U.S. patent number 3,742,113 [Application Number 05/138,213] was granted by the patent office on 1973-06-26 for stringed musical instrument with electrical feedback.
Invention is credited to Marcus S. Cohen.
United States Patent |
3,742,113 |
Cohen |
June 26, 1973 |
STRINGED MUSICAL INSTRUMENT WITH ELECTRICAL FEEDBACK
Abstract
This application discloses a stringed musical instrument
including a means responsive to the vibrations of a string as it is
plucked or hit, and a means for driving the oscillations of the
string electromechanically. The means responsive to the vibration
generates an electrical signal which is amplified and supplied to
the driving means which causes continued vibration of the string.
Means are disclosed wherein the string's vibrations can be
sustained for an arbitrary period of time at its fundamental
frequency or at higher harmonics thereof, or at a mixture of
fundamental and harmonics, producing a sound rich in overtones
unique to this instrument. Also disclosed are means whereby the
player selects at will which of a plurality of strings will be
driven continuously as drones, which will be driven only when
played upon, and which will remain undriven. The effect is to make
possible the creation of aesthetically pleasing sounds which are
entirely different than those created by standard stringed
instruments.
Inventors: |
Cohen; Marcus S. (Winnetka,
IL) |
Family
ID: |
22480979 |
Appl.
No.: |
05/138,213 |
Filed: |
April 28, 1971 |
Current U.S.
Class: |
84/726; 84/741;
84/738; 84/742; 984/375 |
Current CPC
Class: |
G10H
3/26 (20130101) |
Current International
Class: |
G10H
3/00 (20060101); G10H 3/26 (20060101); G01h
003/00 () |
Field of
Search: |
;84/1.13-1.16,1.26,1.06,1.19,267,1.04 ;323/22 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Wilkinson; Richard B.
Assistant Examiner: Weldon; U.
Claims
What is claimed as new and desired to be secured as Letters Patent
is:
1. A musical instrument including: a plurality of strings; means
securing each of the strings at their opposite ends to maintain
them in tension whereby they can be vibrated to produce a musical
sound; a transducer in proximity with a string for generating an
electrical signal proportional to the vibration of the string as it
is vibrated; means for amplifying the electrical signal so
generated; the improvement comprising driving means positioned
adjacent the string for producing a varying magnetic field
effective to vibrate the string; means for connecting said driving
means to said amplifying means to cause the string to vibrate in
response to the output of said amplifying means; a volume control
resistor connected to the output of said transducer and to the
input of said amplifying means; an adjustable impedance circuit
connected between said amplifying means and said resistor for
passing desired harmonic components of the electrical signal across
said resistor to said amplifying means; switching means for
selectively connecting and disconnecting said amplifier to said
driving means; a controllable bidirectional semiconductor switching
device having a first pair of terminals and a gate control
terminal; means connecting one of said first pair of terminals to
said switching means and the other of said pair of terminals to
said driving means; and means including a switch for selectively
connecting and disconnecting said gate control terminal to a source
of gate control voltage.
2. A musical instrument as set forth in claim 1 wherein the movable
contact of said switch selectively connecting said gate control
terminal to said source of gate control voltage is constituted by
said string and the fixed contact thereof is constituted by a
fret.
3. A musical instrument as set forth in claim 2 including a pair of
oppositely poled rectifiers selectively connectable between said
switching means and said driving means.
4. A musical instrument including: a plurality of strings; means
securing each of the strings at their opposite ends to maintain
them in tension whereby they can be vibrated to produce a musical
sound; a feedback circuit comprising a transducer in proximity with
a string for generating an electrical signal proportional to the
vibration of the string as the string is vibrated; amplifying means
having an input connected to the output of said transducer; driving
means positioned adjacent the string and connected to the output of
said amplifying means for producing a force effective to vibrate
the string at frequencies and amplitudes proportional to those of
the signal supplied thereto; means connected to the said input of
said amplifying means for controlling the amplitude of the said
signal supplied to said driving means; and separate means serially
connected in said feedback circuit for modifying the frequency
content of the signal supplied to said driving means.
