"I am so happy that i am in the analog business." - DOMINIK
Oh, sorry, that probably came across as snark - I just don't think I have the temperament to handle all of the challenges associated with analog Theremins.
Anyone who can design any kind of Theremin well has my complete respect (I'm looking at you DOMINIK!).
The one capacitor bridging R23 coupling reduction mod is very interesting, I'm considering a modification with an airgap trim capacitor of which I have a couple around (if I remember correctly one is 30 pF and the other 56 pF at maximum capacity). In another discussion about the Etherwave's sound and the ESPE01 module, Oldtemecula suggested another one capacitor mod, bridging the detector diode D4 with a 47 pF capacitor, but I'm not sure if I understand what this does electronically, doesn't this make more unrectified HF go into the AF circuit after the diode in a frequency dependent way? Or does it work out as a kind of negative feedback to the heterodyned RF signal that somehow reduces oscillator coupling too? I'm also more reluctant to solder something to the diode, I remember semiconductors can get damaged easier by heat than resistors and capacitors.
Dreadvox, yes, both one-capacitor-variants are rf voltage dividers and thus reduce the coupling between the two resonance circuits. The large 4.7nF in series has very little impedance here. Indeed, I preferred to bypass the resistor to protect the diode from hot soldering (avoids complaints).
Thanks for your insight, JPascal. If I understand it correctly I could start with just bridging the 10k R23 with 2 pieces of isolated wire twisted together to create a small capacitor which would even be somewhat variable. I don't have an oscilloscope, but think I will primarily let my ears guide me, and I could record the output and look at it in Audacity to see the waveform and analyse the audio spectrum.
JPascal wrote:
“German engineer A. Klaus used a bridge for the Etherwave with a R-C-connection between the hot sides of the two 15 pF coupling condensers. R=150 kOhm, C=1...5 pF”
Sorry - I’m late ; but you actually found me 
After quite a long time (actually years) I found this simple solution which allows to freely set the coupling compensation amount when adding the very small adjustable capacitor and the resistor. The two twisted wires are just a temporary solution when no ceramic or plastic trimmer capacitor is at hand. The adjusting knob of the trimmer can be extended to make it accessible from outside the housing; I already mounted the trimmer below the PCB and a special little knob counterbore at the bottom wood panel of the EWS. During the last months I compiled this and other Theremin related findings (like the over/undershooting low-pass-filter for improved staccato volume response) to document them mainly for myself – but if this can encourage others to do their own research which (hopefully) confirm the issues some day, this is highly appreciated.
I would give an interim status of my theremin. Construction sites are mainly the suppress of modulations due to captured home installations hum via the pitch antenna and via the high Q coils. And secondly the resulting SNR at audio out is not yet what I want. What benchmark should be reached, 45, 60 db or higher?
For test I used a loop pedal DD7 to record some sequences I played one after the other. While the whole recording all knobs was remaining unchanged. The volume acceleration function is so demonstrated. And an unusal timbre.
"Sorry - I’m late ; but you actually found me" - Neutrodon
Alexander, here you have wrote :
"In Leon Theremin’s original design of e.g. the Rockmore instrument, the over/undershoot is mainly defined by the bandwidth of the volume-control detector resonator-tank, which produces ringing at fast frequency changes, causing voltage variations of the resonator and thus the detector tube’s output. This resonator-tank is missing in Robert Moog’s EWS design."
Trying to look at the Rockmore instrument schematics I can't see an equivalent of "Overshoot LP filter" or anything similar.
Could you clarify which components compose the "ringing " (1..10 Hz) circuit?
What are the parameters which determine the ringing frequency and "overshoot" degree?
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