Results
After a brief worrisome period during which I worked out the two major bugs with the final design, the GT Theremin worked marvelously. Unfortunately I did not have the opportunity to put it through rigorous measurements, so I can only cite rough performance statistics.
We considered the playable pitch range of the GT Theremin to be roughly 200 Hz–2 kHz (fundamental), depending on how carefully it is tuned. It will produce tones outside this range, but due to the high harmonic content at very low frequencies and poor control linearity at high frequencies, I believe the cited 2.7 octave range to be most useful for playing purposes. The dynamic volume range (i.e., the largest volume difference which can be produced by volume hand movement) is around 30 dB when the volume section is tuned properly. We found this to be adequate for performance, though some improvement on this figure would better silence the Theremin when playing through a loud speaker system. The maximum hand sense distance was measured at 12-16 inches depending on the tuning, player stance, and environment. Maximum audio output levels were 1.5 Vpp using a 12AX7 tube for the mixer (this figure will change significantly if a different tube is used in this capacity). The entire Theremin dissipates about 17 W of power (estimated, not directly measured), which varies little once fully heated. The tube heaters account for nearly 16 W of this, and the high voltage circuits use the rest. It should therefore be apparent that the tubes become sufficiently warm while operating that physical contact is ill-advised. This is especially the case for the 6X4 power rectifying tube, whose heater dissipates twice the power of the other tubes’ despite its comparable outer surface area.
Demonstration video
This is fairly self-explanatory. I wish I could have filmed another (more fun and less wooden) demonstration, but I really didn’t have any time left to practice and perform a song. This will have to suffice until such a time as I can commandeer the device from its current keepers.
Conclusion
I have tried to provide a reasonable idea of how the GT Theremin works, sufficient as a starting point for someone looking to explore similar concepts. Rather than giving step-by-step instructions I have attempted to explain the thought process behind the design, including the rationale for each choice we made. I believe this is far more valuable than a list of assembly directions since it should offer the competent designer a solid basis upon which to tweak or improve our design as they see fit.
The GT Theremin is currently on display in the lobby of the Van Leer building at Georgia Tech. If you’re really nice to Edgar Jones or James Steinberg up on the third floor, they might take it out for you and let you give it a whirl.
If you have any questions or comments about the design, or found any part of my explanation unclear, please don’t hesitate to contact me. Once again, I would like to thank James Lewis for putting up with my demands during the circuit design process as well as for his superb work on the physical design and construction. Thanks to Ryan Adams for taking care of administrative and documentation details; which allowed me to devote most of my time to developing and building the circuitry. Yuri Yelizarov worked on some ancillary projects related to the Theremin, assisted in its development some, and lent us ideas and audio equipment for our demonstration. Edgar Jones and James Steinberg provided us with constant assistance in sourcing parts and materials, as well as invaluable construction advice and help. Professors Whit Smith, W. Marshall Leach, and Gabriel A. Rincón-Mora all assisted me in various aspects of the design, and my appreciation is extended to them as well. Finally, a big thank-you to Dr. Brewer for lending me his copy of Clara Rockmore’s Theremin performances and for generally being the funniest person I’ve ever encountered.