Electrical pickup for a stringed musical instrument

[57] ABSTRACT

An electrical pickup for a stringed musical instrument, such as a guitar, comprises a pair of axially spaced coils, a ferromagnetic pole piece spanning the space between the coils and extending into each coil and having a portion between the coils which extends toward a string, magnet means having a pole face directed toward the pole piece and directing a magnetic field into the pole piece of a single magnetic polarity, the magnet means preferably comprising two permanent magnets disposed at opposite sides of the coils and directing magnetic fields into the pole piece from opposite sides thereof of the same magnetic polarity.

Claims, 5 Drawing Figures

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U.S. Patent Nov. 4,1975

3,916,751

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ELECTRICAL PICKUP FOR A STRINGED MUSICAL INSTRUMENT

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an electrical pickup for a stringed musical instrument, such as a guitar.

2. Prior Art

It has been conventional to use a coil underlying the strings of a guitar with the axis of the coil being perpendicular to the top side of the guitar and perpendicular to the strings. This construction causes hum when there are stray magnetic fields nearby, such as emanate from an amplifier transformer, fluorescent lights, or the like.

A prior solution has been proposed to use a second coaxial pickup disposed beneath the main pickup and separated therefrom by a magnetic shield. Such a pickup has been effective in eliminating such hum, but it has a disadvantage in that it doubles the resistance of the wiring through which the signal must pass, and therefore the signal is weakened.

It has also been proposed to use a pickup with two coils side by side with their axes parallel. Constructions of this type have eliminated hum, but they have a disadvantage in that string vibrations are not sensed at a narrow point on the string but along a rather wide range of string length. Both such coils can pick up or sense the fundamental vibration, but for higher frequencies, the phase difference in the signals induced in the spaced coils can cause partial or even total cancellation of signals at these frequencies. These pickups thus have good fundamentals but are poor in reproducing desired overtones and sometimes have a resonance Or boost effect in the midfrequency range.

SUMMARY OF THE INVENTION

In this invention two coils are employed to eliminate hum but they are so arranged and disposed so that they are both Active, preventing loss of signal from use of the second coil. The boost and cancellation problems resulting from spaced coils are eliminated by sensing string vibration at a rather narrow spot on the string.

To effect the foregoing, a pair of coils are' arranged to have their axes extend in the same direction as the string, a ferromagnetic pole piece is disposed in the coils and spans the space between them, it having a portion between the coils projecting toward each string, and permanent magnet means are disposed at opposite sides of the coils and direct a magnetic field of a single magnetic polarity into the pole piece from opposite sides thereof.

Accordingly, it is an object of the present invention to provide an improved pickup for a stringed musical instrument.

Another object of the present invention is to provide a pickup construction which will eliminate hum, but which will not sacrifice signal level in doing so.

Another object of the present invention is to provide a pickup construction for a stringed musical instrument which has a substantially uniform response over the full frequency range of the instrument.

Many other advantages, features and additional objects of the present invention will become manifest to those versed in the art upon making reference to the detailed description and the accompanying sheet of drawings in which a preferred structural embodiment incorporating the principles of the present invention is shown by way of illustrative example.

ON THE DRAWING

FIG. 1 is a cross-sectional view of a pickup constructed in accordance with the present invention;

FIG. 1A is a partial cross-sectional view of the pickup of FIG. 1, illustrating a preferred mounting means for the pickup;

FIG. 2 is a partially cut away top view of the pickup shown in FIG. 1;

FIG. 3 is a diagram showing connection of the coils; and

FIG. 4 illustrates alternate mounting means.

AS SHOWN ON THE DRAWING

The principles of the present invention are particularly useful when embodied in an electric pickup for a stringed musical instrument, such as a guitar. The pickup, as shown in FIG. 1, is generally indicated by the numeral 10. The pickup 10 includes a pair of axially spaced coils 11, 12 each comprising a bobbin 13 with an appropriate number of turns of wire of a proper size discussed below. The coils 11, 12 have axes which are concentric with each other and thus extend along the length of one of the instrument strings 14 of which there may be several as shown. A pole piece 15 formed of ferromagnetic material extends from the left edge of the coil 11 to the right edge of the coil 12, thus spanning the space between the coils 11, 12 and being disposed therein. The pole piece 15 has a portion 16 between the coils 11, 12 which extends toward the string 14. In this embodiment, the portion 16 comprises an adjustable screw of a ferromagnetic material disposed to be accessible for adjustment when assembled on the instrument, whereby the spacing between such screw 16 and the string 14 can be adjusted to get the desired degree of balance soundwise for the various ones of the several strings 14.

