Patent Publication Number: US-3880039-A

Title: Sample and hold circuit for an electronic organ

Description:
United States Patent Studer [451 Apr. 29, 1975 l54l SAMPLE AND HOLD ClRCL&#39;lT FOR AN 3.733.955 5/1973 Rcinagcl ci nl. 84/l.0l ELECTRONIC ORGAN 3.776.087 l2/l973 84H24 3.78am |/i974 84mm [75l Inventor: Richard L. Studer. Villa Hills. Ky. R219@ 4/l974 Stearns 34Min [73] Assignee: D. H. Baldwin Company, Cincinnati.  
  ohio Primary Brandner-Stephen J. Tomsky .-tssismn! Emminer-Stanley J. Witkowski [22 Filed: Feb. 26. 1974 [2l] Appl. No.: 445,985 {57} ABSTRACT A sample and hold circuit for an electronic organ. in [52| L&#39;.S. CI. 8-l/l.0l; 84/DIG. 2; t-l/DIG. 8; which the voltage across a cascade of resistances. se-  
  84/DlG. l0: 84/DlG. 20 lected points of which may be caused to equal ground [5 l] Int. Cl Gl0h l/00: GlOh 5/00 by key switch controlled logic circuitry, is maintained [58] Field of Search 84/I .01. 1.03. L24. DIG. 2. at a fixed predetermined value by means ofa feedback 84/DIG. 8. DIG. l0. DIG. 2O circuit. the voltage output of which is used to control the frequency of a voltage controlled tone signal oscil- [56l References Cited lator. the value of the feedback voltage required being UNITED STATES PATENTS a function of the identity of the closed key switch 25681797 M951 Hand mima m which represents the highest note played. if plural key 1933.699 who BmhdmnY {s4/D162() switches are closed. The logic circuitryl selects both 3.233,90.; 1 |956 Georg.: n i4/1 0] the highest octave containing any selected switch. and 3.535.430 10/1970 Maynard.. 8f4/1.24 X provides a virtual ground physically ungrounded at a 3.538.804 l l/l970 George 84/1 .0l point ofthe cascade of resistances corresponding with 3.569.603 3/1971 Kern 84/DIG. l0 that closed switch of several which represents the 3.609.201 9/1971 Antichi.. #i4/1.0i highest me Pklyed, 3.609.203 9/1971 Adachi 84/l.()l 3.659.031 4/1972 Adachi P14/L0! i9 Claims. 2 Drawing Figures l&#39;&#34;&#34; &#34;&#34;T im im&#34; i 4 -1R39 m9@ n2; :24 22a im ezisnlmfmafnnsniamsLm Las @i2 zu am ne n1 Re Rs n4. Ra $4;  
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 SHEET 2 E? 2 N.HW muunom 53E E 1 I mfom 5&amp;5 1U: w m v .w.m n ./WJ w x/ Hn am Sw |J 1. .1 1- 1- 1521-511 INI I I.. J; k. .n 111: m m m m m m m m m m c. C.. c.. Q.. 9 a m u r r m 1 .1 H.1 1 l L z a @N uf xwm mma @ma E, n *1:91 1i mu 2m? mw .om :9162 n llllnllll||||l|||||l||ollalllillnsllnln. mkh@ utmrwx rl&#39;ll&#39; l &#39;Il&#39; l--. ll-ll&#34;lllll- II-&#39;II&#39;II&#34;-Illlllllllllll&#39;-&#34;lllllll.llllnl-|-l&#39;. l l l I I l I l l IIIIII&#39;IIJ ml om #om 10.539,5 .mi .$51535 Bmxwmn :i I lillwl l. l nl I. I. l. 1 I lill-1I Illlll I ll JWIHJWJH I J ..||.|.|l YJ. 1 1 1 1 1 z .IT z m m m u m u 1 u n n 1 l&#39; |||l w 1 1 Nu u n n u n n IJLHI.. 1.. L Ilnmhwmwwwmmwwmmw uw mnmr SAMPLE AND HOLD CIRCUIT FOR AN ELECTRONIC ORGAN BAC KG ROUN D In a previous application for US. patent. Ser. No. 263,649, filed June lo, |972, in the name of David Bunger, assigned to the assignee of the present invention, there is disclosed a monophonic organ having provision for sounding only the highest note played. and for controlling the frequency of a single tone signal source in response to a voltage which is determined by that highest note played. The Bunger system includes a sample and hold circuit which is interposed between the source of control voltage which ultimately controls the tone signal source. and the voltage control terminaI directly applied to that source for effecting frequency control. That sample and hold circuit must be precise, i.e., its output voltage must equal its input voltage. However, various diode drops occur in the sample and hold circuit. which must be compensated for. The diode drops, i.e., the voltage between base and emitter for a transistor. or the voltage across a diode, are temperature sensitive` and there occurs a failure of precise tracking as a function of temperature, which in turn rcsults in imprecision of tuning.  
