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TVOX Stories
Of the Theremin and Futurist Stringed Instrument Making
Page 8

Let us now look at the original diagram of an RCA theremin. It also used a 24A tetrode (the RCA equivalent was called UX224), but there is no control of the intensity of the signals coming from the oscillators nor the variable capacitor for forcing the binding; furthermore, the interstage transformer has a different tranformation ratio. It is these particulars that lead to the substantial difference between the sound of the RCA theremins and the better sound of Clara Rockmore’s theremin.

Detail of RCA's theremin.
Detail of RCA's theremin.

The need for the second project (the transistor theremin) arose from the periodic requests of people consulting my site and asking me for a simple and reliable circuit that they could build. In the past, I provided them with the diagram of the theremin created by TECI (Theremin Enthusiasts Club International). I had personally never tried it, but various correspondence had convinced me that it could be a good starting point for a self-builder. Unfortunately a number of people were unable to complete the proposed schema, and so I girded up my patience and started testing the circuit. I soon found myself faced by two problems: the components are difficult to find in Italy, and the final result using equivalent components is far from being a good starting point.
The next decison was to adjust the circuit radically in an attempt to obtain one that worked as it should.

Tetrode 36.
Tetrode 36.

My objective was to give the instrument an appreciable sound: using transitor circuits of simple conception, we now know that you often have to make compromises that are difficult to digest. The main problem is to design a mixer that allows a good distortion of the audio signal without becoming too metallic and unnatural, and without limiting the range of the lower frequencies.
I achieved this by constructing a mixer that provides an optimal signal range at all frequencies, but drastically limits the distortion, which is provided at a later stage represented by a transistorised preamplifier that blandly works in the saturation area. As described, this solution could make one screw up one’s nose because we find ourselves in the presence of the generation of odd harmonics, but I can assure you that the result is excellent and, furthermore, by controlling the point of saturation using a knob, it is possible to regulate the desired sound well.
The second objective was obviously to provide a good range of pitch and good linearity. Not wanting to make the design over-heavy by using self-built coils (often an insurmountable difficulty for amateur builders), I limited myself to using standard inductors connected in series to the pitch antenna. The result was an increase in range from two to six octaves and a linearity which, although not optimal, I considered to be sufficient for an instrument intended to be a starting point.
At the time of writing, the project is still being tested, but I shall soon publish an account that should allow any amateur builder to meet the challenge.

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