examples of key systems in accordance with the invention will now be described with reference to circuit diagrams and explanatory diagrams in the accompanying drawings , in which : fig1 is a circuit diagram of a first embodiment of the invention ; fig1 a is a modified circuit diagram of the embodiment of fig1 . fig2 comprises two graphs showing the time - dependent characteristics of the control voltage and of the control current in the circuit according to fig1 ; fig6 is a circuit diagram of a further embodiment of the invention for producing two control voltages having a different course in time , and which are combined to form one single control voltage u s ; fig6 a is a modified circuit diagram of the embodiment of fig6 . fig7 is a graph showing the time - dependent characteristics of the control voltage u s of the circuit according to fig6 and of the sound voltage u t . fig1 shows a first embodiment of the key system according to the invention , in a simplified circuit diagram . the system comprises a key circuit a and a plurality of sound control circuits b ( only one of which is shown in the interest of clarity ), one circuit b being provided for each note of a register . the circuit a comprises a key 11 and a switch 12 connected to one terminal of a voltage supply source u a , the other terminal of which is earthed . by pressing the key 11 , the switch 12 is closed supplying the voltage u a to the circuit . a capacitor 14 is charged to the voltage u a through resistance 13 connected in series with the switch 12 . the voltage across the capacitor 14 , reduced by the emitter - base voltage of a transistor 15 is supplied to a resistance 16 , which is connected to the emitter of the transistor 15 . the circuits b are controlled by the voltage across the resistance 16 or the current from the emitter . in fact each circuit b is a network comprising a transistor 19 , a base resistance 17 and an emitter - resistance 18 in this embodiment . a sound frequency signal u t ( with direct current part ) is supplied to the collector of each transistor 19 and the output voltage is taken from across a resistance 20 . in fig2 the left - hand diagram illustrates variations of the value of the control voltage u s with respect to time and the right - hand diagram illustrates the variations of the control current i for different values of the coupling resistances 17 in dependence on time ( these curves are called i curves ). the horizontal line i z indicates the base current at which the resistance of transistor 19 becomes negligibly small in relation to the resistance 18 . the intersection points of the horizontal i z line with the curved lines give , on the t - abscissa , the transient periods t 1 to t 3 needed to reach i z when the circuit b includes different coupling resistances 17 . referring again to fig1 and to the left - hand diagram of fig2 the asymptotic end point of each i curve ( in the left - hand diagram ) is dependent upon the supply voltage u a . increasing u a results in steeper i curves in the right - hand diagram of fig2 and thereby in a more rapid change in the resistance of transistor 19 . thus it is possible , by varying the supply voltage u a in fig1 ( or the hf initial voltage of a generator 25 in fig4 and 5 , which will be discussed later ) to make the tone response of the instrument softer or harder , without blurring the differences within the instrument . if one looks at the sound control circuit b in fig1 then it can be seen that the base current i z flowing via the resistance 17 also produces an undesirable increased voltage drop across the resistance 20 . therefore the base current i z should not be allowed to become too great . the circuits b which are associated with the left - hand keys of a keyboard open more slowly then do those at the right - hand end of the keyboard . this can be taken into account in the key circuit a for each key . for this purpose the resistance 13 and / or the capacitor 14 , of each key circuit are of different values or are adjustable to control the current i z . instead of the current - controlled circuit b of fig1 it is also possible to use the voltage - controlled circuit b &# 39 ; of fig1 a , in which case , the coupling resistance 17 must have a further resistance 17a added to it , so as to form a voltage divider . in an advantageous development of the circuit according to fig1 the switch 12 is contactless . thereby one obtains a more progressive increase in sound , which can be controlled by the player . in fig3 and 5 , three devices are shown . each produces a control voltage in dependence upon the position of the key . in fig3 this is done by means of a device comprising a lamp 21 , a shutter 22 connected to and movable with the key 11 , and a photo - resistance 23 which replaces the resistances 10 and 13 and is connected to the base of transistor 15 and to capacitor 14 in fig1 . pressure on the key moves the shutter from between the lamp 21 and photoresistance 23 , the amount of light reaching the photoresistance 23 and therefore the current supply to the base of transistor 15 varying in dependence on the position of the key 11 . fig4 shows a magnetic key circuit . a permanent magnet 24 with poles n , s is coupled with the key 11 and varies , in dependence on the key position , the permeability of a transformer 26 . voltage is supplied to the transformer by a high frequency generator 25 and the output from the transformer is rectified by a diode 27 . a capacitor 28 is in parallel with the secondary winding of the transformer and the diode and the voltage across this capacitor varies in dependence on the permeability of the transformer . in fig5 the hf generator 25 supplies voltage to a resistor 31 via a capacitor 29 , 30 , one of whose plates 29 is movable and is connected with the key 11 . after rectification by the diode 27 the hf voltage from the capacitor charges the capacitor 28 to a voltage whose value is dependent upon the position of the key 11 . a tone - response similar to that of pipe organs can be obtained by means of sound control circuits which , in dependence upon the control voltage or of the control current , vary not only the amplitude of the sound voltage , but also its waveform ; for example by amplitude limiting and / or phase intersection . fig6 shows an example of an embodiment of the key system wherein two control voltages are produced . as in fig1 pressing the key 11 closes the switch 12 and the capacitor 9 is charged rapidly via the resistance 8 to the voltage u a , whereas the capacitor 14 is charged much more slowly via the resistance 13 . a resistance 7 is connected from the connection point between a resistance 8 and a capacitor 9 directly to a control line which is connected to the emitter resistance 16 and to the emitter of the transistor 15 . thus the potential at the connection point between elements 8 , 9 undergoes an additional voltage division due to the resistances 7 and 16 , so that the sound valve b is initially controlled by the voltage from the connecting point . this lasts until the charging of the capacitor 14 reaches a higher potential and thereby determines the value of the control voltage u s which is composed of the two individual control voltages . the capacitors 9 and 14 are discharged via diodes 5 and 6 and the resistance 10 after the key 11 is released . fig6 a shows a further embodiment of the circuit of fig6 in which a voltage - controlled circuit b &# 39 ; is substituted for the current - controlled circuit b . fig7 depicts , in dependence upon time , the total control voltage u s composed from the two control voltages , and also the sound voltage across the resistance 20 . the sound voltage across the resistance 20 is almost horizontal at the time t 1 and then between t 1 and t 2 , changes to adopt the curved waveform of the voltage across resistance 16 . the contactless switches of fig3 and 5 are also usable here in place of the circuit elements 5 , 8 , 9 and ( if necessary ) 10 , the contactlessly produced voltages being suitably led directly to the connecting point between the resistances 7 and 13 . a source supplying a voltage u v is connected between the earth point of the resistance 20 and the earth point of the resistance 16 in fig6 and serves to keep the circuit b non - conductive when in the non - keyed state . by varying the voltage u v the degree of distortion of the waveform at the time t 1 can be controlled .