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Claims  |
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I claim:
1. An electrical pickup for a stringed musical instrument, said pickup
comprising a hollow housing of electrically conductive non-magnetic
material, first and second coils each having an opening with an axis and
each having a front end and a rear end, said two coils being arranged side
by side and next to one another within said housing with said axes of
their openings parallel to one another, a first permanent magnet element
extending axially through the opening of said first coil, a second
permanent magnet element extending axially through the opening of said
second coil, said first magnet element having a front end with a north
magnetic pole adjacent said front end of said first coil and a rear end
with a south magnetic pole adjacent said rear end of said first coil, and
said second magnet element having a front end with a south magnetic pole
adjacent said front end of said second coil and a rear end with a north
magnetic pole adjacent said rear end of said second coil, said two coils
having substantially equal numbers of turns and each having two end leads,
and means electrically connecting one end lead of said first coil solely
to one end lead of said second coil so that the electrical circuit between
the other two of said end leads consists solely of said two coils
connected in series with one another in such a way that an electric
current in passing serially through said two coils passes clockwise
through one coil and counterclockwise through the other of said coils as
viewed looking toward said front ends of said coils, said rear ends of
said first and second permanent magnet elements being magnetically
separated from one another by material having substantially the
permeability of air and said front ends of said first and second permanent
magnet elements being magnetically separated from one another by material
having substantially the permeability of air, and two bobbins of
non-magnetic material on each of which a respective one of said two coils
is wound, each of said bobbins having a body portion passing through the
opening of its associated coil and said body portion having a hole
receiving a respective one of said first and second permanent magnet
elements, each of said holes being larger in cross section than its
associated permanent magnet element and being defined by a major interior
surface and a plurality of projections extending radially inwardly from
said major interior surface and engaging the associated one of said
permanent magnet elements to hold it in spaced relation to said major
interior surface, and a material having the resiliency of silicone rubber
filling the spaces between each of said permanent elements and said major
interior surface of its associated bobbin hole for dampening vibration of
said magnet elements relative to said bobbins.
2. An electrical pickup for a stringed musical instrument as defined in
claim 1 further characterized by each of said bobbins having a body
portion passing through the opening of its associated coil and two flanges
located at opposite ends of said body portion, said housing including a
base plate of electrically conductive material against which one flange of
each of said bobbins rests, and a plurality of threaded fasteners for each
of said bobbins which fasteners pass through said base plate and
threadably engage said body portion of the associated one of said bobbins,
said two bobbins and said two coils forming two bobbin and coil
subassemblies which are identical with one another, each of said
subassemblies having a front and a rear coil lead at one end of said base
plate, said base plate having a first set of openings for the threaded
fasteners associated with the first of said bobbins and a second set of
openings for the threaded fasteners associated with the second of said
bobbins, the arrangement of said first set of base plate openings relative
to one another being a mirror image taken along a line passing between
said two bobbins of the arrangement of said second set of base plate
openings relative to one another, each of said bobbins having a set of
fastener holes passing axially completely through its body portion so as
to form a set of openings on each end of the bobbin, said fastener holes
being so arranged that when said bobbin has one end adjacent said base
plate the set of openings formed thereby in said one end match with only
one of said sets of base plate openings and when said bobbin has its other
end adjacent said base plate the set of openings formed by said fastener
holes in said other end match with only the other of said sets of base
plate openings.
3. An electrical pickup of the magnetic induction type for a stringed
musical instrument, said electrical pickup comprising a coil, a bobbin of
non-magnetic material having a body portion passing through the opening of
said coil and on which said coil is wound, said body portion of said
bobbin having a hole therein extending parallel to the axis of said coil,
an element of magnetic material received in said bobbin hole, said hole of
said bobbin being larger in cross section than said element, a material
having the resiliency of silicone rubber filling the space between said
element and the surface of said bobbin defining said bobbin hole for
dampening vibration of said element relative to said bobbin, said surface
defining said bobbin hole including a major portion spaced from said
element and a plurality of projections extending radially inwardly from
said major portion and engaging said element to locate it relative to said
major portion.
4. An electrical pickup as defined in claim 3 further characterized by said
material having the resiliency of silicone rubber being a silicone rubber.