5. A musical instrument as set forth in claim 4 wherein said
separate means is connected between the output of said transducer
and the input of said amplifying means.
6. A musical instrument as set forth in claim 4 wherein said
separate means comprises a rectifier connected between the output
of said amplifier and said driving means.
7. A musical instrument as set forth in claim 4 wherein said means
for controlling the amplitude of the signal supplied to said
driving means is constituted by a volume control resistor connected
to the output of said transducer and said separate means is
constituted by an adjustable impedance circuit connected between
said amplifying means and said resistor.
8. A musical instrument as set forth in claim 4 wherein said
separate means is constituted by a wave-shaping circuit.
9. A musical instrument as set forth in claim 4 wherein said
separate means is constituted by a harmonic selector.
10. A musical instrument as set forth in claim 5 including a
rectifier connected between the output of said amplifier and said
driving means.
11. A musical instrument as set forth in claim 5 including a pair
of oppositely poled rectifiers connected between the output of said
amplifier and said driving means.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to stringed musical instruments.
More specifically, it relates to a musical instrument using
feedback to enhance or change the characteristic sound of the
instrument.
Stringed musical instruments such as guitars, banjos, pianos,
harpsichords or the like all have characteristic sounds which have
each fit into their own musical tradition. Contemporary musicians
have sought and are seeking to achieve effects which are different
from those to which audiences are accustomed. These efforts are
generally welcomed and have resulted in the creation of new musical
forms and techniques.
A particular instrument wherein this effect has been extensive is
the guitar. With the advent of electrical amplification it has
become possible to produce sounds from a guitar which are
substantially different than those produced by conventional
unamplified instruments. For instance, to the distress of some, but
welcomed by others, it is possible by substantial amplification of
a guitar to achieve a kind of acoustical feedback and in doing so
change the basic sounds produced by the instrument. Acoustical
feedback occurs when the energy in the sound output of the speakers
strikes the strings, driving their vibrations, and if the sound
output is high enough the vibration of the string will be
sustained, or other strings induced to vibrate sympathetically. The
vibration sustained or induced may be at the fundamental frequency
or at some harmonic component thereof. Some novel and aesthetic
sounds can be produced in this manner, but the effect is hard to
control by the player, varies with the pitch of the note being
played, and relies on extremely high volume which is displeasing to
many and distorts the sound in unpleasant ways. It is impossible to
integrate an instrument relying on acoustical feedback into a
symphony orchestra or any other group of acoustical instruments,
due to the discrepancy in volume.
This invention provides a new and aesthetic sound comprised of the
indefinite sustainance of notes and the generation of harmonics,
which relies on the principle of electromechanical rather than
acoustical feedback and hence is liberated from the necessity for
high volumes. It can as easily play with a chamber group as with an
electric band; indeed, as the instrument can produce a sound
similar to the violin, cello, or both playing together an octave
apart, or can imitate the sound of the Indian sitar, it is believed
it will gain acceptance as an orchestral instrument with musicians
and audiences alike.
Therefore, it is an object of this invention to provide a novel
musical instrument which is capable of producing new and pleasing
musical sounds.
It is another object of this invention to provide a novel musical
instrument which can be selectively played so as to produce
conventional musical sounds or to produce a wide range of novel
musical sounds.
It is a further object of this invention to provide a novel musical
instrument which can produce new and unusual musical effects
without suffering from the disadvantage of having to be played at
volumes which may be unsatisfying or even unpleasant to some.
It is a still further object of this invention to provide a novel
musical instrument which is similar in method of playing to
conventional musical instruments but is capable of producing a wide
range of novel and aesthetically pleasing musical sounds.