  In a prior application for U.S. patent, filed in the name of Munch` Uetrecht and Studer, Ser. No. 370,283. filed .lune l5, l973. and entitled &#34;Sample and Hold Circuit.&#34; which is assigned to the assignee of the present application` a set of mechanical switches is em ployed to ground points of a resistance divider chain when keys of an organ manual were actuated. It has been found that such switches are not of essentially zero resistance when closed, and that their resistances are in fact unpredictable. This property upsets the voltage relations present, and consequently&#39; leads to incorrect tuning ofthe voltage controlled oscillator of the Munch. et al., system. The present system, as well as the Munch` et al.` system, employs three octaves of keys plus one or thirty-seven switches. The disclosure of the Munch et al application is incorporated herein by reference.  
  The sample and hold system ofthe Munch, et al., application represents a solution to the problem of temperature dependence, and also eliminates inaccuracies of tuning which may result from variations of supply voltages, and of imprecision of amplifier gains, in that output voltage of the system, representing a desired tone frequency, is controlled by a negative feedgack loop extending around the entire sample and hold circuit, and in that the feedback loop is reference to one fixed voltage. The present invention additionally solves the problem of variation of switch contact resistance.  
 SUMMARY OF THE INVENTION A sample and hold circuit in a monophonic electric organ, in which organ keys set a zero voltage point along an array of resistances, and in which a negative feedback circuit extends from the output of the sample and hold circuit to the array of resistances and serves to maintain a predetermined point ofthe array of resistances at constant voltage regardless of the resistance to ground to the array when any of the keys is operated.  
  Three octaves of organ keys are hence of resistances are involved and these are in circuit on a per octave basis and the junctions of the resistances are placed at effective ground values via transistor switches instead of via mechanical switches which connect to actual ground.  
 DESCRIPTION OF THE DRAWINGS FIG. I is a circuit diagram of a system as disclosed in Munch, etal., Ser. No. 370,283, tiled lune l5, i973 assigned to the assignee of the present invention.  
  FIG. 2 is a circuit diagram of the system ofthe present, invention, connected to the system of FIG. l.  
 DETAILED DESCRIPTION The system can be divided into three sections. The first is called the sample and hold section. and provides a voltage that corresponds to the highest note key being played, or. if no keys are presently being played, to the last highest not key that was played. This section is illustrated and described in Munch, supra.  