5. An electrical pickup as defined in claim 3 further characterized by said
coil and said bobbin body portion being elongated along a line of
elongation generally perpendicular to the axis of said coil, said hole in
said body portion being one of a plurality of such holes, and said element
of magnetic material being one of a plurality of such elements of magnetic
material, each of said elements being received in a respective one of said
bobbin holes and each of said bobbin holes being larger in cross section
than its associated element, and a material having a resiliency generally
similar to that of silicone rubber filling the space between each of said
elements and the surface of said bobbin defining its associated bobbin
hole for dampening vibration of said element relative to said bobbin.
6. An electrical pickup as defined in claim 5 further characterized by each
of said elements of magnetic material being a permanent magnet.
7. An electrical pickup of the magnetic induction type for a stringed
musical instrument, said pickup comprising a bobbin of non-magnetic
material having a body portion, a coil wound on said body portion of said
bobbin, said body portion of said bobbin and said coil being elongated
along a line of elongation generally perpendicular to the axis of said
coil, said body portion of said bobbin having a plurality of holes therein
with axes generally parallel to the axis of said coil, a plurality of
elements of magnetic material each received in a respective one of said
bobbin holes, a base plate to which said bobbin is fixed with the axis of
said coil generally perpendicular to said base plate, and a cup-shaped
housing member of electrically conductive non-magnetic material attached
to said base plate and with said base plate forming a cavity within which
said bobbin, coil and elements of magnetic material are received, said
housing having two side walls, two end walls and a top wall, said coil
having one side face facing one of said housing side walls, and a pad of
material having the resiliency of silicone rubber interposed in a slightly
compressed state between said one side of said coil and the adjacent side
wall of said housing member to dampen vibration of said side wall relative
to said coil.
8. An electrical pickup of the magnetic induction type for a stringed
musical instrument, said pickup comprising a bobbin of non-magnetic
material having a body portion, a coil wound on said body portion of said
bobbin, said body portion of said bobbin and said coil being elongated
along a line of elongation generally perpendicular to the axis of said
coil, and said body portion of said bobbin having a plurality of holes
with axes generally parallel to the axis of said coil, a plurality of
elements of magnetic material each received in a respective one of said
bobbin holes, a base plate to which said bobbin is fixed with the axis of
said coil generally perpendicular to said base plate, and a cup-shaped
housing member of electrically conductive non-magnetic material attached
to said base plate and with said base plate forming a cavity within which
said bobbin, coil and elements of magnetic material are received, said
housing member having two side walls, two end walls and a top wall, said
bobbin having one end facing said housing top wall, and a pad of material
having the resiliency of silicone rubber interposed in a slightly
compressed state between said one end of said bobbin and said housing top
wall to dampen vibration of said top wall relative to said bobbin.
9. An electrical pickup for a stringed musical instrument having a set of
transversely spaced parallel strings, said pickup comprising first and
second coils each having an opening with an axis, said two coils being
arranged side by side with said axes of their openings parallel to one
another, said two coils extending axially between parallel front and rear
planes common to both coils so that each coil has one axial end located in
said front plane and the other axial end located in said rear plane, a
first permanent magnet element extending axially through the opening of
said first coil, a second permanent magnet element extending axially
through the opening of said second coil, said first magnet element having
an end with a north magnetic pole adjacent said front plane and another
end with a south magnetic pole adjacent said rear plane and said second
magnet element having an end with a south magnetic pole adjacent said
front plane and another end with a north magnetic pole adjacent said rear
plane whereby said two magnet elements establish a magnetic flux circuit
which passes axially in opposite directions through said two coils and
which flux circuit has a rear air portion extending in one direction
between the rear plane ends of said magnet elements and a front air
portion extending in the opposite direction between the front plane ends
of said magnet elements, and means electrically connecting said two coils
in series with one another in such a manner that an electric current in
passing serially through said two coils passes clockwise through one coil
and counterclockwise through the other of said coils, each of said coil
openings being elongated along a line of elongation perpendicular to the
associated opening axis, the two lines of elongation of the two coils
being parallel to one another, said first permanent magnet element being
one of a plurality of first permanent magnet elements extending axially
through the opening of said first coil and spaced from one another along
the associated line of coil elongation, said second permanent magnet
element likewise being one of a plurality of second permanent magnet
elements extending axially through the opening of said second coil and
spaced from one another along the associated line of coil elongation, each
of said coils being wound on a bobbin of non-magnetic material, each of
said bobbins having a body portion passing through the opening of its
associated coil and said body portion of each bobbin having a plurality of
holes each receiving a respective one of said permanent magnet elements,
each of said holes being larger in cross section than its associated
permanent magnet element and being defined by a major interior surface and
a plurality of projections extending radially inwardly from said major
interior surface and enagaging said associated permanent magnet element to
hole it in spaced relation to said major interior surface, and a material
havng the resiliency of silicone rubber filling the spaces between each of
said permanent magnet elements and said major interior surface of its
associated bobbin hole.
