U.S. patent number 3,733,953 [Application Number 05/213,935] was granted by the patent office on 1973-05-22 for stringed musical instrument with optoelectronic pickup sound amplifier.
Invention is credited to Dennis A. Ferber.
United States Patent |
3,733,953 |
Ferber |
May 22, 1973 |
STRINGED MUSICAL INSTRUMENT WITH OPTOELECTRONIC PICKUP SOUND
AMPLIFIER
Abstract
A stringed instrument, such as an acoustical guitar, is provided
with a compact, battery powered sound amplification unit attached
thereto in an arrangement including cooperating optoelectronic
devices situated adjacent the instrument's strings. Each string
intersects with a path of a light beam sent from a first
light-emitting device toward a second light-detecting device so
that vibration of a string modulates the intensity of light from
the first device impinging on the second device to produce an
electronic signal corresponding to the musical tone associated with
any particular string vibration rate. The unit can be affixed to an
acoustical guitar and utilized to drive a speaker or equivalent
acoustic transducer situated within the sound box of the guitar so
as to cause the transducer to produce amplified musical tones
corresponding to the musical notes played on the strings of the
guitar. The source of electric power for operating this device
shall be situated on or within the guitar to provide a
self-contained device that is not dependent on a source of domestic
electric power.
Inventors: |
Ferber; Dennis A. (Los
Alamitos, CA) |
Family
ID: |
22797101 |
Appl.
No.: |
05/213,935 |
Filed: |
December 30, 1971 |
Current U.S.
Class: |
84/724; 984/368;
84/743 |
Current CPC
Class: |
G10H
3/181 (20130101); G01H 9/00 (20130101) |
Current International
Class: |
G10H
3/18 (20060101); G10H 3/00 (20060101); G01H
9/00 (20060101); G10h 003/00 () |
Field of
Search: |
;84/1.14-1.16,1.04,1.06,1.18,DIG.19 ;317/235N |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
F W. Gutzwiller, "G. E., SCR Manual" Page 289, 1967, G. E.,
Electronics Park, Syracuse 1, N.Y..
|
Primary Examiner: Wilkinson; Richard B.
Assistant Examiner: Weldon; U.
Claims
I claim:
1. In combination with a musical instrument having a sound box in
which an opening is formed over which a plurality of parallel,
laterally spaced, tensioned strings extend that are formed from a
material that is less light conductive than air, an assembly
operatively associated with said musical instrument for amplifying
the sound from said strings when the latter are vibrated, said
assembly including:
a. electrically operated, sound reproduction means;
b. a source of electric power;
c. electric amplifier circuit means that connects said sound
reproduction means and said source of electric power; and
d. a plurality of electrically operated light emitting and light
sensing means located on opposite sides of said strings and
connected to said electric amplifier circuit means, with said
strings so situated as to at least partially obstruct the beams of
light between said light emitting and light sensing means, said
strings when vibrating cooperating with said light emitting means
to subject said light sensing means to a plurality of spaced pulses
of light of the same frequencies as those at which said strings
vibrate, and said light sensing means imparting electric signals to
said electric amplifier circuit means to cause said sound
reproduction means to reproduce the sounds of said vibrating
springs at an amplified level, with said light sensing means
including:
1. first and second NPN junctions that each includes a base,
collector and emitter;
2. first, second and third resistors;
3. a first plurality of conductors that connect the base of said
first NPN junction to said first resistor, the emitter of said
second NPN junction to said second resistor, the collector of said
second emitter to said third resistor, the collector of said first
NPN junction to the collector of said second PN junction, and the
emitter of said first NPN junction to the base of said second NPN
junction;
4. a second conduit that connects said first and second resistors
to the ground;
5. a third conductor connected to said third resistor, with said
second and third conductors connected to said source of power;
and
6. a fourth conductor connected to said collector of said second
NPN junction, and said signal being emitted by said third and
fourth conductors.
2. An assembly as defined in claim 1 in which said instrument is an
acoustic guitar, and with said sound reproduction means so
positioned within said sound box as to direct the reproduced sounds
of said strings away from said opening, and said sound box acting
as a resonator to further amplify the reproduced amplified sounds
of said strings from said sound reproduction means.
3. An assembly as defined in claim 2 in which said sound
reproduction means is an acoustic transducer.
4. An assembly as defined in claim 2 in which said sound
reproduction means is a dynamic speaker.