  FIG. I contains integrated circuits ICI() and lCll which are operational amplifiers. The current sources Il through I4, which are constructed with transistors and resistors, are described in Munch Ser. No. 370,283. The current sources are under the control of the voltage Vl. When the voltage Vl is below 5 volts, current sources I3 and I4 are turned on. When the voltage VI is between 5 volts and 20 volts, all four current sources are essentially turned off (open circuit). When the voltage VI is above 2() volts, current sources II and I2 are turned on. In addition to providing an On Off control` the voltage VI determines the magnitude of these current sources. The ratio of I2 to Il when these current sources are turned on is equal to the ratio of I4 to I3 when the latter current sources are turned onA The operation of the sample and hold under various conditions is as follows. With any&#39; switch SI-S37 closed` transistor 0105 is turned off which allows diode D51 to become forward biased through R549 and R550 if Vl does not fall below 0.8 volt. This condition on Vl is always satisfied. Transistor 0106 is off because its emitter is held at 5 volts and its base is a diode drop above that voltage. These two transistors are now essentially out of the circuit.  
  The desired output voltage V3 with any key switch closed is the value that will cause the voltage V2 to be 5 volts. The voltage V2 is determined by the resistor voltage divider consising of resistance R571 and all switch resistors from RI to the resistor immediately preceding the switch that is closed, and the output voltage V3. For example, with switch S3 closed and II through I4 equal to zero, which occurs while VI is between 20 and 5 volts, the voltage V2 will be related to the output voltage by the following expression:  
 Since the desired value of V3 is that which results in V2 equal to 5 volts the following expression gives its value:  
  lf prior to the closing of switch S3. the output voltage V3 is higher than the desired value as defined by equation (2), equation l indicates that the voltage V2 will be higher than 5 volts. The voltage I/2` however, will be prevented from rising by I3. For a very small increase in V2` Vl will fall below 5 volts and turn on current source I3 which will provide a shunt path around the switch resistors to lower V2. This negative feedback will cause V2 to assume the value that will maintain thc current source I3 at the proper level. The above de scription uses the fact that if the positive and negative inputs of the operational amplifier are at the same voltage` the output voltage is l5 volts. Therefore. to maintain Vl below 5 volts, the negative input of the operational amplifier must be at a higher voltage than the positive input. Since the required change in voltage from nominal at vl is l volts l5-5), the voltage difference required between the positive and negative inputs must be this voltage divided by the gain of the operational amplifier. This gain is infinity in theory and in practice is typically above l00,000. This results in a required voltage difference between the operational amplifier inputs of l0 `volts 100,000 vous &#34;01ml &#34;&#34;l This voltage Av is called the differential input voltage and is added to the voltage at the positive input terminal to obtain the voltage V2, i.e., V2 5 volts 0.000l volts 5.00I volts. The small value of this differential input voltage implies that for&#39;practical purposes. the differential voltage is zero and both terminals are at the same voltage. This assumption remains true whenever negative feedback like that provided by I3 is present.  
  The initial value of the current souce I3 is now calculablc because the value of V2 has been determined to be +5 volts. The current through R571 is known because its value is specified and the voltage across it is known. Likewise the current through the switch resistors is known and I3 must be the difference between these two.  
 Since V2 5 volts,  
 Equations (2) and (4) show that I3 equals zero if V3 is at the desired voltage and that I3 is dependent upon the difference between the actual value of the output voltage and the desired value. Current source I4 is dependent upon I3. Its value therefore also depends upon this error voltage. Current source I4 discharges capacitor C523 whose voltage is amplified by transistor 0114 and lCll to become the output voltage V3. The output voltage V3. therefore, begins to fall and approaches the desired value. As the voltage V3 approaches the desired value, I3 and I4 are reduced which slows the discharge of C523. When the output voltage V3 reaches the desired value, both current sources are turned off and the discharge of C523 stops. The speed with which this process takes place is determined by the ratio of current source I4 to I3. If this ratio is large` I4 is large and the output voltage converges quickly to the desired value. If, however. this ratio is small the output voltage converges slowly to the desired value. This voltage change has the same transient characteristics as a series RC combination (exponential) due to the feedback provided by I3 and I4.  