10. An electrical pickup as defined in claim 9 further characterized by
said material having the resiliency of silicone rubber being a silicone
rubber.
11. An electrical pickup for a stringed musical instrument having a set of
transversely spaced parallel strings, said pickup comprising first and
second coils each having an opening with an axis, said two coils being
arranged side by side with said axes of their openings parallel to one
another, said two coils extending axially between parallel front and rear
planes common to both coils so that each coil has one axial end located in
said front plane and the other axial end located in said rear plane, a
first permanent magnet element extending axially through the opening of
said first coil, a second permanent magnet element extending axially
through the opening of said second coil, said first magnet element having
an end with a north magnetic pole adjacent said front plane and another
end with a south magnetic pole adjacent said rear plane and said second
magnet element having an end with a south magnetic pole adjacent said
front plane and another end with a north magnetic pole adjacent said rear
plane whereby said two magnet elements establish a magnetic flux circuit
which passes axially in opposite directions through said two coils and
which flux circuit has a rear air portion extending in one direction
between the rear plane ends of said magnet elements and a front air
portion extending in the opposite direction between the front plane ends
of said magnet elements, and means electrically connecting said two coils
in series with one another in such a manner that an electric current in
passing serially through said two coils passes clockwise through one coil
and counterclockwise through the other of said coils, each of said coil
openings being elongated along a line of elongation perpendicular to the
associated opening axis, the two lines of elongation of the two coils
being parallel to one another, said first permanent magnet element being
the only one of such received in the opening of said first coil and being
elongated along the associated line of elongation, said second permanent
magnet element being the only one of such received in the opening of said
second coil and being elongated along the associated line of elongation,
each of said coils being wound on a bobbin of non-magnetic material, each
of said bobbins having a body portion passing through the opening of its
associated coil and said body portion of each bobbin having a hole for
receiving the associated one of said permanent magnet elements, said hole
of each bobbin being larger in cross section than its associated permanent
magnet element and being defined by a major interior surface and a
plurality of projections extending radially inwardly from said major
interior surface and engaging the associated permanent magnet element to
hold it in spaced relation to said major interior surface, and a material
having the resiliency of silicone rubber filling the space between each of
said permanent magnet elements and said major interior surface of its
associated bobbin hole.
12. An electrical pickup as defined in claim 11 further characterized by
said material having the resiliency of silicone rubber being a silicone
rubber.
13. An electrical pickup for a stringed musical instrument having a set of
transversely spaced parallel strings, said pickup comprising a hollow
housing of electrically conductive non-magnetic material, first and second
coils each having an opening with an axis, said two coils being axially
between parallel front and side by side and next to one another within
said housing with said axes of their openings parallel to one another,
said two coils extending axially between parallel front and rear planes
common to both coils so that each coil has one axial end located in said
front plane and the other axial end located in said rear plane, a first
permanent magnet element extending axially through the opening of said
first coil, a second permanent magnet element extending axially through
the opening of said second coil each of said coils being wound on a bobbin
of non-magnetic material, each of said bobbins having a body portion
passing through the opening of its associated coil and said body portion
of each bobbin having a hole for receiving the associated one of said
permanent magnet elements, said first magnet element having an end with a
north magnetic pole adjacent said front plane and another end with a south
magnetic pole adjacent said reak plane and said second magnet element
having an end with a south magnetic pole adjacent said front plane and
another end with a north magnetic pole adjacent said rear plane whereby
said two magnet elements establish a magnetic flux circuit which passes
axially in opposite directions through said two coils and which flux
circuit has a rear air portion extending in one direction between the rear
plane ends of said magnet elements and a front air portion extending in
the opposite direction between the front plane ends of said magnet
elements, said two coils having substantially equal numbers of turns and
each having two end leads, and means electrically connecting one end lead
of said first coil solely to one end lead of said second coil so that the
electrical circuit between the other two of said end leads consists solely
of said two coils connected in series with one another in such a way that
an electric current in passing serially through said two coils passes
clockwise through one coil and counterclockwise through the other of said
coils as viewed looking toward said front ends of said coils, said housing
forming a cavity receiving said coils, bobbins and permanent magnet
elements, said housing having a front wall and each of said bobbins having
a front end facing said front wall of said housing, and a pad of material
having the resiliency of silicone rubber located between said front walls
of said housing and said front ends of said bobbins.
