US 3742113 A
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.
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Description (OCR text may contain errors)
United States Patent [191 Cohen 1 June 26, 1973 STRINGED MUSICAL INSTRUMENT WITH ELECTRICAL FEEDBACK  Inventor: Marcus S. Cohen, 134 Woodland Avenue, Winnetka, 111. 60093  Filed: Apr. 28, 1971  Appl. No.: 138,213
 U.S. Cl 84/1.04, 84/l.06, 84/1.16, 84/1.15, 84/1.19  Int. Cl. G0lh 3/00  Field of Search 84/1.l3-1.16, 1.26, 84/1.06, 1.19,267, 1.04; 323/22  References Cited UNITED STATES PATENTS R26,533 3/1969 Cookerly et a1. 84/1.l5 X 1,838,886 12/1931 Tuininga 84/1.15 2,119,584 6/1938 Knoblaugh... 84/1.l5 2,896,491 7/1959 Lover 84/l.l5 3,177,283 4/1965 Fender ..84/1.15 X 3,291,888 12/1966 Meazz et a1 84/1.16 3,325,580 6/1967 Barcus et a1. 84/1.16 3,456,063 7/1969 Teranishi et al.... 84/1.16 3,493,669 2/1970 Elbrecht et a1. 84/1.16 2,784,631 3/1957 Fender 84/l.15 2,600,870 6/1952 Hathaway et a1. 84/1.l6 X 3,185,755 5/1965 Williams et a1, 84/l.16 X 3,551,787 12/1970 Walters 323/22 FOREIGN PATENTS OR APPLICATIONS 673,375 6/1930 France 84/l.l5
Primary Examiner-Richard B. Wilkinson Assistant Examiner-U. Weldon AtlomeyMcDougal1, Hersh & Scott  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 strings 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.
11 Claims, 8 Drawing Figures PATENTEDJUNZS 191s 3; 742.1 13
sum 1 or a FIG, 1 4
INVENI'OR 4 977201712215 Sg'oherz, dziys STRINGED MUSICAL INSTRUMENT WITH ELECTRICAL FEEDBACK 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 havecharacteristic 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 awide 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 embodi ment 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 controlthe 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 thebody 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 spacedfrom 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 thatof 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. I
Referring now to FIG.2, there maybe 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 ofa stringed instrument. t
In this configuration a steel string 42 extends between two fixed points 44 and 46 and is under tension I sothat it will vibrate to produce a musical sound when plucked or hit. Whether the string is in a guitar, piano, harpsichord orthe 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 ofa coil wound on an iron core and when excited by the amplifier 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 cmploying an amplifier with zero phase shift, the driving force is in phase with the strings 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-v 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 thedriver 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 toselectively connect the output of the amplifier throughthe rectifier 58, terminal 62 or rectifier 60 to the driver'52.
When the switch64 isin 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 waveformdi se tortion in the circuit the effect of the feedback is to sustain the vibrationof the string so as to produce .the fundamental note.
if the switch 64 is connected to the diode 53, 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 sclector 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 oddor 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 strings vibration under feedback. 2
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 strings 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 lllb.
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 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 strings 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, conmeets 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 l4 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 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 1,70. Suit able 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 airloop 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.
What is claimed as new and desired to be secured as Letters Patent is:
l. 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 elec trical 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 meansv 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 waveshaping 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.