  If the output voltage is lower than the desired value before the switch is closed, or ifa switch with a lower index number is closed after the output voltage has settled to the desired value. the system will correct itself in a similar manner. For these circumstances, however, V2 will attempt to be lower than. 5 volts, which will force Vl above 20 volts thereby turning on current sources Il and I2. Current source Il provides the negative feedback to control [C10 and corresponds to I3 in the previous discussion. Current source l2 provides charging current for capacitor C523 to allow the output voltage to reach the desired value, and corresponds to I4 in the previous discussion.  
  The above description may be said to be of the transient portions of the sample mode. After the transient portion is complete` which indicates that the output voltage is at the desired value for the particular switch closed, the system continues to maintain the correct output level. Any noise, leakage currents, or other disturbances that attempt to change the output level will appear at V2 and adjust Vl to turn on the proper cur rent source so that the proper correction can be made. Thus, while a key is being held` Vl can be at any voltage between 5 and 20 volts. Capacitor C519 provides additional negative feedback from VI to V2 which slows the rate of voltage change at Vl. This is necessary to both protect against large noise spikes and to protect against noise from the key switches during opening and closing operations.  
  When all key switches are released it is necessary that the sample and hold circuit to transferred into the hold mode. This is accomplished by using the 5 volts at V2 through all key switch resistors connected in series to saturate 0105. With all key switches open the output voltage V3 wouldunless prevented by control circuits` fall to a lower voltage than with any key switch closed. The voltage at Vl would fall and attempt to turn on I3 and I4. However, with Q saturated, when the voltage at Vl is equal to l0 volts` transistor 0106 turns on via R548 and provides negative feedback to V2, which will hold Vl at l0 volts. With Vl at l0 volts all current sources Il through I4 and zero and there exists no charging or discharging current for C523. The output voltage therefore remains at the value corresponding to the last key played. The hold time is determined by the ability of capacitor C523 to hold charge` which explains the need for the buffer amplifier consisting of 0114 and ICI l. This amplifier has a DC gain of 2 with respect to l5 volts which means that a voltage of I8 volts at the positive terminal of ICll will produce an output voltage of 2l volts. This gain reduces the voltage swing required at capacitor C523 to produce a given voltage range at the output. Resistors R568 and R569 provide the resistor divider necessary to supply an effective l5 volts with a 10K impedance to the junction of R570 and the negative input of ICI l, which is required to give the amplifier this gain. Resistor R567 provides bias current for transistor QI I4 and resistors R545, R547, R546 and R544 are current limiting resistors, which prevent destruction of their associated active components under accidental short circuit conditions. Capacitors C518 and C520 provide additional noise and switch bounce protection.  
  The description of the operation of these current sources, I1 I4 is not provided herein, because reference can be made to Munch. et al.. identified supra, for such description, and because the details of the mode of operation of these current sources is not pertinent to the present improvement.  
  FlG. 2, the connector plugs 1, 2, 3 are connected. respectively, to corresponding plugs in FIG. 1 and to resistance R544, resistance R545, and lead D of FIG. 1 respectively.  
  Instead of connecting the junctions of resistance R1 R37 directly to a ground lead as in Munch, et al` supra, via mechanical switches S1S37 transistors 0127 0163 are substituted and connected to adjust the junctions to virtual ground, and these transistors have their bases connected to grounded base lead 50 via the switches S 1-S37. When a switch, as S1, is closed, its associated transistor. as 0127, is rendered conductive, since its emitter is connected to a negative voltage point, i.e., the collector of 0164, which has its emitter connected to a negative lead 51.  
  The emitters of transistors 0127 to 0138, corresponding with one octave of notes, are interconnected via diodes as Dl to D11, only D11 being illustrated, which serve to isolate these emitters from each other and to provide certain voltage drops. 0127 employs no diodes, and in general the highest note transistor of each octave includes no diode while all other note transistors do, the circuit of 0150 being typical.  