5. An assembly as defined in claim 1 which in addition
includes:
e. means for effecting relative lateral adjustment between said
strings and said light emitting and light sensing means whereby
said strings obstruct at least portions of said beams of light
prior to the latter impinging on said light sensing means.
6. An assembly as defined in claim 1 in which said musical
instrument includes a bridge over which said tensioned strings
extend, and said assembly in addition including:
e. a plurality of laterally adjustable supports mounted on said
sound box adjacent said bridge, with each of said supports having
one of said light emitting means and light sensing means mounted in
spaced relationship thereon, and each of said supports being
movable relative to one of said strings to a position where said
string obstructs at least a portion of the beam of light from said
light emitting means when said string is stationary.
7. An assembly as defined in claim 1 in which said source of
electric power is at least one battery removably situated at a
fixed position relative to said sound box.
Description
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to an optoelectronic unit utilizable
with stringed musical instruments and responsive to string
vibrations when the instrument is played in conventional
fashion.
The present invention relates also to a sound amplifier system
designed so that all of its component parts are situated within a
portable stringed instrument such as an acoustical guitar.
The present invention in a detailed sense relates to a pickup
device including cooperating optoelectronic devices, electrical
amplifier circuitry, and associated speaker in combination with the
strings of a musical instrument in an arrangement wherein string
vibrations are sensed (picked up) and transduced into corresponding
electrical signals that are, in turn, transduced into corresponding
audible tones after having been electronically amplified.
Transducer pickups are utilized for transducing sounds produced in
playing stringed musical instruments into corresponding electronic
signals that can be electronically amplified and fed to a
loudspeaker to cause it to produce audible sounds that can be heard
from great distances. Modernly, pizeoelectric transducers have
found popular usage with such stringed instruments as the electric
guitar and the bass fiddle. Pizeoelectric and other kinds of
transducer pickups cannot afford precisely the same results and
advantages that are afforded by the optoelectronic transducer
pickup of the present invention, nor are the former utilizable with
all of the various kinds of stringed instruments with which the
latter may be utilized.
The modern electric guitar utilizes a pizeoelectric transducer
arrangement connected by electrical wires in a cord to a separate
amplifier, power source and loudspeaker unit. Such a guitar differs
from an acoustical guitar which, unlike the former, has a sound box
defined in the body thereof for providing acoustical amplification,
via a sound opening in the body, of the musical tones or sounds
actually produced by plucking or strumming the guitar strings. In
contrast to the foregoing, an actual embodiment of the present
invention can be utilized in conjunction with an acoustical guitar,
wherein the acoustic transducer makes advantageous use of the
soundbox, with all of the components thereof -- optoelectronic
transducer pickup arrangement, amplifier system, and power system
-- contained in a detachable unit secured to the acoustical guitar
forming a self-contained unit.
OBJECTS OF THE INVENTION
It is accordingly an object of the present invention to provide a
compact pickup and amplifier unit, small enough to be affixed to a
stringed musical instrument such as an acoustical or electrical
guitar.
It is another object of the present invention to provide an
optoelectronic transducer pickup system adapted for utilization
with stringed musical instruments to produce optoelectronically
generated electronic signals corresponding in frequency to the
frequency of the musical tones produced by effecting vibration of
the instrument's strings at various rates. It is another object of
the present invention to provide a battery powered unit of the kind
herein described that is designed to allow for the mechanical
adjustment of the relative positions of the instrument's strings in
relation to its optoelectronic devices so that the unit can be
utilized in conjunction with different, but similar, stringed
musical instruments having different spacing between strings and
other physical differences.
It is another object of the invention to provide an acoustical
guitar or like instrument with an acoustic transducer in the sound
box thereof and with a composite optoelectronic transducer, power
and amplifier unit affixed to the outside front face of the guitar
body near the bridge of the guitar.
These and other further and related objects, advantages, and
features of the present invention may be made apparent in light of
the following illustrative description of the present invention
taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a six-string acoustical guitar
provided with an acoustic transducer situated inside the sound box
of the guitar body and an optoelectronic transducer/amplifier/power
unit removably attached to the outside front face of the body over
the guitar bridge at one end of the guitar strings;
FIG. 2 is a view taken generally along line 2--2 of FIG. 1
illustrating an arrangement for photoelectrically sensing
vibrations of any guitar string so as to produce corresponding
electronic signals that are amplified and then fed to the
aforementioned loudspeaker;
FIG. 3 is a view taken along line 3--3 of FIG. 2 showing a single
guitar string in relation to a light source manifold and a
photodetector and other elements;
FIG. 4 is a plan view taken along line 4--4 of FIG. 2 of a
longitudinally slotted bar-shaped member and two threaded bolts.
This Figure, together with FIGS. 2 and 3, is illustrative of one
way in which the optoelectronic transducers for each string may be
arranged relative thereto.
FIGS. 5 and 6 are illustrative views taken orthogonal to each
other, showing an acoustic transducer mounted inside the sound box
of the guitar and located rearwardly of the guitar's sound opening,
with FIG. 5 being taken along line 5--5 of FIG. 1;
FIG. 7 is an electrical schematic of circuitry for
photoelectrically sensing string vibrations so as to generate
corresponding electronic signals, amplifying such signals, and
feeding the amplified signals to an acoustic transducer; and
FIG. 8 is an electrical schematic of circuitry sensitive to light
of variable intensity.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring initially to FIG. 1 there is shown an acoustical guitar
10 which is representative of various stringed musical instruments
with which device arrangements embodying the present invention may
be utilized, and which has six strings. The guitar 10 is
conventional and includes a sound opening 12 in the front face of
its main portion or body 14 which is hollow between its front and
back faces so as to define therein a sound box. The guitar 10, in
accordance with the present invention, includes a unit 16 that
contains all of the electrical components and circuitry depicted in
FIG. 7 with the exception of the sound reproduction device that may
be either a dynamic speaker or acoustic transducer which is
preferably mounted within the sound box of the guitar body 14.
The unit 16, as illustrated, includes a casing 18 which, as will be
explained in greater detail hereinafter, encloses six sets of
optoelectronic devices, amplifier circuitry, and a battery power
supply. The casing 18 has four side walls forming an enclosure and
is open at its frontmost side and its bottom side which is next to
the front face of the guitar body 14. There is a slidable switch
button 20 extending from the rearmost side wall of the unit which
can be actuated when desired to either turn on or turn off the
supply of battery power to the circuitry components disposed within
the unit. Two opposite side walls of the casing straddle the guitar
bridge, to which one end of each of the six strings is secured. The
casing 18 is secured to the body front face in a suitable fashion,
as by means of a machine screw 22 threaded into a threaded hole in
the top casing wall and extending downwardly between the two middle
guitar strings in a threaded hole in the body front face so as to
secure the casing to the guitar body in such a way that the casing
can be detached readily from the guitar body when desired.
Located within the casing 18 is a mechanical support assembly for
six sets of optoelectronic devices which can be seen in FIGS. 2 and
3. The support assembly includes two bar-shaped frame members 24
and 26 held in spaced relation by machine screws extending
therebetween. Attached to the topmost frame member 24 and suspended
therefrom are six separate support members 28, 30, 32, 34, 36, and
38 that are each generally C-shaped when seen from the view shown
in FIG. 2. Each of the support members 28 to 38 are suspended from
the frame member 24 in the same way and in a manner that is shown
at the left in FIG. 2, wherein a partial cross section is taken
through the support member 28 in order to show how it is suspended
from the frame member 24. The support member 28, as are the other
support members, is suspended in place by the utilization of a
first machine screw 39 with a slotted head 40 that passes through a
longitudinal slot 42 (partially shown in FIG. 4) into a threaded
hole 44 in the support member 28. By loosening the screw 39 with a
screwdriver it is possible to loosen the support member 28 from the
frame member 24 so that the support member 28 and frame member 24
no longer securely abut one another, to allow the support member to
be moved parallel to the longitudinal direction of the frame member
24. In this way the support member 28 may be repositioned relative
to string S1 which extends between top and bottom ends of the
support member 28 in a direction normal to an imaginary line drawn
between the support member ends. This feature, just described,
allows the support member to be accurately positioned relative to
the string S1 and also adapts the support member and frame member
arrangement for utilization with different guitars and other
stringed instruments wherein the spacing of the strings of the
different instruments may vary to some extent. The other support
members 30, 32 34, 36, and 38 are suspended from the frame member
24 in the same way that support member 28 is suspended by the
utilization of respective machine screws 46, 48, 50, 52, and
54.
The frame members 24 and 26 are held in spaced parallel relation by
means of two frame machine screws 56 and 58 at opposite ends of the
frame members that are each threaded into corresponding threaded
holds in the frame members 24 and 26. A first pair of threaded nuts
60 and 62 and a second pair of threaded nuts 64 and 66 are utilized
to determine the spacing of frame members 24 and 26. The nuts 60
and 62 are threaded onto the frame machine screw 56 on opposite
sides of the frame member 24 and the nuts 64 and 66 are threaded
onto the frame machine screw 58 on opposite sides of the frame
member 24 so that by tightening the nuts against the frame member
24 it is possible to maintain the frame member 24 at any desired
spacing from frame member 26. FIG. 4 is a plan view of the left end
of the frame member 24 showing, for further illustrative purposes,
the arrangement of the frame machine screw 56, threaded nut 60, and
the heads 40 and 68 of the machine screws 39 and 46 relative to the
frame member slot 42.
Referring further to FIG. 2, electrically operated light emitters
D1, D2, D3, D4, D5, and D6 are secured to the top ends of the
support members 28 and 38 and light detecting sensors D7, D8, D9,
D10, D11, and D12 are secured to the bottom ends of the support
members 28 to 38 so that the light emitter D1 faces the light
sensor photodiode D7, the emitter D4 faces the sensor D10, the
emitter D5 faces the sensor D11, and the emitter D6 faces the
sensor D12. The light emitter and light sensors D1 to D12 are
optoelectronic devices that function in a manner to be explained
hereinafter. The emitters D1 to D6 are respectively aligned with
the sensors D7 to D12 along respective axes that are transverse to
the guitar strings S1 and other guitar strings S2, S3, S4, S5, and
S6 so that light rays emitted by each of the emitters D1 through D6
form respective light beams that impinge respectively on the
sensors D7 through D12. The strings S1 through S6, being
respectively located between the emitters D1 through D6 and the
sensors D7 through D12, intersect with the respective light beams
to control the amount of light energy from the emitters D1 through
D6 reaching the respective sensors D7 through D12 in accordance
with string vibrations produced when the strings are plucked or
strummed during the playing of the guitar.
When a musical note is played on any of the guitar strings, the
string vibrates at a rate or frequency depending upon the
particular note played. Each vibrating string moves sideways back
and forth in directions generally normal to the light beam passing
between the associated emitter and sensor so as to chop the light
beam at the same rate as the string vibration rate. The chopping of
any light beam produces a signal voltage in the particular
photodiode illuminated by the light beam, since the voltage
produced in a sensor by impinging light is proportional to the
light energy contained in the light ray and such voltage signal is
of a frequency corresponding to the vibration rate or frequency of
the associated string. The voltage signals produced in the
foregoing manner are electronically amplified, as will be
explained, and fed to a sound reproducing device, earlier
mentioned, mounted on or inside the guitar body, the device
broadcasting the musical tones produced when musical notes are
played on the guitar strings.
The optoelectronic devices in pairs D1-D7, D2-D8, D3-D9, D4-D10,
D5-D11, and D6-D12 respectively function as optoelectronic
transducer pickups in that each mentioned pair (e.g., D1-D7) serve
to transduce the optical energy of a light beam into an electronic
signal. These devices, together with the guitar strings S1 through
S6 each constitute a mechanoelectronic or mechano-optoelectronic
transducer in that each arrangement of diode, photodiode and string
(e.g., D1, D7 and S1) transduce the mechanical motion of a guitar
string into a corresponding electronic signal.
Referring now to FIG. 3, there is shown an elevational view taken
along line 3--3 of FIG. 2 of the emitter D6 and sensor D12 on the
support member 38 as arranged relative to the string S6. The
support member 38, as stated earlier, is suspended between the
frame members 24 and 26 (which are shown in cross section), the
frame member 26 abutting the outside front face of the guitar body
therebeneath. The string S6 extends to the right as shown in FIG. 3
into a vertical groove of a guitar bridge GB to an anchoring
element A situated to the right of the guitar bridge.
