The 145 Theremin

Posted on Feb 5, 2014

Since the publication of the type 144 theremin project article in September, 1998, many of them have been successfully constructed. As in the 144, and its forerunner, the Southwest Technical Products model 142, the 145 theremin retains a heterodyne topology similar to that used by Mr. Theremin in his original instruments. The 145 theremin is similar to the 144 in most respects, excepting the types of oscillators used.

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The 145 theremin oscillators are differential-pair types, in comparison to the Colpitts types in the 144. The approximate operating frequencies for the 145 theremin are 270kHz for the volume circuit and 360kHz for the pitch circuit. Compared to the 144, the 145 has improved pitch-arc linearity, with A440 occurring at about 10 inches of hand distance, compared to about 5 inches for the 144. The practical pitch range for the 145 is about 4 1/2 octaves. Inexpensive silicon rectifiers are used as varactors to provide a volume oscillator tuning range of about 1200Hz and a pitch reference oscillator tuning range of about 700Hz. These frequencies will typically vary within ±10 per-cent among instruments, due to tolerances. In the new design, oscillator outputs are taken from the resonant L-C pairs, and are low-distortion sine waves. Weak coupling is provided between the two pitch oscillators via C8 and R10 to induce a "lock" condition for zero-beat when the pitch hand is absent. The pitch oscillator levels at the mixer input are unequal, and the mixer output amplitude is significantly reduced in comparison to the 144. This reduct

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