14. An electrical pickup for a stringed musical instrument having a set of
transversely spaced parallel strings, said pickup comprising a hollow
housing of electrically conductive non-magnetic material, first and second
coils each having an opening with an axis, said two coils being arranged
side by side and next to one another within said housing with said axes of
their openings parallel to one another, said two coils extending axially
between parallel front and rear planes common to both coils so that each
coil has one axial end located in said front plane and the other axial end
located in said rear plane, a first permanent magnet element extending
axially through the opening of said first coil, a second permanent magnet
element extending axially through the opening of said second coil, said
first magnet element having an end with a north magnetic pole adjacent
said front plane and another end with a south magnetic pole adjacent said
rear plane and said second magnet element having an end with a south
magnetic pole adjacent said front plane and another end with a north
magnetic pole adjacent said rear plane whereby said two magnet elements
establish a magnetic flux circuit which passes axially in opposite
directions through said two coils and which flux circuit has a rear air
portion extending in one direction between the rear plane ends of said
magnet elements and a front air portion extending in the opposite
direction between the front plane ends of said magnet elements, said two
coils having substantially equal numbers of turns and each having two end
leads, and means electrically connecting one end lead of said first coil
solely to one end lead of said second coil so that the electrical circuit
between the other two of said end leads consists solely of said two coils
connected in series with one another in such a way that an electric
current in passing serially through said two coils passes clockwise
through one coil and counterclockwise through the other of said coils as
viewed looking toward said front ends of said coils, a housing of
electrically conducting non-magnetic material forming a cavity receiving
said coils, bobbins and permanent magnet elements, said housing having two
side walls and each of said coils having one side facing a respective one
of said side walls of said housing, and a pad of material having the
resiliency of silicone rubber interposed between said one said of each
coil and the adjacent one of said housing side walls. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
This invention relates to magnetic induction type pickups for guitars and
other stringed musical instruments, and deals more particularly with such
a pickup having an improved ability to suppress hum or other noise arising
from stray magnetic fields and also to suppress the production of
microphonic type output signals.
Magnetic induction type pickups establish a magnetic flux circuit passing
in part both through a string of the associated instrument and through an
associated coil, and operate on the principle that as the string vibrates,
the reluctance of the flux circuit is varied to vary the magnetic flux
through the coil and to thereby generate a signal voltage in the coil.
Unfortunately, electric motors, and many other pieces of common electrical
apparatus tend to emit stray magnetic fields, and when magnetic induction
type pickups are used in their vicinity, the stray fields often tend to
couple with the pickup to produce a noise factor in the output signal of
the pickup reproduced as audible noise in the output from the amplifier
and speaker system driven by the pickup. Due to the very common use of
60Hz alternating current power, the stray magnetic fields in question
commonly have a 60Hz frequency and in coupling with the pickup produce a
60Hz audible hum from the associated speaker.
Another problem often encountered with magnetic induction pickups is that
sound vibrations in the air may be transmitted to the pickup and may set
one or more internal parts of the pickup into resonant vibration to
produce accompanying flux changes and output signals at the resonant
frequency which, through positive feedback, sustain the resonant vibration
and produce microphonic or squealing sounds from the speaker or speakers.
The general object of this invention is, therefore, to provide a magnetic
induction type pickup for a stringed musical instrument which minimizes
hum and other noise in its output signal by suppressing the effect of
stray magnetic fields regardless of the direction of such fields and which
pickup despite such noise suppressing quality nevertheless yields a high
quality output signal voltage in response to the vibration of the
instrument strings.
