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July 1l, 1950
C. R. DOUGLASS, JR
ELECTRODYNAMIC RECORDING HEAD
Filed Aug- 25, 1947
Om
H.mil
Patented July 11, 1950
2,514,379
UNITED STATES PATENT oFFicE
255151379;`
CharlesiRgDouglass,
ELECTRODYNAMICJr.,
RECORDING
Los Angeles, Calif.
Application August 25, 1947, Serial`No..770,3`68
'4 Claims;
(Granted under the act oi’ March: 3, 1883,:-as
amended April-3,0, 1928;: Í370 0. G. 757)
Thisinventi'on relatesingeneral to transducers
and: more particularly: to a recording headv> for
thealateral cut disci type of phonograph record.
The recording head? ofk this vinvention is >of the
magnetic type, but as distinguished:'fromy those
ofthe prior art, ithas only aziraction of the mass
of those in commonv use.
The` discovery ofthe
2
that is in common> use. This reduction. inmass‘,
reduces, theA inertia of _ the; moving, system whichv
results inthe use of- a much less stiff spring cen
tering andl return system. This reductionv iny
mass,l also reduces the amount of damping re
quired which greatly increases the overall ef
ficiency ofthe moving system. The use of this
structure-of this inventionthatenables a material
light moving system, displaces the mechanical
reduction in the massof the-.electromagnetic unit'
resonantpeak above the useful -frequencyrange
giveszrise toa >number of advantages of the'unit 10. of the recording head, and; reduces thetransient
of thisinventionloverthoseoi the .prior art.
distortion by virtue of the reductionv of the in
Accordingly, it isy an object. voi? this invention
ertia of the‘moving parts.>
to' provide a. recording unit` of: the magnetic' type>
Throughthe use of a moving coil as the actuat
in which the moving mechanism has a fraction of
ing` medium, the: harmonic.; distortion`> is reduced
the mass of the moving mechanism of vthe mag
toa great degree over 'that of thegmagnetio-record
netic recorders-ofthe prior art.
ing~ head, >Where magnetic saturationA takesvv placeI
in-smail-moving armatures and pole-pieces. Ilìi iS-~
reduction in the inertia ofthe moving mechanism>
here pointed out, and' will .be described indetail'
which results in the use of less stiiî^~spring>cen~
later, that the type of moving coil used in the>
20 recording head of> this invention is` different from
tering and returnL mechanism.
Another object :isato'y providet- a-,recorder with a
the moving coil of`> the »prior art, andfthatit »has
moving mechanism that> requires less dampingv
several-l` advantages over the conventionalY moving
and/results in an overallincreasgein efiiciencyofl
coil system.
Another object isv to provide a recorder with a
the moving system.
'
The recording head of~> thi-s inventionl is her
Another object is to provide a recorder with 2.5 metical'lly. sealed and instead of 'using' the con
a.- moving> mechanism that displaces Ythe mechan
ventional rubber or gelatin composition for a
icallyA resonant peak above 'the useful frequency>
range of the recording head.
Another object is tofprovide> a .recorder that
has less transient distortion due to the reduction
of inertia.
Another object is toprovide a recorder in which
the harmonic distortion has beenureduced to Va
greatv degree over that present in magnetic .ren
damping material; whose characteristics vary>
considerablydue-to temperature changes, a fluid'
is used for damping. This fluid'is capable oi
withstanding Wide temperature variations With
outy changing vitsV Visco-sity. The case is'ñiled- to
twoeth-irds‘of its height Withv fluid" of Vproper vis
ccsity-»to offer the required damping for proper
recording response.
corders of theprior art.
’ The main cutting needle actuating shaft 'l in
Another objectv is to `provide a hermetically
Figure‘2, -projectscut in front ofthe case,'thereby
sealedrecorder; in- which fluid damping. is'ëutilized'.
allowingythe cutting `needle to operate in a posi
Another object is the provision of a-‘recorder
tion AWithout obstructions on either side. This is
Which Willbe economicalt to manufacture, >reliable
an importantv advantage, for the vacuum shaving
in operation and which possesses all oftheíqual 40: collection tube 20, which is'used toy pick up shav
ities of ruggedness» and dependability. in service.
ing I8 cu-t'by needle IT, is mounted adjacent to
Figure 1Y illustrates affront` elevation viewof'the „
recording unit.
vFigure ZiHuStrates a side elevation view of the.
recording unit.
