Theremin modelling

Posted: 4/14/2011 2:47:40 AM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as . ...................................

Joined: 12/7/2007

Some final comments on this design:

The original ( is interesting because of its "upside down" configuration.. In fact, if one changes the design to the way you have in your "mirrored" circuit, one ends up with a circuit which is instantly recognisable as a variant on Bob Moog's EW oscillators!

I can see absolutely no reason for flipping the circuit the way that has been done on the original - as in, replacing the NPN's with PNP's and putting the tank components on the bottom rather than at the top - other than to obscure the fact that this section of the circuit is a variant on the EW oscillator circuits - there is absolutely no advantage technically in doing the circuit this way.

The tuning mechanism (using power diodes as varicaps) is awful - The Moog tuning scheme is FAR superior! The capacitance is a function of the width of the depletion zone in the diode - this width (which acts as an adjustable width dialectric, effectively moving the 'plates' further apart as reverse bias voltage is increased, and therebye changing the diodes capacitance) is severely affected by temperature.. I am sure that this theremin will suffer from severe drift, both on pitch and volume settings.

I actually see nothing of merit in this design - There is no equalization, it will drift, its mixer is crude.. It is only slightly less complex than the EW design..

My advice? Spend the time you are wasting on simulation, on building an EM / EW instead!


Oh - there is one slight improvement on the EW design - The VFO and REF oscillators are buffered.. This is to drive the mixer - but could also drive other mixer circuits.. and there is seperate oscillator coupling [R10 + C8] which is better IMHO than the Moog scheme where coupling is integral to the mixer.. And some equalization will be caused by this coupling..
Also - The single supply will make this design more suitable for portable applications.

So this design does have some useful features.
Posted: 1/14/2023 11:44:31 PM

Joined: 11/2/2022

Got it working! I have posted my LT Spice model of Art's 145 Theremins pitch oscillators up there ( well.Now I want to feed the audio frequencies through a parametric equalizer and feed the signal level back to the radio frequency to see if I can control the linearity.

Nearly 12 years later... just want to say thank you! Troubleshooting the exact same behavior, with the exact same part of the exact same circuit. Excited to see how you figured it out.

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