Mike Beauchamp of Therovox is offering custom-built electro-theremins, a type of mechanically-controlled electronic musical instruments.
The Therevox electro-theremin uses a mechanical linkage to control an audio oscillator. This makes the instrument very accurate. The traditional theremin is a space-controlled electronic musical instrument, which makes learning to control it difficult.
In addition to the physical differences, the electro-theremin and traditional theremin differ in the tones they produce. The traditional theremin’s tone is rich in harmonic content, producing an almost string-like quality in the low to mid registers, and a human soprano-like quality in the upper register. The Electro-Theremin, on the other hand, utilizes a sine wave as its tone. The theremin’s tone is earthy and organic. The Electro-Theremin’s tone more easily conjures up imagery of space and of the heavens.
The Therevox site has photos and samples of the electro-theremin in action.