Another general object of the invention is to provide a magnetic induction
type pickup for a stringed musical instrument wherein resonance of various
internal parts is suppressed to avoid microphonics.
Other objects and advantages of the invention will be apparent from the
accompanying drawings and from the following written description.
SUMMARY OF THE INVENTION
This invention resides in a magnetic induction type electrical pickup for a
stringed musical instrument consisting of two coils which are arranged
side by side between common front and rear planes with the axes of their
openings being parallel to one another and perpendicular to such planes.
Each coil has at least one permanent magnet element extending axially
through its opening. The permanent magnet elements of the two coils are
oppositely magnetically poled. That is, the permanent magnet element of
one coil has a north magnetic pole adjacent the front plane and a south
magnetic pole adjacent the rear plane and the permanent magnet element of
the other coil has a south magnetic pole adjacent the front plane and a
north magnetic pole adjacent the rear plane, so that the magnet elements
establish a magnetic flux circuit passing through both coils via the
magnet elements and passing through the air from one magnet element to the
other at the front and rear ends of the magnet elements. Additionally, the
two coils are electrically connected in series with one another and so
wound that an electrical current passing serially through them passes
clockwise through one coil and counterclockwise through the other coil. As
a result, flux variations due to a vibrating string of magnetic material
extending through the front air portion of the flux circuit induces
similar voltages in the two coils which are serially added to one another
and appear across the two output leads of the pickup. On the other hand,
flux from a stray magnetic field and having a direction parallel to the
coil axes passes through both coils in the same direction, and the
voltages which are induced thereby in the two coils oppose and cancel one
another so as to produce no output voltage across the two output leads of
the pickup. Also, a stray magnetic field having any direction in a plane
perpendicular to the coil axes links no turns of either coil and,
therefore, induces no output voltage in either coil. Further, the
direction of the flux in the front air portion of the flux circuit is
opposite to its direction in the rear air portion of the circuit so that
any stray magnetic flux having a direction such as to assist or oppose the
flux of one air circuit will have an opposite and cancelling effect on the
flux of the other air circuit and will, therefore, have little or no
effect on the flux passing through the two coils.
Preferably, the two coils are elongated in a direction perpendicular to
their axes so that their openings extend transversely across the full set
of strings of the instrument and preferably each opening includes either
one magnet element for each of the strings or a single transversely
elongated magnet providing a single magnetic pole face extending
transversely across the full set of strings.
The invention also resides in assembling the parts of the aforesaid pickup
in such a manner as to dampen the vibration of certain parts to suppress
microphonics. In particular, the coils are wound on bobbins having
openings for the associated magnets, and each magnet is separated from the
major portion of its opening wall by a vibration dampening layer of
silicone rubber or other resilient material. Resilient pads are also
placed between the pickup housing and the pickup parts enclosed thereby
for the same purpose.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a partial plan view of a guitar having two electrical pickups
each embodying this invention.
FIG. 2 is an enlarged plan view of one of the pickups of FIG. 1 with
portions of the pickup being shown broken away to reveal other details.
FIG. 3 is a still further enlarged cross sectional view taken on the line
3--3 of FIG. 2.
FIG. 4 is a fragmentary sectional view taken on the line 4--4 of FIG. 3.
FIG. 5 is an exploded perspective view of the pickup of FIG. 2.
FIG. 6 is a rear view of the pickup of FIG. 2.
FIG. 7 is a fragmentary sectional view taken on the line 6--6 of FIG. 6.
FIG. 8 is a schematic diagram showing the direction of winding and
electrical interconnection of the two coils of the pickup of FIG. 2.
FIG. 9 is a perspective view of a coil, bobbin and magnet assembly which
may be used in a pickup similar to that of FIGS. 2 - 7, and in place of
the corresponding assemblies there shown, to make an alternative
embodiment of the invention.
