole pieces, which gaps are lled with non-magnetic material, and are larger than the corresponding side air gaps between the movable bottom armature and the pole pieces, so as to produce a greater magnetic flux through the bot- A further object of this invention is to provide an improved electromagnetic sound recording head of the above nature-which will be relatively simple in construction, inexpensive to manufacture, compact, ornamental in appearance, and very eicient and durable `in use.

With these and other objects 'in View, which will appear as this description proceeds, there has been illustrated on 'the accompanying drawing one form in which the'invention may conveniently be embodied in practice.

In the drawing,

Fig. 1 is a schematic view of a sound recording apparatus embodying the invention.

Fig. 2 is a longitudinal sectional view recording head'of such apparatus.

of the Fig. 3 is av transverse .sectional View, taken along the line 3-3 of Fig. 2, looking in the direction of the arrows. l

Fig. 4 is a transverse sectional View taken along the line 4-4 of Fig. 2, looking in the direction of the arrows. y

Fig. 5 is a bottom view of the recording head with the base plate and casing removed to show the interior parts.

Referring now to the drawing in which like E reference numerals ydenote corresponding parts throughout the several views, the numeral I0 indicates a vertical lower langular movable armature sectionof magnetic material, said lower armature section I0 having a horizontal rearwardly-extending dat section II, the ends of which have a pair of horizontal opposed stiii' torsional pins I2, I3 provided with a pair of front and rear squared blocks I4, I5 integral therewith. The blocks I4, I5 are rigidly secured, as by soldering, in a non-magnetic elliptical ring plate I6, preferably of brass.

The movable armature section IIJ is provided with a square ldepending leg I1 having a hollow socket I8 for receiving the shank of an inclined diamond-tipped recording stylus I9 which is adapted to indent sound grooves 20 in a disk record 2| mounted on a turntable 22.

The lower movable armature section I0 eooperates with an upper stationary flat armature section 23 spaced vertically therefrom by a small air gap 24,k The armature sections I0 and 23 are embraced by a U-shaped magnetincluding a pair of fixed L-shaped magnetic pole pieces 25, 26, between the front of which is a rectangular upper aperture 21 enclosing a relatively short solenoid coil 28 which surrounds the upper armature section 23. Above the coil 28, the pole pieces 25, 26 have inturned arms 29, 30, and

below said coil inturned arms 3 I, 32 are provided.

The coil 28 is energized from an amplifier 33 which is adapted to receive audio-frequency currents from a microphone 34. The ring plate I6 is secured to the lower pole piece arms 3l, 32 by a pair of screws 35, 35.

vThe upper fixed armature section 23 is spaced from the pole piece arms 29, 30 by a pair of relatively largeside gaps 31, 38 ywhich are lilled by a pair of non-magnetic spacer blocks 39, 40 asshown in Figs. 3 and 4, said blocks being soldered in position. The lower movable armature section I0 is spaced from the pole piece sections 3l, 32 by a pair' of relatively small side open air gaps 4I, 42. By means of this construction, kthe steady magnetic flux passing through the mov,- able stylus-carrying armaturesection I0 will be,

3 greatly enhanced, causing the sensitivity of the recording head to be, at the maximum and resulting in a superior performance thereof.

It has been found that the sensitivity of the recording head will reach a maximum when the upper side gaps 31, 33 are about fty per cent greater than the lower side air gaps 4I, 42. If the upper side gapsrare made larger `than this, however; the sensitivity of the recording head will start to diminish because of the introduction of too much reluctance to the flow of the alternating magnetic flux. It has been'found'that if the upper side gaps 31, 38 are made larger with respect to the lower'gaps 43I, -4'2 lmore off'the steady flux due to the permanent magnetsv will flow through the lower gaps 4 I, 42, than throug said upper gaps. l

It has also been found that there is an optimum relationship between the upper gaps 3l, v38"a"ndY the lower gaps 4I, 42 for maximum eilciency of the recording ihead. Thus,if the upperfgaps .31, 38 are made too large, however, a criti'calpoint will' be vreached where the reluctance of 4said upper Vgaps Awill reduce the uX `in the entire vmagnetic circuit. This is illustrated bythe `following u tab-le which :give the results of tests made with a recording "head embodying Vthe present invention and using signals at 400 cycles. The :size of `the upper :and lower :gaps isexpress'ed in inches-and the :power offsensitivityis expressed in decibels p Upper gaps Lower 'gaps Povfsgnsl' Inches Inches b. 007 '-2 O10 007 -2 012 007 4 .'014 007 0 027 007 It will be noted from this table Ythat the peak sensitivity is obtained when the upper `gaps are .0'1'2 inch, `and that for larger upper'gaps the sensitivity levels ofi due to the increase in the reluctance 'of-'the magnetic circuit.

The recording head is provided with a protectiveca'sing143 within which'is secured a'lrear block 44' of permanently magnetized material, said bl'ockFbeing'held -withi-n the sides foi fthe L--shaped pole pieces 25, 2'6 'by a horizontal elongated sc'rewwtl In order to dampen undesired resonant vibrations yof the movable armature section I0, provision is Amade of a thick rectangular bloc-kl of soft yiel'dable resilient rubber-"like material havingfa square central aperture 741 which is fitted tightly about the depending leg I? of the l'ower movable armature It and preferably cemented thereto. The block 46 is held in :place in a rectangular recess 'il'of -a non-magnetic base'vplate 49, -detachably secured `to the "pole piecesf25,- '-2`B by'means of-small screws 59, 5l. The block 45 hasf'afpairofside bottom ribs52, 53 which engage the base plate-"49 and serveto space the central part voi ssaid block therefrom.