A case C secured to the guitar bridge GB defines an enclosed space
V outline by dashed lines, into which the battery B1 can be
inserted. Attached to the frame member 24 is a printed circuit
board (PCB), with printed circuit conductors formed on its top and
bottom faces. Electrical lead wires interconnect the emitter D6 and
the sensor D12 to electrical conductors on the printed circuit
board PCB. Disposed atop the printed circuit board PCB are the
circuit components of the circuit of FIG. 7, these components being
represented in FIG. 3 by the components C1 and C2. The circuit
components are interconnected by wires to the printed circuit
conductors and also to battery output terminals (not shown)
extending from the battery case C.
Referring now to FIGS. 5 and 6, there is shown in each a
conventional acoustic transducer VC1 mounted inside the guitar body
14. A dynamic speaker may be used in lieu of the transducer if
desired. In FIG. 5 a plan view is shown of the rear end portion of
the guitar body 14, corresponding to that portion to the right of
the sound opening 12 in FIG. 1, with the front face of the body
taken away. The transducer VC1 is shown mounted on one of two
internal guitar frame members 70 that transversely extend inside
the body of the guitar. These members 70, together with the front
wall 14a, back wall 14b and side wall 14c of guitar body 14, formed
an enclosed space 72 into which the acoustic transducer portion 73
may broadcast. The broadcasting takes place by moving air in the
confined space to take advantage of the acoustical resonance and
acoustical amplification that is inherent to the guitar body 14
when the guitar strings are plucked or strummed.
Referring now to the circuitry shown in FIG. 7, the circuitry
includes optoelectronic transducer circuitry 76 electrically
coupled to a preamplifier circuit 78 that in turn is electrically
coupled to a power amplifier circuit Q1 which has its output
terminals connected to the loudspeaker VC1. The circuitry has a
direct current power source B1, preferably a battery inside the
guitar body 14, that supplies direct current thereto when a switch
W1, operated by the switch button 20 (see FIG. 1), is closed.
The circuit components shown in FIG. 7 are labelled for
identification, and are identified and rated as follows: D1 through
D6 are all light emitters; D7 through D12 are all light sensors;
D13 through D18 are 1N67 rectifying diodes; R1 through R25 are
ohmic resistors; R1 and R2 are 0.50 watt 330 ohm resistors; R3, R5,
R7, R9, R11, and R13 are 0.25 watt 100,000 ohm resistors; R4, R6,
R8, R10, R12, and R14 are 0.25 watt 1,500,000 ohm resistors; R15 is
a 0.25 watt 47,000 ohm resistor; R16 and R17 are 0.25 watt
3,300,000 ohm resistors; R18 is a 0.25 watt 10,000 ohm resistor;
R19 is a 500,000 ohm potentiometer; R20 is a 0.25 watt 500,000 ohm
resistor; R21 is a 0.25 watt 56,000 ohm resistor; R22 is a 0.25
watt 18,000 ohm resistor; R23 is a 0.25 watt 330,000 ohm resistor;
R24 is a 0.25 watt 56,000 ohm resistor; R25 is a 0.25 watt 5,600
ohm resistor; C1 through C8 are capacitors rated in microfarads as
follows: C1 is 0.005 microfarads, C2 is 2.2, C3 is 47, C4 is 1,000,
C5 is 30, C6 is 500, C7 is 4.7 and C8 is 0.002 microfarads. B1 is a
battery supply consisting of 10 series-connected nickel cadmium
battery cells. Q1 is a General Electric integrated circuit (PA 237)
power amplifier circuit. VC1 is either an acoustic transducer or a
dynamic speaker with an 8 ohm voice coil. Q2 is an NPN
emitter-base-collector transistor.
The light emitters D1, D2, and D3 are connected in series with the
resistor R1, and the light emitters D4, D5, and D6 are connected in
series with the resistor R2. The emitters D1, D2 and D3 and the
resistor R1 are connected in parallel with the emitters D4, D5, and
D6 and the resistor R2. The emitters D1 through D6 are all poled in
the same direction so that each is normally forward biased during
circuit operation so as to each emit a light beam. The light
sensors D7 through D12 are connected in respective series circuits
that are parallel to one another and also in series with the
resistor R15 that is itself in parallel with the capacitor C1.
The resistor R3 and the diode D13 are connected in series with the
photodiode D7. The resistor R5 and the diode D14 are connected in
series with the photodiode D8. The resistor R7 and the diode D15
are connected in series with the photodiode D9. The resistor R9 and
the diode D16 are connected in series with the photodiode D10. The
resistor R11 and the diode D17 are connected in series with the
photodiode D11. The resistor R13 and the diode D18 are connected in
series with the photodiode D12.
