Question on the problems of timbre formation of the theremin sound

(Sorry if there is already a thread about this and I didn't see it)
It has long been noticed that the sound coming after the mixer of two high-frequency generators of the theremin almost cannot be changed by the usual means of timbre formation.
Both passive RC timbre-forming chains and T-filters on operational amplifiers.
If you apply sound to these filters from a conventional sound generator, then timbre formation works predictably, logically, correctly, and efficiently. And the sound of the theremin is not.
What is the reason? Help me understand, please.

There are many high-frequency components in the theremin signal, which probably are not completely filtered out by one capacitor to ground, or to plus. And is this the reason?
It is necessary to convert the theremin sinusoid into a meander, with a Schmidt trigger, and only then will the filters begin to change the sound?
I ask respected electronic engineers to share here their findings in the field of sound timbre formation in simple analog theremin circuits.
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(my numerous experiments do not give an impressive effect. Well, except that this option gives a noticeable change in timbre, when the outputs from the generators go to a tuning resistor and by changing it sometimes a noticeable result is achieved. (softening a sharp timbre, for example, to a soft one)

A simplified idea for changing the timbre in the picture: (more precisely, a more subtle solution is needed - two resistors. A constant resistor for 15 ... 18 k and a tuning resistor for 1.5 k).

-But, as I found out, this solution does not work for every type of theremin circuit. (

"I ask respected electronic engineers to share here their findings in the field of sound timbre formation in simple analog theremin circuits." - