Electrical pickup for a stringed instrument having ferromagnetic strings

An electrical pickup device for a stringed musical instrument having ferromagnetic strings comprises a pair of superposed coaxial bobbins, each axially wound with a coil having its axis perpendicular to the instrument strings. An integral plate of magnetic material is provided comprising a base disposed between the two bobbins perpendicular to the coil axis and two side walls extending upwardly and perpendicularly from the base to at least immediately below the top face of the upper bobbin. A plurality of rod-like permanent magnets extend through at least the upper coil parallel to the axis thereof and contact the base of the integral plate and the magnets have like polarities at the tops thereof.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an electrical pickup for a stringed 
instrument having ferromagnetic strings, such as a guitar. 
2. Prior Art 
Single coil electrical pickups operating by means of magnetic induction 
such as those disclosed in U.S. Pat. Nos. 3,588,311 and 3,711,619 tend to 
couple with the stray magnetic fields produced by electrical devices such 
as motors, transformers and fluorescent lights. This coupling commonly 
produces an audible hum in the amplifier and speaker driven by the pickup. 
This hum commonly has a 60 Hz frequency, due to the use of 60 Hz 
alternating current power. Prior attempts at the reduction or elimination 
of this hum have been proposed which utilize two coils disposed side by 
side with parallel axes such as in U.S. Pat. Nos. 2,896,461, 3,902,394, 
3,916,751 and 3,983,777, or with the two coils disposed one above the 
other, separated by a flat magnetic shield such as in U.S. Pat. No. 
3,657,461. The first method causes string vibrations to be sensed over a 
relatively broad length of string, and results in cancellation of various 
frequencies, due to the spacing of the coils under the strings. The second 
method functions effectively as regards reproduction of frequencies 
generated by the instruments' strings, but has the disadvantage of not 
producing a strong signal. Additionally, pole legs extending around the 
strings in single coil pickups have been proposed to reduce hum in U.S. 
Pat. Nos. 3,236,930 and 4,026,178. 
SUMMARY OF THE INVENTION 
The main object of the present invention is to provide a two coil pickup 
device which avoids the disadvantages of the prior art while simply 
achieving the elimination of hum. 
These and other objects of the invention are achieved in accordance with 
the invention by an electrical pickup device for a stringed musical 
instrument having ferromagnetic strings, comprising a pair of superposed 
coaxial bobbins, each axially wound with a coil having its axis 
perpendicular to the instrument strings, an integral plate of magnetic 
material comprising a base disposed between the two bobbins perpendicular 
to the coil axis and two side walls extending upwardly and perpendicularly 
from the base to at least immediately below the top face of the upper 
bobbin and a plurality of rod-like permanent magnets extending through at 
least the upper coil parallel to the axis thereof and in contact with the 
base of the integral plate and wherein the magnets have like polarities at 
the tops thereof. 
In a preferred embodiment, the base of the integral plate has a plurality 
of apertures therein and wherein the rod-like magnets extend through the 
apertures and the lower coil. 
Additionally, the two coils are preferably connected in series or parallel, 
such that current flowing clockwise through one coil will travel 
counterclockwise through the other. The thin plate of magnetic material is 
inserted between the two coils with the magnets abutting same or passing 
therethrough and the plate extends beyond the sides of the bobbins, and is 
bent perpendicularly to extend upwards to the top face, or immediately 
below the top face of the upper bobbin. The plate by this means becomes a 
transmission medium for the directed flow of magnetic force creating an 
efficient field interaction with the magnets and instrument strings since 
the two coils are connected together in such a way that an electrical 
current passing through one coil clockwise will pass through the other 
coil counterclockwise, and because both copies are so wound as to induce 
similar voltages, externally generated hum will be substantially reduced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIGS. 1-4, the invention as shown and previously stated is 
particularly intended for use with an electric guitar. The pickup includes 
two superposed coaxial bobbins or coil forms, 2 and 3 with bobbin 2 being 
the upper bobbin and bobbin 3 being the lower bobbin relative to the 
strings 51-56. 2 and 3 are wound with an appropriate gauge and amount of 
wire to produce coils 21 and 31 respectively and so that the induced 
voltage in one coil is equal to the other. The bobbins are separated by, 
and attached to, plate 4. The plate is constructed from a single piece of 
metal, preferably iron or another magnetic material, and includes base 41 
and is bent so that the sides 42, 43 are perpendicular to the base 41 of 
the plate, as shown in FIG. 2. The magnets 11-16 are arranged so that they 
fit through the plate 4, by means of pre-drilled holes 41a-f, and both 
bobbins through holes 2a-f. The polarity is lengthwise, so that in the 
embodiment shown, the north pole of each magnet faces upwards, as shown in 
FIG. 3. The magnetic field resulting from the arrangement of the magnets 
11-16 and plate 4 is shown in FIG. 3. It should be noted that the 
perpendicular sides 42, 43 of 4 are critical in creating a focused 
magnetic field immediately above the pickup, so that a string 51-56 
vibrating in the field will induce a sufficiently powerful electric 
current to create high output. Coils 21 and 31 are connected together in 
series or in parallel so that the current flowing clockwise in 21 will 
flow counterclockwise in 31. This will enable the cancellation of 
externally generated hum. 
The side walls 42, 43 should extend upwardly to encompass coil 21 to at 
least immediately below the top surface 2' of bobbin 2. 
FIG. 5 illustrates an alternative embodiment wherein plate 4' has a base 
41' with no apertures and bobbin 3' has no apertures. In this embodiment 
magnets 11'-16' are of sufficient length to only extend through holes 2a-f 
in bobbin 2 and thus coil 21 with their bottoms contacting base 41'. In 
this embodiment, base 41' is also configured to completely separate coils 
21 and 31. 
The terms and expressions which are employed herein are used as terms of 
description only and it is recognized that various modifications are 
possible within the scope of the invention claimed.