The Enkelaar Theremin is a modified version of the simple Silicon Chip 2004 Theremin, it uses the S/C Circuit board.
Major changes from the original S/C design are the inclusion of user controls to tune the pitch (reference) and volume oscillators, and using the power amplifier to implement distortion of the waveform to provide two selectable tones. The power amplifier is not used to drive a loudspeaker, and is used to provide adjustable line-level output.
The simplest modification one can make to the Enkelaar is to replace the tone switch with a potentiometer - disconnect R35 and place a 100k potentiometer (wired as a variable resistor) in place of the closed contacts shown for SW1.. You may wish to put a resistor or trimmer in parralel with this variable resistor to reduce to maximum resistance down to about 80k to make control tighter, but a 100k pot works fine.
I will post more stuff about the Enkelaar soon.