Figure 3, illustrates .a plan. view of the `recording
unit.
The recording head of this inventionl is of a
different structure from that of'any other record
ing headît-hat has beenusedtor the purposesof re
cording sounde on disc. It «embodies .many im
provements that' have been made to overcome ob;«
jections to the magnetic` type» of recording head.
The massk of the moving," mechanism is only a
fraction. of> the> mass> of; the magneticL type head
the needle.
This is an ’improvement that allows
greater dependability in collecting this shaving,
which has proven-to be a hazard With recording
heads of conventional construction, Where the
cutting needle >projects ‘from the bottom of'the
cutting head`
In many currentlyl used recording heads the
needle protrudes through the bottom of thev case.
» The-¿needle is usually lsurrounded by a rubber
washer whosev purpose is to exclude foreign ma
terial from the opening. The foreign material
is usually iron` filings- attracted. by the magnetic
ñeld. This construction is not satisfactory, Afor
these small pieces of foreign material1 rWork upl
2,514,379
4
3
into the opening during needle replacement.
viously pointed out, the permanent magnets are
This difficulty is overcome in the recording head
of this invention due to the needle chucking de
vice being completely on the outside of the case.
Referring to Figures 1 to 3, the recording head
receives its audio excitation from the proper
source impedance or terminals,` 2l. lThese ex
citing currents flow through coil> I which is an
torque in the resulting reaction between the mag
netic field of the permanent magnets and the
magnetic ñeld set up by the movable coil in re
sponse to the varying currents received. The
reactions of the magnetic fields tend to displace
the coil from the air gap of the magnets, first
extremely light insulating composition bobbin`
on one side and then on the other, depending
so poled as to produce the maximum amount of
wound with the required number of turns of 10 upon whether the currents and the correspond
ing magnetic field produced thereby are increas
wire. This coil is oblong in» shape as shown
ing or decreasing. This developed torque tends
at 22, so that the greatest percentage of turns
to rotate the coil about a pivot formed by the
shall be in the magnetic` field. There are two
springs 8, and it also rotates the cutting needle
complete magnetic circuits, one generated by
magnet I3 and one by magnet lll. The magnets 15 I'I about said pivot. It is clear that the tip of
the needle has a lateral motion, the amplitude
are poled so that the flux paths I5 and I6 will
of which is proportional to the strength of the
be as shown, which Will result in additive lateral
currents supplied. When the moving needle is
forces being exerted by eachi‘side of coil I. Coil
imbedded in a moving record, it cuts a groove
I is mounted on aluminum alloy pins E which
are supported by main drive shaft 1, Main 20 whose lateral variations are proportional to the
variations of the currents supplied. If the >sup
drive shaft 'I is pivoted in two places by hardened
plied currents carry intelligence such as a speak
steel wire pivot springs 8. Steel pivot springs
ing voice, the record so made by said currents
8 are vimbedded ñrmly in cleats 9. This- method
will reproduce the voice upon reproduction as
of suspension in itself is unique,- for the center
ing and stiifening springs il are used as the 25 a phonograph record.
The novel structure set forth in this inventio
pivoting medium also. The length of springs 8
enables high fidelity recordings to be made under
is- adjusted during‘manufacture to oifer the re
conditions where said recordings were runable to
quired mechanical stiffness to the moving sys
be made prior to this invention.
tem. The cutting needle I'I is chucked in the
The invention described herein may be manu
main shaft 'I by means of screw 26. The turn 30
factured and used .by or for the Government of
ing motions of the main shaft are hereby trans
the United States for governmental purposes
lated to lateral motions at the tip of the cut
without the payment to me of any royalties
ting needle I‘I, which further reduces the mass
of the moving system by eliminating the usual
heavy lower needle -chucking device. The hel’ 35
thereon or therefor.
What is claimed is:
.
,
.