SUMMARY OF THE INVENTION
The foregoing and other objects are achieved in one embodiment of
the invention wherein a pickup is provided which produces a varying
electrical signal in response to the vibration of a string. The
electrical signal is then amplified and supplied to a means for
driving or vibrating the string at the fundamental frequency of the
signal, at some selected harmonic component thereof, or at a
mixture of the fundamental and its higher harmonics.
BRIEF DESCRIPTION OF THE DRAWING
The invention itself is set forth in the claims appended hereto and
forming a part of this specification. The structure and mode of
operation of various embodiments thereof may be understood by
reference to the detailed description taken in conjunction with the
accompanying drawings in which:
FIG. 1 is a plan view of a guitar embodying this invention;
FIG. 2 is a schematic illustration of one embodiment of the
invention;
FIG. 3 is a schematic illustration of a second embodiment of the
invention;
FIG. 4 is a schematic illustration of a third embodiment of the
invention;
FIG. 5 is a schematic illustration of a fourth embodiment of the
invention;
FIG. 6 is a schematic illustration of another embodiment of the
invention;
FIG. 7 is a circuit diagram of still another embodiment of the
invention as included in the guitar of FIG. 1; and
FIG. 8 is a sectional view of an embodiment of the driving means
forming a part of the invention.
DETAILED DESCRIPTION
FIG. 1 of the drawing illustrates a guitar body and a portion of
the neck thereof and shows how the invention of this application
may be incorporated into such an instrument.
A guitar body 2 has extending therefrom a neck 4 on which are
mounted in a conventional fashion the usual frets 6. A rear string
attachment assembly 8 is provided on the body and anchors one end
of each of a plurality of strings 10. In a preferred embodiment the
strings are formed of steel and are electrically isolated from each
other. In this case, the number of strings shown is six. Obviously,
the number of strings can be varied for there are guitars that are
in use which have as many as 12 strings. The strings extend from
the attachment assembly 8 and pass over a bridge 12 which raises
them from the surface of the body. At their opposite ends the
strings are secured by conventional adjustable tensioning means
mounted at the end of the neck, which means are well-known in the
art.
The guitar illustrated is of the type wherein the vibrations of the
strings are transduced into an electrical signal. For this purpose
pickups 14, 26 are provided adjacent the strings. The pickups are
constituted by coils wound around magnetized cores to as to produce
electrical signals responding to the vibrations of the strings
which electrical signals may be amplified and converted into sounds
by suitable loudspeakers.
The guitar may be provided with volume and tone controls for the
bass and treble portions of the musical spectrum as desired. Thus,
a knob 16 is arranged to control the volume of the rear pickup 26
while the knob 18 controls the tone of its sounds. Likewise, a knob
20 controls the volume of the forward pickup 14 while the knob 22
controls its tone. An output jack 24 is provided on the side of the
body to receive a plug coupling the pickup outputs to the
amplifier.
What has been described heretofore, is the general configuration of
a conventional electrically amplified guitar. In accordance with
the invention, the guitar is provided with a differential pickup 26
of the type designed to be insensitive to external magnetic fields.
A magnetic shield 28 surrounds the pickup 26 in a manner and for a
purpose to be explained in greater detail hereinafter. Positioned
adjacent the neck beneath the strings so as to be spaced from the
pickup 26 are the rear driver coil assembly 29 and the forward
driver coil assembly 30. An input jack 32 is mounted on the side of
the guitar body to receive a plug connecting the driving signals to
the driver coil assemblies 29 and 30. A first set of toggle
switches 34 are provided in one row on the body along with a second
set of toggle switches 36 provided in a second row. Also included
is a master feedback toggle switch 38 along with a changeover
switch 40. The purpose and function of these switches will be
explained hereinafter in connection with the explanation of the
circuit shown in FIG. 7.
