Thanks for your comments.

There are many different Theremins nowadays... Some of them might
actually produce sine wave. However, the musically more useful
(including the original Theremin) instruments don't. The reason for
this is explained here:

http://www.paia.com/ProdArticles/thereton.htm
"As coupling increases, the two [high frequency] oscillators try to
pull one another to a common frequency resulting in distortion that
produces an output rich in harmonics at integral multiples of the
difference frequency. The waveform may be saw-like or pulse-like
depending on phase of harmonics..."

It is more complex and musical than (say) a sine wave fed to a tube
amp.

Propably, if the sound of Theremin - or especially Ondes Martenot -
could be done really well with standard synth techniques, we would
have heard it already... I am skeptical though. At least, I think
filtering a standard oscillator isn't enough.

How good is Ondes Martenot sound It is like a Hammond or Rhodes. You
can play it whole your life! Personally, I can't say that about any
synth sound I have come across. If you are interested, please listen
those Ondes Martenot sound examples I linked in my first post. If
someone can make that kind of sounds with any synthesizer, I would
like to hear! - I am soon getting my French Connection...


Best regards
Jari Jokinen