The light sensors and diodes D7 through D18 are all poled in the
same direction so as to be normally forward biased during circuit
operation. The resistors R4, R6, R8, R10, R12, and R14 are
respectively connected in series with the sensors D7 through D12.
The sensors D7 through D12 are respectively illuminated by the
light rays coming from the emitters D1 through D6. Each illuminated
sensor has a voltage induced therein by the light rays impinging
thereon.
Sensor D7 and resistor R4, sensor D8 and resistor R6, sensor D9 and
resistor R8, sensor D10 and resistor R10, sensor D11 and resistor
R12, and sensor D12 and resistor R14, respectively, are connected
in parallel with the series circuit including D1, D2 and D3 and
resistor R1; and also with the series circuit including diodes D4,
D5, and D6 and resistor R2. The light emitters D1 through D6, and
light sensors D7 through D12, resistors R1 through R15 and the
capacitor C1 are connected together to form the optoelectronic
transducer pickup circuit 76.
The preamplifier circuit 78 includes the bias resistors R16, R17
and R18 and the NPN emitter-base-collector transistor Q2. The
resistor R18, connected to the emitter of the transistor Q2, serves
as an emitter follower resistor, the resistor R16 being connected
in parallel with the base and collector of the transistor Q2 serves
to reverse bias the base-collector transistor junction when the
switch W1 is closed. The resistor R17 is connected in parallel with
the base and emitter of the transistor Q2 and the resistor R18
serves as a base resistor for applying a signal voltage to the
transistor emitter-base junction to modulate the forward bias
thereof when a signal voltage at the cathodes of the diodes D13
through D18 is coupled thereto by the capacitor C2. The capacitor
C2 serves as a signal coupling capacitor connected between the
output of the optoelectronic transducer pickup circuit 76 and the
base of the transistor Q2 of the preamplifier circuit 78. the
capacitor C4 connected in parallel with the battery B1 serves to
suppress oscillations that might otherwise occur, but for the
capacitor C4 in the preamplifier circuit.
The capacitor C3 and the potentiometer R19 are series connected in
parallel with the resistor R18 so that amplified alternating
current signal voltages impressed across the resistor R18,
resulting from the application of signal voltage across the
transistor Q2 emitter-base junction, are coupled by the capacitor
C3 to the potentiometer R19. The potentiometer R19 has a movable
tap terminal for pickup of a selected portion of any voltage
impressed on the resistance of the potentiometer R19. The position
of the potentiometer tap terminal is adjustable by turning a volume
control knob K (see FIG. 1) extending from the casing 18 of the
unit 16. The position of this tap terminal determines the volume of
sound that can issue from the loudspeaker VC1.
Resistors R20 and R21 are series connected, and together are
connected in parallel with the resistors R16 and R17. The capacitor
C5 is a coupling capacitor that is connected between the
potentiometer tap terminal and the serial junction of the resistor
R20 and resistor R21. The resistors R20 and R21 serve as a voltage
divider in circuit with the battery B1, and together function to
establish a bias voltage level at an input terminal T14 of the
power amplifier integrated circuit Q1.
The power amplifier circuit Q1 includes, in addition to the input
terminal T14, output terminals T3 and T5 and circuit biasing
terminals T8, T7 and T12. The coupling resistor R22 is connected in
series with the two loudspeaker terminals between the power
amplifier output terminals T3 and T5 to couple the output of the
power amplifier circuit to the loudspeaker VC1.
A biasing circuit network, including the resistors R23, R25 and R25
and the capacitors C6, C7 and C8, is interconnected with the power
amplifier circuit Q1 and the acoustic transducer VC1. The resistor
R23 is connected between the terminals T7 and T12; the resistor R24
is connected between the terminals T12 and T8; and the resistor R25
and the capacitor C7 are series connected across the resistor R24.
The capacitor C8 is connected between the terminals T3 and T7. The
capacitor C6 is connected between one end of the resistor R22 and
the terminal T7. The terminal T5 is connected to the collector of
the transistor Q2. Voltage signals applied across the resistor R21
are amplified by the power amplifier circuit Q2 to provide output
signals of amplified power at the terminals T3 and T5 that are
coupled to the acoustic transducer VC1 by the resistor R22 to drive
the transducer.