While the foregoing description has been that of a guitar it is
contemplated that the invention may be applied to other stringed
instruments such as the harpsichord, or instruments utilizing steel
prongs such as the electronic piano, and enhance their musical
qualities as in the case in the guitar. Also, it is contemplated
that where, as in the embodiment of FIG. 1, the invention has been
applied to each string of the guitar but the pickups 14 and 26 mix
the output of all the strings and feed them into a common output,
it may be found desirable to provide a separate pickup for each
string, the output of which leads into a separate volume and tone
control array and a separate amplifier, thus eliminating or
controlling at will cross coupling or sympathetic vibration between
strings.
Referring now to FIG. 2, there may be seen a first embodiment of
the invention. This figure illustrates in diagrammatic form how the
invention may be applied in one form to any one or more strings of
a stringed instrument.
In this configuration a steel string 42 extends between two fixed
points 44 and 46 and is under tension so that it will vibrate to
produce a musical sound when plucked or hit. Whether the string is
in a guitar, piano, harpsichord or the like the body of the
instrument supports the string to amplify the sound created by its
vibration and to produce a particular tonal quality depending on
the instrument.
In accordance with the invention a pickup 48 comprising a coil
mounted on a magnetized core is positioned adjacent the string 42.
As the string 42 is vibrated as the result of being plucked or hit,
an electrical signal is generated in the pickup unit 48. An
amplifier 50 has its input connected to the output of the pickup 48
and supplies the electrical signal amplified to a driver unit 52.
The driver unit consists of a coil wound on an iron core and when
excited by the amplifier 50 produces a magnetic field which varies
as the original vibration of the string and is effective to sustain
that original vibration. In this configuration, and employing an
amplifier with zero phase shift, the driving force is in phase with
the string's fundamental frequency of oscillation as transduced by
the pickup, the feedback in the loop (string, pickup, amplifier,
driver) is positive, and oscillation is maintained as long as the
player permits the string to vibrate. In this form the invention
could be used to play a drone or sustained note while a melody is
played on the other strings of the instrument. A switch 54 may be
provided to be manipulated by the musician to determine whether or
not such a sustained note is to sound and when its sound is to
cease.
In another form of the invention, as illustrated in FIG. 3, the
feedback loop in addition to the pickup 48, amplifier 50 and driver
52 also includes a harmonic selector 56. Such a harmonic selector
may be constituted by any of the passive or active filter circuits
well known in the art for deriving a desired harmonic or group of
harmonics from an electrical signal having a fundamental frequency.
Such circuits may consist of various combinations of resistance,
inductance and capacitance arranged to permit only a desired signal
to be transmitted, or they may be more complicated
frequency-independent active networks.
With the embodiment of this invention in FIG. 3 the driver 52 may
be supplied with an electrical signal causing it to produce a
magnetic field varying in accordance with a harmonic component of
the fundamental vibration of the string 42. The effect under these
circumstances is to cause the string to produce a musical sound
much richer in that harmonic than it would be under normal
conditions.
In still another form of the invention another means of emphasizing
the harmonic content of the signal supplied to the driver 52 is
shown. In this embodiment, the output of the amplifier 50 is
supplied to a first rectifier 58 poled either in one direction or a
second rectifier 60 poled in the opposite direction, or directly to
the driver 52 via a terminal 62. A four position switch 64 is
provided to be disposed in an off position or to selectively
connect the output of the amplifier through the rectifier 58,
terminal 62 or rectifier 60 to the driver 52.
When the switch 64 is in the off position as shown, the feedback
loop is interrupted and the instrument is played in the
conventional manner. As described previously, when the switch 64 is
connected to the terminal 62 and there is little or no phase shift
or waveform distortion in the circuit the effect of the feedback is
to sustain the vibration of the string so as to produce the
fundamental note.
If the switch 64 is connected to the diode 58, the positive half
cycles of the amplifier output are supplied to the driver 52 and
the effect has been found to produce a vibration of the string
wherein the even-numbered harmonics predominate or appear to a
greater extent. On the other hand, when the switch is connected to
the rectifier 60 it has been found that the odd-numbered harmonic
content is increased.