  The operational amplifier IC12 is supplied with 5 volts at its terminal 7, its terminal 2 is connected via a small resistance R809 and lead 54 to the last resistance R37 of the resistance chain R1 R37, and therefore has a 5 volt voltage applied thereon, if no key is operated` because the input resistance at terminal 2 of IC12 is almost infinite. The terminal 4 is connected to lead 55, which is held at volts. The lead 55 is connected to ground via resistances R807, R806, R804, R802, R801. The junction of resistances R806 and R804 is at l2/2 volts, the junction of resistances R804 and R802 at -l lz volts, and the junction of resistances R802 and R801 at l01&amp; volts, and these voltages are steady voltages, applied to the bases of 0166, 0165 and 0164, respectively. The emitters of these transistors have variable voltages applied to them. For no keys closed. the voltage at point 57 is -20 volts.  
  The bases of 0165 and 0164, but not of 0166, are connected to the points 60, 61 via diodes D57 and D56, respectively, which serve to mutually isolate the bases of the transistors 0165 and 0164 from the -111/2 and -lOl/ volts references.  
  Point A of the resistor chain Rl to R37 is held at -5 volts, as by the circuitry of FIG. 1. lf no key switch is closed, this voltage is applied through R1-R37 to the 2 lead of IC 1 2, which is referenced to zero volts. The output of IC12 therefore approaches -20 volts, since this is the voltage applied to terminal 4 of IC12. These are the conditions if none of switches S1-S37 are closed.  
  If any key switch is closed the base-emitter junction of the associated transistor will be forward biased, and will provide negative feedback for IC12. For example, if S24 is closed the output voltage of IC12 will fall to about -12 volts, which in turn will turn on 0165. The voltage at the collector of 0165 will then fall to approximately 7.2 volts, equal to the voltages across eleven forward baised diodes. as D1 to D11, and one forward biased emitter junction. The forward biased diodes are those connected to the emitters of 0150 to 0140, no diode being present at 0139, and the emitter junction being that of 0150. Transistor 0150 then turns on because it has a current path through 0165.  
  The negative feedback path from the output of IC12 through 0165, eleven diodes, 0150 and R25 to R37 to the input of IC12 will cause 0150 to conduct precisely the correct current to hold the junction of R24 and R25, i.e., the collector of 0150, at zero volts. The result is achieved because IC2 has extremely high input impedance and gain, so that the effect is the same as if the junction of R24 and R25 were directly grounded through a perfect switch.  
  The number of diode drops involved is only that required to assure that 0165 will turn on. Once it is on the feedback path takes over to control voltage and the high impedance of IC12 prevents current flow beyond the point where a conducting transistor` of 0139- 0150, exists.  
  lf S23 were now closed. 0149 would be turned on and the collector voltage of 0165 would rise by one diode drop, since then only ten diodes and the baseemitterjunction of0149 would be forward biased. This removes the forward bias from 0150, and holds the junction of R23, R24, at zero volts, and this junction now represents the highest note played.  
  Each of the three octaves is connected to the output of IC12 through a separate transistor, i.e., 0166, 0165 and 0164. These three transistors provide highest octave preference due to the diverse voltages applied to their bases. The highest note played will always force its associated one of transistors 0164` 0165 and 0166 to turn on. which will prevent a lower octave from turning on.  
  There are. accordingly, two preference actions in the present system, one is per octave and the other per note within an octave. Once a zero voltage point has been established along R 1-R37, any lower note is inoperative, because the associated lower octave transistor or transistors of0l64, 0165, 0166, if any exist. cannot become conductive, and because lower note transistors within an octave cannot become conductive.  
  Resistances R38 to R44 are to provide precision of resistance values for the resistances R1 to R37. The latter are l.7f resistances and are connected in groups of six across resistances R38 to R44. It can then be the case, absent R38-R44, that a 6.7: error of resistance will exist. The resistances R38-R44 are selected to provide precisely the required resistance for the six resistor group. R38 merely parallels R1, and these can be considered a single precise resistance` i.e., R38 compensates for error in R1 only.  
