The Theremax manual goes into much greater detail than the following brief design analysis.
Hand and body motions in proximity to the sensing antennas produce frequency changes in a Hartley oscillator operating at approximately 750kHz. The signal from this variable oscillator is mixed with a constant reference frequency in a ring modulator and the result passed through a single pole of low pass filter to leave only the difference between the variable and reference oscillator frequencies. The sensing circuitry is duplicated for Pitch and Volume sensor antennas.
The output of the ring modulators is boosted in level by discrete transistor amps which also provide a second pole of low pass filtering because of the the feedback capacitors from collector to base. The sine wave output of the pitch sensing circuitry is routed to the VCA () for volume processing.
The fully filtered signal is converted to a square wave by a Schmitt trigger, then differentiated to produce narrow pulses which are integrated to produce control voltages. The Volume Control Voltage is used as the control signal for the internal VCA.
The front panel “Timbre” control allows a blend of sine and/or square wave to serve as the input to the VCA which consists of discrete transistors and a single stage op-amp. VCA gain structures are arranged so the stage will distort when Velocity is set to maximum.
The Volume Control Voltage is differentiated to produce a Velocity Control Voltage. Velocity CV is also routed to the diff. amp input of the VCA to produce asymmetrical distortions in response to rapid volume changes (see Panel Controls).
The Velocity CV is thresholded by a Schmitt trigger to produce Gate and S-trigger outputs.