The effects produced by the embodiments of FIGS. 3 and 4 can be
combined in the embodiment of FIG. 5. In this embodiment, the loop
includes the harmonic selector 56 as well as the oppositely poled
diodes 58 and 60. Thus, the harmonic selector may be controlled to
pass desired harmonic components while the diode is chosen by
operation of the switch 64 to emphasize either the odd- or
even-numbered harmonics or both.
FIG. 6 shows the pickups connected through a clipper 65 to the
amplifier. The clipper turns the sine wave into a square wave and
is thus a further method of enhancing the harmonic content of the
signal. It also acts as a limiter to the amplitude of the string's
vibration under feedback.
FIG. 7 illustrates an embodiment of the invention as applied to a
guitar and which has proven satisfactory in use. The mode switches
36, the harmonic switches 34, along with the diodes 58 and 60, the
driver coils 156 and 158 and their cores 152 and 154 are duplicated
six times, for each of the six strings. In this embodiment, the
pickup 48 is a differential pickup of a type well known in the art,
and is constituted by a magnetized core 70 and a non-magnetized
iron core 72, while on the cores 70 and 72 are a pair of coils 74
and 76, respectively. The magnetized core 70 is adjacent to the
strings, and transduces their vibration into an electrical signal,
while the non-magnetized core 72 does not respond to the string's
vibration. However, both coils respond to ambient magnetic fields
identically. The coils are wound oppositely, so their difference
signal is taken substantially cancelling ambient magnetic fields,
while retaining the signal from the vibrating string. In this way,
direct pickup of the field from the driver coils is minimized,
helping to eliminate unwanted feedback through the magnetic field
in the air, an effect which can cause a very unpleasant "howling,"
analogous to that produced when a microphone is placed too close to
a loudspeaker in a public address system. Further measures taken to
eliminate feedback in the air loop are use of magnetic shielding 28
about the differential pickup 26, and the enclosure of the driver
coils in an iron shielding box 160. Both of these are explained
below.
Another pickup comprising a core 78 and coil 80 is also provided.
The pickup constituted by the cores 70, 72 and coils 74 and 76 is
denominated as the rear pickup and is that designated by reference
numeral 26 in FIG. 1. The pickup constituted by the core 78 and
coil 80 is denominated the forward pickup and is that designated by
the reference numeral 14 in FIG. 1.
The output of the forward pickup is supplied to a potentiometer 82,
the adjustment of which is controlled by the forward pickup volume
control knob 20 of FIG. 1. The output of the potentiometer derived
by the wiper 84 is connected to a tone control comprising a
potentiometer 86, the wiper 88 of which is connected to one end of
a capacitor 90, the other end of which is grounded. The wiper 88 is
controlled by the tone control knob 22 in FIG. 1. The output of the
rear pickup is supplied to a potentiometer 92 having a wiper 94
which connects its output to a potentiometer 96 connected in turn
through its wiper 98 to a grounded capacitor 100.
By adjustment of the wipers 84 and 94 the volume or amplitude of
the signals passed to the amplifier from the pickups may be
controlled. At the same time the wipers of the tone control
potentiometers may be adjusted to determine the frequency content
of the signal transmitted. The potentiometers 86 and 96, together
with their respective capacitors 90 and 100, constitute what are
called loser type tone controls, since they vary the amount of
treble, or high frequencies, dumped through the capacitor to
ground. Since the forward pickup is situated near the vibrational
antinodes of the fundamental frequency of vibration of the string,
the output of the forward pickup tends more toward the lower, or
bass end of the musical spectrum. Conversely, the rear pickup is
situated near the vibrational antinodes of the higher harmonics,
and tends more toward the treble end of the spectrum. Thus, by
suitable combinations of adjustments of the forward and rear
pickups' volume and tone controls, a wide range of tones, i.e.,
harmonic balance, can be obtained.
The output of the pickups 14 and 26 is supplied via shielded
conductors 102 and 106 to terminals 104 and 108, respectively, of
the changeover switch 40.