The pickup 10 further includes permanent magnet means, here comprising two permanent magnets 17, 18 disposed at opposite sides of the coils 11,12 and directing their magnetic fields into the pole piece 15 from opposite sides thereof. As shown, only one magnetic polarity, shown here by way of example as North, is directed to the pole piece 15. Each magnet pole having North polarity is directed away from each magnet pole having South polarity, and such direction or facing is along the length of the string 14.

Although the invention is operative and can be used with a single string, it preferably is so constructed that the coils 11,12, the magnets 17,18, and the pole piece 15 are elongated in a direction transverse to the length of the string 14 sb as to be responsive to a plurality of such parallel strings. Magnets 17 and 18 should be of sufficient length and be positioned so as to provide a substantially uniform magnetic field of a single magnetic polarity through said pole piece.

As best seen in FIG. 2, the magnetic pole piece has four shoulders 19 against which the bobbins 13 of the coil assemblies 11, 12 abut. After the coils 11, 12 have been placed on the pole piece 15, whenn the magnets 17, 18 are placed there against, they act so strongly on the pole piece as to actually trap and hold the components in assembled relationship. However, tape may be added if desired, particularly to assure that the components will stay together until they are encased.

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b. a ferromagnetic pole piece spanning the space between said coils and extending into each of said coils, said pole piece having a portion between said coils extending toward the string; and

c. permanent magnet means having a pole face directed toward said pole piece, and directing a magnetic field into said pole piece of a single magnetic polarity.

2. A pickup according to claim 1 in which said per10 manent magnet means comprises two permanent magnets disposed at opposite sides of said coils and directing magnetic fields into said pole piece from opposite sides thereof of the same magnetic polarity.

3. A pickup according to claim 1 in which said per¹⁵ manent magnet means has two pole faces of opposite polarity, said means being oriented so that only a neutral side between the poles is facing an instrument string.

4. A pickup according to claim 1 in which said coils ²⁰ are connected in series in out-of-phase relation to each other.

5. A pickup according to claim 1, in which said coils, said magnets, and said pole piece are elongated in a direction transverse to string length so as to be responsive to a plurality of parallel strings.

6. A pickup according to claim 1 in which said pole piece has oppositely facing shoulders engaging said coils and thus defining the axial spacing therebetween.

7. A pickup according to claim 1 in which said portion extending toward the string comprises an adjustable screw disposed to be accessible for adjustment when assembled on the instrument.

8. A pickup according to claim 5 which includes a ₃₅ partially hollow mounting base receiving said coils, said magnets and said pole piece, said base having an external configuration of uniform cross section and a cover closing said base.

9. An electrical pickup for a stringed musical instru₄₀ ment comprising:

a. a pair of axially spaced coils arranged to have their axes extend along the length of an instrument string; said coils being connected in series out of phase relation with each other;

₄₅ b. a common pole piece for said coils, said pole piece being shaped to an adjacent each string of said instrument at only a simple point which point is between said coils; and

c. means for providing a magnetic field of a single magnetic polarity which is substantially uniform through said pole piece.

* ♦ ♦ ♦ ♦

One type of encasement is shown in FIGS. 1 and 1A which includes a flanged base 20 which supports the pole piece 15 by means of screws 21 and spacers 22 (see FIG. 1A). A cover 23 is secured to the base 20 and is suitably apertured as at 24 to provide access to the adjustable magnetic screws 16.

In place of the structure shown in FIG. 1A, an extruded plastic base 25 shown in FIG. 4 may be utilized, the same being partially hollow to receive nestingly the structure otherwise shown in FIG. 1. Use of an extrusion to provide a partially hollow mounting base yields an external configuration of uniform cross section to which the cover 23 may be attached.

The coils 11,12 are identical and the left end of each coil in FIG. 3 denotes the starting end of the winding, whereby, as shown, they are connected in out-of-phase relationship with each other. One coil is as active as the other in connection with the induction of signals and yet they sense at a single point for each string (the end of the corresponding screw 16) because of the construction of the pickup in its entirety.

In one embodiment, I have used 4,250 turns of No. 42 wire per coil. This produces an impedance which is suitable for most present day typical guitar amplifiers. If a greater or lesser impedance is desired, the number 25 of turns is respectively increased or reduced correspondingly.

A pickup 10 constructed as shown and described senses the vibration of a string only at the pole piece portion 16 and thus produces a sound response which 30 is substantially uniform over the full frequency range of the instrument. (The bottom side representing the lower end of the screw 16 can also sense inphase, but is not utilized in the present installation illustrated.) It is to be noted that each magnet 17, 18 is oriented so that a side between the poles, which is neutral, rather than a pole, faces toward the string, and therefore there is no sensing by such magnets at these neutral areas. Thus, the pickup truly picks up only at a single point for each string.

Although various minor modifications might be made or suggested by those versed in the art, it should be understood that I wish to embody within the scope of the patent warranted hereon, all such embodiments as reasonably and properly come within the scope of my contribution to the art.