>I would dearly love to be proved wrong about it being difficult to design a good theremin.
I'm curious, Gordon - what is it that you believe makes the task difficult? What is lacking in existing designs that makes them not good? By "good", do you mean somehow easier to play?
I designed and built a theremin back in the late 1970's when I was in high school. It was stable, linear, and (to my ear) pleasantly rich in harmonic content. I lost interest in trying to master a good playing technique, so it was cannibalized for parts to feed other projects. That was before the Internet was available to the general public, before I was even aware of the theremin being used for any purpose other than as a sci-fi movie backdrop.
Recently I designed and built another one. This one, although quite different in design due to an extra 30 years of experience, is also stable, linear, and pleasant sounding. I built it because I'm interested in developing ways of recording performance data. That is, recording not the audio produced during a performance, but the performer's manipulation of the instrument. That way, I can take the data and play back the performance as-is, transpose to another key, speed it up, slow it down, or even pump the data into an entirely different instrument without losing any of the performance subtleties. Designing the hardware was (by far) the easy part. Translating performance data between instrument types is far more challenging...
[EDIT] - pardon me for hijacking the thread. I'll shut up now and let you guys return to answering the original question. :)