FIG. 10 is a perspective view showing two coil, bobbin and magnet
assemblies which may be substituted for those of FIGS. 1 - 7 to make
another alternative embodiment of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a solid body guitar 10 having two electrical magnetic pickups
12 and 14 located in cavities in the guitar body 16 covered by a pick
guard or plate 18 attached to the front surface of the body. Each pickup
projects outwardly from its cavity through an opening in the pick guard
toward the overlying set of strings 20, 20 extending along the length of a
neck 22 and attached at their lower ends to the guitar body 16 by a bridge
and tail piece unit 24. The manner in which each pickup is mounted from
the guitar body may vary widely, but preferably the mounting system is
such as shown in the copending patent application filed simultaneously
herewith and entitled PICKUP MOUNTING FOR STRINGED INSTRUMENT, Ser. No.
556,897 to which reference may be had for further details of its
construction.
A solid body guitar having two pickups has been shown in FIG. 1 by way of
example only, and it should be understood that the pickup of this
invention may be used with various different styles of guitars including
those having only a single pickup or more than two pickups. In FIGS. 2 -
5, the illustrated pickup is taken to be the pickup 12 of FIG. 1.
Referring to FIGS. 2 - 5, the pickup 12 there illustrated includes a hollow
housing consisting of flat base plate 26 and a cup-shaped housing member
28 having two side walls 30, 30, two end walls 32, 32 and a top wall 34.
The housing member 28 and base plate 26 are both made of brass or similar
electrically conducting nonmagnetic material and they are fastened and
electrically connected to one another by spots of solder 36, 36 on the
rear surface of the base plate. Together the housing member and base
plate, therefore, define an electrically shielded cavity in which the
remainder of the pickup components are housed. At each end of the housing
member, the base plate extends beyond the housing member to provide a
mounting flange 41 used to attach the pickup to the guitar body, the two
illustrated mounting flanges 41, 41 being shaped to adapt the pickup to
the mounting system of the aforesaid co-pending patent application.
Within the housing cavity of the pickup are two electrical coils 38 and 40
which are arranged side by side on the base plate 26 with the axes of
their openings parallel to one another and generally perpendicular to the
base plate 26. Each coil is elongated along a line of elongation extending
generally transversely of the strings 20, 20 so that each coil opening has
a length slightly greater than the spacing between the two farthest apart
strings. The two coils are received respectively on two bobbins 42 and 43
made of a nonmagnetic material such as Delrin or other plastic. Each
bobbin in turn includes an elongated body portion 44 around which the
associated coil is wound and two end flanges 46, 46 at opposite ends of
the body portion between which the associated coil is located. Each flange
46 has a generally flat outwardly facing surface. Each one end flange of
each bobbin rests flatly against the base plate 26 as shown best in FIG.
3.
The body portion 44 of each bobbin 42 or 43 includes a plurality of
openings 45, 45 extending axially therethrough, there being one such
opening for each of the strings 20, 20 and each opening, in use, being
arranged to lie directly beneath a respective one of the strings. Each
opening is of generally circular cross section and each receives a
generally cylindrical permanent magnet element 48.
The base plate 26 defines a rear plane for the pickup and the front ends or
flanges of the two bobbins define a front plane for the pickup. Each
permanent magnet element 48 extends between such front and rear planes.
Further, all of the permanent magnet elements 48, 48 of the coil 38 and
bobbin 42 have south magnetic poles adjacent the rear plane or base plate
36 and north magnetic poles adjacent the front plane; whereas, all of the
permanent magnet elements 48, 48 of the coil 40 and bobbin 43 have north
magnetic poles adjacent the rear plane and south magnetic poles adjacent
the front plane.
Each coil 38 and 40 has one side facing one of the side walls 30, 30 of the
housing member 28 and between each such coil side and housing side wall 30
is an interposed pad 50 of rubber or similar resilient material. Each pad
50 is held in a slightly compressed state by the coil and the adjacent
side wall 30 and, therefore, acts to dampen vibration of the housing side
wall relative to the coil. Without such pad, the housing may, as a result
of exciting vibrations in the air, tend to resonant or ring at certain
frequencies and through such vibration produce changing capacitances which
become coupled with the electrical signal produced by the coils to produce
a microphonic or squealing output from the associated sound system. For
the same purpose, another pad 52 of rubber or similar resilient material
is interposed between the top wall 34 of the housing and the front ends of
the bobbins 42 and 43, the pad 52 also being held in a slightly compressed
state to dampen vibration of the top wall.
