How to test the volume control circuit? I use a 555 timer with a double potentiometer to change the frequency of an asymmetrical square-wave generator. So I found the capacitors "in" and "out" of the voltage controlled attenuator to be to small. Replaced them by 3 µF non polar ones.
A YT video shows the work in progress. It was a bit challenging to adjust the camera with my right hand and to do the rest with the left hand. So it's not for a movie award, but enjoy it anyway.
I've tried a few things lately with the volume control unit. The use of mono-flop or Schmitt trigger circuits to influence the initial part leads to plucking effects. However, a static effect that is not enough accelerated by the varying speed of my hand. Here is potential remaining yet for brain work.
In this context some thoughts to the walking bass effect. Pamelia Stickney said in a interview concerning plucking the hand outside but near the volume antenna, how I heard that out: "...how do you get the staccato and I thought much more efficient with the hand to move away from the antenna and use the thumb as a stopping place for the other fingers and then just control how slowly you want to open these fingers, bring down the sustain to silence, so you can choose how long you want your note to hold that you are plucking." This discribes the effect of an ADSR controller caused by her hand movements.
One more reason to spend more time learning to play the theremin before circuits are developed! 
The problem with the theremin “walking bass” is that even when it is skillfully done, it is extremely “pitchy” but we don’t notice it because it’s all happening in a register where the human ear is less pitch sensitive. The bass line merrily bom-bom-boms along creating a wonderful illusion of an acoustic bass from the standpoint of timbre and amplification. It’s a great theremin novelty effect and it impresses the hell out of people, but it is not a serious tool for music making.
It is interesting for me. The pitch in the bass range would be disturbed by the volume dynamic? Anyway, seems to be an artifact of ordinary theremines. But this play technique described could principally also be used in higher pitch ranges.
I made an Excel pitch calculator based on physical formulas. The influences of the main parameters can so be analyzed. Inputs are here the resonant inductance and the antenna geometry. The zero beat frequency can be fixed by varying the sum of all capacitances (C,add) contributing to the oscillator frequency . The hand capapacitance is calculated with the hand area in front to the rod, the distance and the thickness of the rod. Here is a screen shot with parameters I found from old RCA Theremin here in TW.
Now I would like to test it a bit with real parameters of theremins. I need herefore the oscillator frequency, antenna length and radius, the resonant inductance. In case of using EQ Coil the value is to be added to the oscillator circuit coil. C,add is then lower.
The D-Lev (DIY Dewster) can operate anywhere between maybe 100kHz to 2MHz and will behave more or less identically after FPGA parameter change and recompile. Drastic antenna / plate area / geometry changes may require a bit of touch-up of the linearity knob, and the other axis knobs might need a bit of adjustment due to changes in gain. I've tested inductors between 0.5mH (~2MHz) and 8mH (~500kHz).
You can get some information from configuration files of Theremin Explorer (zip archive)
New version (1.17) is still expected, but updated config files you can download here:
https://yadi.sk/d/FM2a3EGllfoinQ
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