 What is claimed is:  
  1. An electric organ, comprising an array of keys of said organ. a resistance having points corresponding respectively with said keys, means responsive to actuation of any selected key of said array of keys for providing a physically undgrounded point of zero voltage at a point of said resistance corresponding with the selected key, said resistance having an invariable point of reference potential, means including a feedback loop for maintaining the voltage of said invariable point of reference potential constant where any of said keys is actuated by feeding back current through said resistance in amplitude sufficient to maintain constant said 7 voltage of said invariable point of reference potential. said feedback loop including a storage capacity. a voltage controlled tone signal oscillator responsive to the voltage of a point of said feedback loop, and means for acoustically transducing said tone signal.  
  2. The combination according to claim l` wherein is included. a further high gain high input resistance feedback loop, means responsive to closure of any selected one of said key switches for controlling the voltage drops along said array&#39; of series connected resistances to provide said zero voltage at a junction of said array corresponding with the selected one of said key switches in response to the feedback action of said further feedback loop.  
  3. The combination according to claim l, wherein is included a further high gain high input resistance feedback loop. means responsive to closure of any selected plurality of said key switches for controlling the voltage drops along said array of series connected resistances to provide said zero voltage point at a junction of said array corresponding with that one of said key switches corresponding with the highest note represented by said selected pluralityl of said key switches in response to the action of said further feedback loop.  
  4. The combination according to claim l, wherein said means responsive to any selected key of said array of keys includes a voltage divider having points each corresponding with one of said first mentioned points, normally open electronic switches closable in response to actuations of said keys and each connecting one of said points of said resistance with one of said points of said voltage divider, and a further feedback loop between said voltage divider and said resistance.  
  5. An electric organ, comprising an array of key switches. a resistance, means responsive to operation of any combination of said key switches for adjusting the value of said resistance by providing one physically undergrounded point of zero voltage at any of plural preselected discrete points of said resistance corresponding with only one of said key switches` circuit means including a voltage comparator extending from an invariable point of said resistance to an output point, a resistive feedback loop extending from said output point to said further invariable point. a source of reference voltage, means for developing an input error signal for said voltage comparator as the difference of said reference voltage and the voltage of said invariable point, a voltage controlled tone signal source connected to said output point` and means for acoustically transducing the signal output of said tone signal source.  
  6. The combination according to claim 5, wherein is provided a storage capacitor coupled to the output circuit of said amplifier to store the output voltage of said voltage comparator, and means responsive to said error signal for supplying charge to and abstracting charge from said storage capacitor as a function of the voltage of said invariable point.  
  7. The combination according to claim 5, wherein said resistance includes plural arrays of series connected resistances, said arrays being connected in series. said arrays having junctions corresponding onefor-one with said key switches, further feedback loops, each including one of said arrays and all including a common high gain high input resistance operational amplifier, means responsive to closure of any selected plurality of said key switches concurrently for completing only that one of said feedback loops corresponding with that one of said arrays containing a junction corresponding with the highest pitched one of said key switches. and means further responsive to said closure of said selected plurality&#39; of said key switches to provide 5 said undergrounded point of zero voltage at the last mentioned junction in response to the action of said further feedback loop.  
  8. The combination according to claim 5, wherein said means responsive to operation of said key switches includes a separate normally open solid state switch operatively associated with each of said key switches, means responsive to closure of. each of said key switches for rendering conductive the solid state switch associated with that key switch, a diode voltage divider chain interconnecting first corresponding terminals of said solid state switches to discrete points of said diode divider chain, said diode divider chain including identically poled diodes in series, a load circuit terminating said diode divider chain, a voltage comparator device including a source of reference potential and an input terminal for applying voltage for comparison with said reference potential by said voltage comparator device, and means responsive to the output of said comparison device for controlling the potential at said variable load circuit terminating said diode voltage divider chain` and means supplying voltage from one end of said resistance to said input terminal of said comparison device, said voltage comparison device providing a negative feedback loop extending from said one end of said resistance to said variable load circuit, said voltage comparison device being arranged to draw substantially zero current from said resistance via said input terminal` and means connecting second terminals of said solid state switches to said discrete points of said resistance, said variable load circuit being a voltage responsive device responsive to control voltage to provide a variable load as a function of magnitude of said control voltage.  