'
_
l. In a transducer of the electromagnetic type
metc seal between the main shaft 'l and front
plate I2 of the case is effected by a conical rub
having a moving mechanism of low mass for use
turn is bolted to the cast aluminum case II,
having opposite poles adjacent each other, bridge
mounted on the tone or carrying arm by the
members on both sides of one pair of opposite
in high fidelity recording comprising, a pair of
ber gasket IU which is at all times pressed in by
magnets each having pole pieces positioned to
spacing washer 2l'. All mechanism in the record.
ing head is bolted to the front plate l2 which in 40 form a centrally located airgap, said magnets
poles for spacing said magnets, a coil adapted
to be energized by the signal to be recorded posi
A' number of advantages have resulted in using
the recording head of this invention. In the 45 tioned in said airgap, a horizontally positioned
shaft, support pins depending vertically from said
past several years, especially during war time,
coil, said support pins being rigidly attached at
many thousands of hours of disk recording have
their other ends to the aforementioned shaft,
been made both in the studio and in the field.
vertically positioned spring pins mounted be
Much difficulty has been encountered in record
tween said shaft and said bridge members, said
ing high fidelity records, especially in the field.
spring pins acting as pivot mountings for said
These difficulties can be traced primarily to the
shaft, and means for mounting a stylus at one
recording heads used` The heads used in the
end of said shaft.
field in the past have suffered variations in fre
2. A transducer similar to that of claim l, and a
quency response and efliciency due to the wide
range of temperature and humidity changes en 55 housing means enclosing said magnets, coil and
shaft, said housing means having an opening
countered. Furthermore, in order to record
therein through which the shaft supporting the
high fidelity with currently used recording heads,
stylus enters.
an audio emplifier power output of 50 watts is
3. A transducer similar to that of claim 1, and
required. By using the recording head of this
invention, high ndelity response can be had with 60 a fluid-tight housing means completely enclosing
said magnets, coil and shaft, said housing means
a power output of 12 watts, which effects a de
having an opening therein through which the
sirable saving of weight in the portable record
shaft supporting the stylus enters, said opening
ing units.
having a conical rubber gasket associated there
Through the use of hermetic sealing in the
recording head of this invention, consistent high 65 with to eifect a hermetic seal between the shaft
and the housing means, the housing means being
fidelity recording can be accomplished under
filled with liquid to aid >the damping of the said
conditions of varying temperature and humidity
transducer.
without danger of failure due to corrosion and
4. In a transducer of the electromagnetic‘type
fungus growths.
In operation, the currents varying in strength 70 having a moving mechanism of low mass for 'use
in high fidelity recording comprising a pair of
in accordance with the intelligence to be recorded
magnets each having pole pieces positioned to
are supplied through lead-in terminals 2l, which
form a centrally located airgap, said magnets
conduct the currents to coil I where they set up
having opposite poles adjacent each other, bridge
a magnetic iield whose strength varies propor
tionally to the strength of the currents. As pre 75 members on both sides of one pair of opposite
tapped holes 28.
2,514,379
poles for spacing said magnets, an electrodynamic
Winding positioned in said airgap, a horizontally
positioned shaft, supporting means for said wind
ing located normal to said shaft and integral
therewith, spring pins positioned parallel to said
supporting means and integral with said shaft,
said pins being rigidly attached to said bridge
members for free movement of said shaft and
Winding, means at one end of said shaft for
mounting a stylus, and a ñuid tight housing
means completely enclosing said magnets, Wind
ing and shaft, said housing means having an
REFERENCES CITED
The following references are of record in the
ñle of this patent:
v
UNTI‘ED STATES PATENTS
Number
Name
Date
1,689,339
1,760,527
1,831,787
1,870,446
2,213,936
Harrison _______ __ Oct. 30,
Robinson ________ __ May 27,
Goudy __________ __ Nov. 10,
FoleyV ____ __ ______ __ Aug. 9,
Thompson _______ __ Sept. 3,
1928
1930
1931
1932
1940
opening therein through which protrudes the end
Number
of said shaft to which the stylus is mounted, the
housing means being fined with uquid to aid the 15
335,290
FOREIGN PATENTS
damping of said transducer.
Germany ______ ____ Jan. 28, 1937
CHARLES R. DOUGLASS, JR,
641,339
Country
Date
Great Britain ____ __ Sept. 25, 1930
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