The center terminal 110 of this switch may be connected to either
the terminal 104 or the terminal 108 or both of them to provide a
signal input to a feedback amplifier 112 and an audio amplifier
114. The audio amplifier has its input, consisting of the signals
from the forward pickup, rear pickup, or from both connected to a
loud speaker system indicated by the reference numeral 116. The
signal is supplied to the amplifiers via output jack 24 and split
via shielded cables 111a and 111b.
The output of the feedback amplifier 112 is supplied via a
conductor 118 and input jack 32 to a terminal 120. As shown, the
conductor 118 is shielded to minimize cross-talk and other
interference. The terminal 120 is connected to the master feedback
toggle switch 38 which connects it in turn to a conductor 122 and a
voltage divider constituted by a pair of resistors 124 and 126. An
output is derived from the voltage divider at the junction of the
resistors 124 and 126 through a rectifier 128 and smoothed by a
capacitor 138 to ground to provide a gate control voltage to a
triac 130. The voltage output of the divider is held relatively
constant by a zener diode 132, the cathode of which is connected to
the cathode of the diode 128 while its anode is connected through a
resistor 134 to ground. The gate control voltage of the triac 130
is provided via a conductor 140 connected through resistor 141 to
the appropriate string 10. The strings are electrically isolated
from each other. The circuit to the gate of the triac 130 is
completed through the conductor 136 connected to the frets 6 only
when the appropriate string 10 is pressed into contact with the
frets 6 during the course of playing. Thus, in this mode of
operation, called the "melody" mode, the triac is made to conduct
and supply the feedback signal to the string via its driver coils
only when that string is fretted. In this mode of operation, a
string's vibration is fed back and sustained only if that string is
being fretted at the time; this mode eliminates unwanted feedback
of open strings, which may not be in key with the melody being
played. This mode occurs only when the mode switch for that string
is in the "melody" position, as explained below. An alternative
position of a mode switch 36 is the "drone" position, in which a
string sustains continuously whether it is open or fretted. This is
also explained below.
The appropriate one of the six mode toggle switches 36 connects the
conductor 122 to an off position 142 or a selected one of the
terminals 144 or 146. A conductor 148 connects the output of the
mode switching circuit to the oppositely poled diodes 58, 60 and
the terminal 62. The appropriate one of the toggle switches 34,
which may be termed the harmonic switches, connects the selected
one of the rectifiers 58 or 60 or terminal 62 via a conductor 150
to a driver constituted by a pair of cores 152 and 154 on which are
oppositely wound coils 156 and 158, respectively, connected in
series but with opposite polarities so as to produce oppositely
directed magnetic fields.
The operation of this embodiment of the invention is as follows.
The stringed instrument, in this case a guitar, is played by
plucking the strings. The particular manner in which it is to be
played is determined by the musician by setting the switches in the
position in which he desires them and by adjusting the volume and
tone controls also as desired. Thus, if it is assumed that it is
desired to play the instrument wherein one of the strings performs
the function of a drone, that is, produces a sustained note while a
melody is played on the other strings, the toggle switch 38 is
closed while the mode switch 36 for that string is moved to contact
the terminal 144. Assume further that it is desired that the drone
sound be primarily produced by the fundamental or lowest frequency
components of the vibration of the string. The harmonic switch 34
is then moved to contact terminal 62, while the changeover switch
40 may be moved to contact the terminal 104. In addition, the tone
control is adjusted by rotating the knob 22 so as to move the wiper
88 along the potentiometer 86 towards the bass end of the tonal
range. As the string is plucked, it vibrates generating an
electrical signal in the forward pickup which is derived by the
wiper 84 and supplied through the terminal 104 and switch 40 to the
amplifier 112. The output of the amplifier 112 is supplied through
the conductor 118, terminal 120, switch 38, conductor 122, switch
36, terminal 144, conductor 148, terminal 62 of switch 34 and
conductor 150 to the driver coils 156 and 158. The signal in the
driver coils is effective to create an oscillating magnetic field
which exerts a driving force on the string to sustain its
vibration. The particular arrangement just described is one wherein
the string will tend to be driven by the lowest harmonic component,
i.e., the fundamental, so as to produce a pure drone sound.