As shown best in FIGS. 3 and 5, each permanent magnet element 48 extends a
small distance beyond the front end of its associated bobbin. To
accommodate this, the pad 52 includes a plurality of openings 54, 54, and
the housing top wall 34 includes a plurality of registering openings 56,
56 which receive the forwardly projecting portions of the magnet elements.
The permanent magnet elements 48, 48 are also mounted in their associated
bobbins in such a manner as to dampen vibration between them and the
bobbins to inhibit microphonic noise arising from such vibration. For this
purpose, each bobbin opening 45 has a cross section slightly larger than
the cross section of its associated magnet element and a layer 49 of
resilient material, such as a silicone rubber potting agent, is interposed
between the magnet element and the bobbin, as shown best in FIG. 4.
Preferably, and as shown, each opening 45 has three axial ribs 47, 47
which project radially inwardly from the major portion of the interior
surface of the opening. These ribs engage and center the associated
permanent magnet element 48 in spaced relationship with the opening.
Therefore, in assembling the elements 48, 48 with the bobbins, they may
first be inserted into the openings 45, 45 and then the resilient material
of the layers 49, 49 may be poured in liquid form into the spaces between
the elements and their opening walls, the ribs holding the elements in
their proper positions while the resilient material cures.
As shown best in FIG. 5, each coil 38 and 40 has two end leads at one of
its rounded transverse ends, and the two coils, and their associated
bobbins, are placed on the base plate 26 so that each has its leads at the
same end of the base plate. The coils are electrically connected in series
with one another in such a manner that an electric current in passing
through one coil flows in a clockwise direction around its associated set
of permanent magnet elements 48, 48 and in passing through the other coil
flows around its set of permanent magnet elements 48, 48 in the opposite
or counterclockwise direction. This is shown schematically in FIG. 8 which
for convenience shows only one of the magnets 48 of the coil 38 and one of
the magnets 48 of the coil 40. The connection is actually made, as shown
in FIG. 5, by connecting the two upper leads of the two coils together as
at 58. The two bottom leads 60, 60 pass through a hole in the base plate
26 and become the output leads for the pickup across which the pickup
signal appears. Another lead 62 is also connected to the base plate 26, as
by one of the solder spots 36, as shown in FIG. 6, for grounding the
pickup.
Each bobbin 42 and 43 is fixed to the base plate 26 by two screws 64, 64
which pass through openings in the base plate and threadably engage a
corresponding pair of fastener openings in the associated bobbin body
portion. The arrangement of the base plate openings and the fastener
openings in the bobbins is such that the two coil and bobbin assemblies
are identical with one another and the only way in which they can be
attached to the base plate, with their leads at the same end of the base
plate, is in such a relative arrangement that when the two upper coil
leads (or alternatively the two bottom coil leads) are connected, as at
58, the current necessarily flows through the two coils in opposite
directions. This is an aid to the manufacture and assembly of the pickup
since both of the coil and bobbin assemblies may be made identical with
each other and it assures that the correct interconnection of the two
coils of the pickup will occur if two coil and bobbin assemblies are
attached to the base plate with their leads at the same end of the base
plate and the two upper (or two lower leads) are connected to one another.
The means for assuring that the two bobbin and coil assemblies are attached
to the base plate in the proper relationship is as follows. Referring to
FIGS. 6 and 7, it will be understood that the four screws 64, 64 pass
through openings in the base plate, and from FIG. 6 it will be seen that
the two base plate openings associated with one of the bobbins have a
relative arrangement which is the mirror image, taken along a line
extending between the two bobbins, of the relative arrangement of the two
base plate openings associated with the other bobbin. Each bobbin, as
evident from FIGS. 5 and 7, has two axial fastener holes extending
completely through the body portion of the bobbin and, therefore, the two
holes form two openings 68, 68 on each of the two end faces of the bobbin.
The arrangement of the fastener holes is such that when a bobbin and coil
assembly is placed on the base plate 26 with the coil leads at the
left-hand end of the base plate, as seen in FIG. 5, if the bobbin is
turned so that one of its end faces is adjacent the base plate then the
openings 68, 68 in that end face of the bobbin will match one but not the
other of the sets of base plate openings, and if the bobbin is turned so
that its other end face is adjacent the base plate, then the openings in
that end face will match with the other set of base plate openings. Thus,
the case of the two coil bobbin assemblies shown in FIG. 5, one of them is
turned "upside down" relative to the other so that when the two upper
leads are connected, as at 58, the desired connection as respect to coil
winding direction is obtained.