, Y Y It will -be understood that the damplnglblock Y 4 such materials is due to the fact that they are somewhat plastic and have a high internal molecular resistance to flow-requiring work to alter their shapes.

The coil 28 is connected to the amplier 33 by means of a pair of conductors 54, 55 which pass out of the casing in a groove 55 in the top of the base plate lIMJ.

The recording head is connected by a flexible leaf spring 51 to a horizontal recording arm 58 which is power-driven by means of a vertical shaft 59 .passing through a chassis 60 above which is located the turntable 22 which is driven by a shaft 965|. Y

' Inor'derto 'supply power to operate the amplifier-33 'and the A'turntable motor (not shown), provision isrmade ora-power plug 62 connected by a pair of conductors 63, 64 to said amplifier. The microphone 34 is likewise connected to the amplier 33 by means of a pair of conductors 65, 66.

'Operation In operation, a Iluxof steady magneticlines of force will pass across'ft-he upper side rgaps 31,138, the lower air gaps 4I, A4-2, and through the upper and lower armature sections 2-8 4and I8, respectively-the magnetic circuit .being completed through the pole pieces 25, 26 and theY magnet block 44. The stylus I9 will becau-sed to vibrate laterally; around the axis vof the pins I2, I3 in responseY to the electrical -oscillation-s Ysupplied to the coil 28 from .themicrophone .34 through the ainplier.

One advantage of the present inventionis that the vend lblocks I4., I5 connected to the torsional pins I2, I3 ofthe movable armature section Iil are rigidly mounted in the -ringplate I6, which inturn is rigidly secured to the pole pieces .25, 25. This constructioncausesthe lower armature section-I1 to be ineXi-ble vertically, and insures that the air gap 24 between the upper and lowerarmature sections '23 yand I0 4will-notnbe diminished vby the pressure of the stylus 'I9 upon .the disk record 2'I.

A still further advantage vdueto the novel stiiT relatively inflexible mounting -cf the movable armature ill inthe recording head, 4is that extremely accurate :groove spacing will be maintained throughout theentire recording. This will prevent undesired displacements of theerecordystylus -under the impact of external shocks which frequently occur vwhen-flexibly mounted recording styluses areA employed.

A'still furtheradvantagefof the rigid mounting of 'movable arma-ture Iliiis that it'results in a high natural-period of vibration `of said armature. ThisV assists vin -accentuating the higher frequencies of the sound waves being recorded and-in attenuating `the .lower lfrequencies 'thereof, the :desirability of which is indicated rin the prior Patent N'o.' 2,239,042 to Kleber et al. of April '22, 1941, 4'entitled4 Wa-ve 'recording and reproduction. This :stiff `construction also avoids over cutting `which sometimes occurs when recording vals'f'slow groove speeds, by vthe apparatus:disclosed'in my prior ReissuerPatent No, f22,.83,issued VSeptember 22,` 1942 (original No; '2200;865 `'oflli/lay le, 1940:), entitled Apparatus i or recording sound on `ithinfdislns.

While there has been disclosed this specification one form `inxwhich .the invention may :be embodied, itis to 'be understoodthat this 'form iseshown for the purpose oiillustrationfonly, .and that the invention :ls'notto :beliinite'd'.to the-speciiic disclosure, but'criayche `zxn'cziified` and vembodied in various other forms without departing from its spirit. In short, the invention includes all the modications and embodiments coming within the scope of the following claims.

Having thus fully described the invention, what is claimed as new, and for which it is desired to secure Letters Patent, is:

1. Inal phonographic sound recording head, a movable lower stylus-holding armature section 0f magnetic material, a stationary upper armature section cf similar material arranged in substantial alignment with said movable armature section leaving a horizontal air gap therebetween, a permanent magnet embracing both of said armature sections leaving two pairs oi upper and lower vertical side gaps therebetween, the upper side gaps being substantially larger than the lower side gaps to produce a greater magnetic iiux through the lower movable armature section than the magnetic flux through the upper fixed armature section.

2. The invention defined in claim 1, in which said upper side gaps are filled with Vertical blocks of non-magnetic material.

3. In a phonographic sound recording head, a movable lower stylus-holding armature section of magnetic material having a pair of front and rear cylindrical torsion rods integral therewith, a, pair of squared end blocks rigidly secured to said rods, a stationary upper amature section ofk similar material arranged in substantial alignment with said movable amature section leav.- f

ing a horizontal air gap therebetween, a permanent magnet embracing both of said armature sections leaving two pairs of upper and lower Vvertical side gaps therebetween, the upper side gaps being substantially larger than the lower side gapsto produce a greater magnetic iiux through the lower movable armature section that the magnetic flux through the upper xed armature section, and an open ring plate secured to said kmagnet and rigidly secured to said end blocks. LINCOLN THOMPSON.

REFERENCES CITED The following references are of record inthe iile of this patent:

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