  9. The combination according to claim 6, wherein said means responsive to actuation of any selected key of said array of keys includes a separate normally open solid state switch connected to be closed in response to operation of each of said keys, an amplifier, and a further feedback circuit extending through any closed ones of said solid state switches to the output of said amplifier and via said resistance to the input of said amplifier.  
  10. ln an organ system` a chain of equal series connected resistances extending from a first point to a second point, a chain of equal impedances providing equal voltage drops thereacross extending from a third point to a fourth point` an amplifier having an input and an output, means applying a voltage to said first point` means connecting said second point to said input` means connecting any selected junctions of said resistances to selected ones ofjunctions of said impedances, means connecting said fourth point to said output, said third point floating, wherein is provided means for maintaining said first point at a fixed voltage, a voltage controlled tone signal oscillator, means responsive to said last means for controlling the frequency of said voltage controlled oscillator. and means for transducing the tone signal output of said oscillator.  
  ll. An electric organ, comprising an array of normally open key switches. an array of electronic switches, each of said key switches when closed rendering conductive one of said electronic switches, a series resistance chain having junctions corresponding one for one and each connected to one terminal of one of said electronic switches, a diode chain including series connected correspondingly poled diodes, means connecting the remaining terminals of said electronic switches one for one with corresponding electrodes of said diodes, a source of control voltage, means applying said control voltage to one end of said resistance chain. an amplifier having an input terminal and an output terminal, means connecting the other end of said resistance chain to said input terminal, means connecting one end of said diode chain to the output terminal of said amplifier, said resistance chain, diode chain and electronic switches being arranged and interconnected to provide a circuit from said source of control voltage through resistances of said resistance chain and via closed ones of said electronic switches and via said diode chain to said output terminal, and said resistance chain alone providing a circuit from said source of con trol voltage to said input terminal.  
  l2. An electric organ, comprising a series connected array of resistances having a reference point, means including key switches for bringing any selected junction of said array of resistances to a physically ungrounded zero potential, said junction being a function of the operated key switches, a comparison circuit responsive to the voltage of said reference point and to a xed reference voltage for generating an error signal` a storage capacitor, current source and drain means responsive to said error signal for controlling the charge of said capacitor, an output terminal, means amplifying the voltage across said storage capacitor to provide an amplitied version of said voltage across said capacitor to said output terminal, a resistive feedback circuit extending from said output terminal to said reference point, a  
 voltage controlled tone signal oscillator connected to said output terminal and responsive to the voltage of said output terminal to generate tone frequency signals of variable frequency, and means for asoustically transducing said tone frequency signals.  
  13. The combination according to claim l2, wherein is included means responsive to opening of all said key switches for disabling said current source and drain means for controlling said charge on said capacitor.  
  14. The combination according to claim 12, wherein said means including key switches for bringing any selected junction of said array of resistances to a physically ungrounded zero potential includes a voltage divider chain. solid sate switches responsive to closure of said key switches for connecting said selected junctions one for one with points of said voltage divider. and a further feedback circuit connecting an end point of said voltage divider with and end point of said array of resistances.  
  l5. The combination according to claim 14, wherein said voltage divider is a series array of diodes each providing a fixed voltage drop.  
  i6. The combination according to claim l5, wherein said switch means are solid state switch means.  
  I7. The combination according to claim 16, wherein said impedances are diodes.  
  18. The combination according to claim 17, wherein said amplifier is a comparator, said voltage of said first point is positive, and the output of said comparator is negative.  
  19. The combination according to claim 18, wherein said chain of series connected resistances encompasses a resistance for each note of plural octaves of notes.  
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