Having now described the construction of the pickup 12, its functioning may
be discussed with reference to FIGS. 2 and 3. As shown in FIG. 2, the
arrangement of the pickup in use is such that each string 20, 20 of the
associated instrument extends over and across two of the permanent magnet
elements 48, 48, one of such two elements being in the coil 38 and the
other element being in the coil 40. As shown in FIG. 3, the two permanent
magnet elements 48, 48 associated with each string 20 set up a magnetic
field having a continuous flux path indicated by the broken lines in FIG.
3. This flux path has two portions in which the lines of flux pass
respectively through the two permanent magnet elements 48, 48 in opposite
directions. It also has two air portions. One of these air portions is
located to the rear of the pickup and in it the lines of flux pass between
the rear ends of the two permanent magnet elements. The other air portion
of the flux path is located to the front of the pickup and in it the lines
of flux pass between the two front ends of the two magnet elements. The
associated string 20 is situated in the front air portion of the flux path
so that some of the flux of this air portion passes through it. The string
is made at least in part of steel or other magnetic material and when set
into vibration, during the playing of the instrument, moves away from and
toward the front end of the pickup as indicated by the broken line
showings 20a and 20b of the string in FIG. 3. More lines of flux of the
front air portion of the flux circuit will pass through the string 20 when
it is close to the pickup than when far from the pickup. Thus, the
vibration of the string effectively changes the reluctance of the front
air portion of the flux circuit and varies the amount of flux passing
through each of the two permanent magnets 48, 48 and thereby induces
signal voltages in the two coils 38 and 40. Since the flux has opposite
directions through the two permanent magnet elements and since the two
coils 38 and 40 are serially connected in the manner shown in FIG. 8, the
signals induced by the string vibration in the two coils will be in series
aiding relation to one another and will combine to produce an output
signal at the two output leads 60, 60.
If a stray magnetic field is present and has a direction parallel to the
axes of the magnet elements 48, 48 such field will pass in the same
direction through all of the permanent magnet elements and induce signals
in the two coils 38 and 40 which are of opposite polarity so as to oppose
and effectively cancel one another to produce no output at the two output
leads 60, 60. If the stray magnetic field has a direction perpendicular to
the axes of the permanent magnet elements and parallel to the strings 20,
20, it will cut no turns of either of the coils 38 and 40. Also, it will
oppose the flux in one of the air portions of the flux circuit and will
aid the flux in the other air portion of the flux circuit, so that the net
effect will be to induce no signals in either of the two coils. If the
stray magnetic flux is perpendicular to the axes of the permanent magnet
elements and perpendicular to the strings 20, 20, then it will neither
oppose nor aid the flux in either of the two air portions nor will it cut
any of the windings of either of the two coils, and no signal will appear
at the two output leads of the pickup. Accordingly, regardless of the
direction of the stray magnetic field, coupling of such field with the
normal output signal generated by the vibrating strings is suppressed.
It may sometimes be desirable to vary the heights of the permanent magnet
elements relative to the associated strings 20, 20 and when this is the
case, a coil, bobbin and magnet assembly similar to that shown in FIG. 9
may be used for each of the two such assemblies in the pickup 12 as shown
and previously described in connection with FIGS. 1 - 7. Referring to FIG.
9, the bobbin 70 and coil 72 there shown are similar to the bobbins and
coils previously described. The permanent magnet elements 74, 74, however,
are externally threaded and provided with slots at their upper ends. The
threads of each magnet in turn threadably engage the locating ribs 47, 47
associated with the opening 45 in the bobbin which receives the magnet.
Therefore, by rotating each permanent magnet element 74, as by means of a
screw driver placed in its slot, the element may be threaded either into
or out of its opening to raise or lower its front end relative to the
string. In this case, each element 74 may be designed to self thread
itself into threaded engagement with the ribs of its opening.
Also, instead of each coil and bobbin assembly being provided with a
separate magnet element, such as the previously described magnet elements
48, 48, for each string, a single magnet may be used for each such
assembly. Such a combination is shown in FIG. 10 wherein each of the
illustrated bobbins has a single elon | | |