If it is desired to utilize the instrument wherein the feedback
arrangement is effective to permit a string to play melody notes
only when that string is fretted, the mode selector switch 36 is
moved to the terminal 146. Since these melodic notes are generally
more pleasing when very rich in the higher overtones, the switch 40
may be moved to the terminal 108 and the tone control 18 adjusted
to emphasize the treble contents of the sound. In addition, the
harmonic switch 34 is moved to contact either rectifier 58,
rectifier 60 or terminal 62, producing, in conjunction with the
treble tone setting, even harmonics, odd harmonics, or both. In
this way, all the previously described richness of sound is
achieved. In this "melody" mode, when the instrument is played, the
output of the amplifier derived through the conductor 122 is
supplied to the triac 130 which will be rendered conductive when a
string 10 is pressed against a fret 6 causing the appropriate
voltage to appear at the gate of the triac whereby it will conduct
and will remain conducting for as long as the string is held in
contact with the fret, it being the nature of such devices to
conduct as long as an anode is positive with respect to a cathode.
Thus, in the "melody" position, the string will be driven only when
it is fretted in the course of playing a melody, assuring that the
open string does not sustain out of key.
Obviously, the arrangement shown in FIG. 7 permits a musician a
wide variety of choices as to how the instrument can be played for
the permutations and the combinations of the various selectively
controllable switches and resistors is very large. The sounds that
can be created are varied and almost numberless. Basically, the
arrangement provides for an electromechanical feedback to the
vibrating string permitting it to produce sound which are rich in
harmonic content and may be sustained or terminated as the musician
desires.
In order to insure that the sounds produced are those desired by
the musician, magnetic "howling", that is, feedback in the air loop
caused by direct pickup of flux from the drivers by the pickups,
must be suppressed. It is thus necessary to avoid the effect of
stray flux from a driver acting on a pickup coil. This objective is
achieved by a combination of measures. First, the driver and pickup
coils are spaced from each other so as to lessen the direct
magnetic coupling between them. Secondly, within the rear pickup 26
the coils 74 and 76 are wound so that any signal produced by stray
flux from the drivers is cancelled out and the signal represents
solely the effect of the vibrating string, as previously described.
In addition, magnetic shielding 28 is provided for the rear pickup
26 consisting of layers of high and low permability materials. As
may be seen in FIG. 1, the drivers 28 and 30 are spaced equidistant
from the forward pickup 14 and their coils are wound in opposite
senses as shown in FIG. 7 so that the field they exert at the locus
of the forward pickup 14 is substantially zero.
Further shielding is obtained as shown in FIG. 8 wherein the
drivers are mounted in a shield box 160 formed of magnetic ingot
iron. A plurality of wells 162 are provided in the iron shielding
box and in each well is mounted a driver coil 166, the core 164 of
which is mated to the bottom of the well. The upper face 168 of the
core is provided with a concave figure so as to focus or
concentrate the flux generated on a string 170. Suitable electrical
connections are provided through the bottom of the shielded box so
that the coils 166 may be connected to the feedback amplifier as
desired.
As stated above, the invention may be used in conjunction with a
wide variety of stringed instruments and when provided with the
selectively operated mode control indicated will permit the
instrument to be played in a large variety of keys. The location of
the pickups and the drivers on the instrument is determined by its
physical configuration and therefore the available space. The
positioning of the elements is also determined by the desire to
eliminate or minimize any air-loop feedback between the drivers and
the pickups.
Obviously, numerous variations and modifications beyond those
discussed can be made while still remaining within the scope of the
invention and it is intended by the claims to cover all such
modifications and variations which fall within that scope.
* * * * *