Theremin


Posted on Feb 5, 2014

THE PRINCIPLE of operation of the RCA Theremin is that of the beat frequency oscillator, The frequency of one oscillator may be varied by the capacity change caused in an associated circuit by the movement of the hand to or from a pitch control rod, Also an additional oscillator provides radio frequency current for heating the filament of a UX-120


Theremin
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arranged so as to control the volume of output, this control being due to the movement of the hand in relation to the volume control loop. A detailed description of the functioning of the various circuits follows: As stated in the foregoing, the musical note of the RCA Theremin is produced by two oscillators of slightly different frequency beating together. This beat note is then amplified by two audio stages. The change of note caused by the movement of the hand is due to the change of capacity across what is known as the pitch coil, this slight change having sufficient effect on an adjacent oscillator circuit to change the frequency and thus the beat note, the amount of change depending on the position of the hand in relation to the pitch control rod. The pitch control rod is connected to a coil having a very high inductance. Connected to this coil is a small condenser and a small concentrated coil. This entire circuit is tuned by the distributed capacity of its coils and resonates at approximately 172 K. C. Not having any fixed capacitor connected across it for tuning the ratio of inductance to capacitance is very high. Thus the small increase of capacity caused by the hand close to the pitch rod will cause the circuit to change its natural period considerably a great deal more than if a large capacity and small inductance were used. This pitch control circuit is connected to the grid side of the variable pitch control...




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