The acoustic transducer VC1, when driven, broadcasts the amplified
sound of the vibrating strings to the confined space 72 to be
further amplified as the guitar body 14 acts as a resonator.
From the above description it will be apparent that the strings S1
through S6 must be formed from a material that is less light
conductive than air in order that the vibrating strings will stop
the beams of light prior to the latter impinging on the light
sensors D6 to D12.
Light of variable intensity resulting from the vibration of the
strings S-1 to S-6 may be used to provide an electrical output that
is proportional thereto by the device shown in FIG. 8 that is
provided for each of the strings. Each of these devices Z includes
two NPN junctions 200 and 202. The NPN junctions 200 and 202 are
placed in an envelope 204 that is transparent to the extent that a
light beam from one of the light emitters D-1 to D-6 impinges on
the junctions 200 and 202. The PN junction formed in silicon, which
is the basic structural element of the semiconductor diode and
transistor, is inherently photosensitive. That is to say that such
a junction, when electrically stressed in the reverse polarity
passes a current which is dependent upon secondary sources of
energy such as heat or light. The greater the amount of light
energy irradiating such a junction the greater the current passing
through the junction. All PN junctions exhibit this phenomena in
varying degrees, and the processes required to enhance this
characteristic are clearly understood throughout the semiconductor
industry.
The base 200a, collector 200b, and emitter 200c of NPN junction 200
are connected by conductors 206, 208 and 210 to a resistor 212,
junction point 214 and base 202a of NPN junction 202. Collector
202b is connected to junction point 214. Emitter 202c is connected
by a conductor 216 to resistor 218. Resistors 212 and 218 are
connected to a conductor 220 that has a junction point 220a therein
that is connected by a conductor 222 to ground 224.
Junction point 214 is connected by a conductor 226 to a junction
point 228, which junction point is connected to a resistor 230.
Resistor 230 is connected to a conductor 232 that extends to a
junction point 234. Connector 220 terminates in a junction point
236. Power is supplied to junction points 234 and 236. The output
voltage that is determined by the intensity of the light beam is
taken off from conductor 220 and a conductor 240 connected to
junction point 228.
Recognizing the fact that any transistor is made up of two PN
junctions arranged in such a manner that they have an electrically
common area (i.e., the "base"), and that under normal operating
conditions one of these junctions is reverse polarized (e.g.,
base-to-collector) while the other is forward polarized (e.g.,
base-to-emitter), it may be recognized that the photo current is
generated within the base-to-collector junction and is passed
through the base-to-emitter junction in series with the load
circuit (i.e., collector-to-supply-to-emitter). Consequently, it is
feasible for the photo-current to forward polarize the
base-to-emitter junction without the use of external electrical
attachment.
In order to continue this description, it must first be clarified
that "forward" polarization of the base-to-emitter junction is
"normal" operation, but as is the case with the base to collector
junction, the base-to-emitter junction may continue to pass current
even when reverse polarized, due to ambient thermal energy. Also,
it should be noted that photo current must pass through the
base-to-emitter junction in order to be amplified by normal
transistor action.
Continuing, by connecting a resistor in the load circuit between
the emitter and the reference (i.e., ground) end of the supply, a
potential is developed across this resistor that is proportional to
the photo-current and is in phase with respect to changes in
illumination when measured with respect to the reference. By
connecting a second resistor between the base electrical attachment
and the reference (i.e., in parallel with the base-to-emitter
junction and emitter to reference resistance) a portion of the
photocurrent is diverted around the base-to-emitter junction. This
current gives rise to a potential of the same polarity and phase as
the potential across the emitter-to-reference resistor. The
difference between these two potentials is the base-to-emitter
junction polarization potential, and will be self-adjusting. That
is to day that the photo current will divide between the two
available paths in such proportions as to establish quiescent
equilibrium under all ambient conditions.
It should be clear at this point that connection of the resistance
between the base electrical attachment and the reference has given
rise to a biasing current that is directly proportional but
180.degree. out of phase with photo-current (i.e., illumination).
This technique of "negative" current feed-back has all of the
advantageous effects of negative feed-back as used in more
conventional circuitry. Specific areas of improvement include (1)
gain stabilization device-to-device as well as with changes in
ambient conditions, (2) extended bandwidth, (3) reduced distortion,
and (4) improved isolation of the collector-to-battery (i.e.,
output) circuit.
* * * * *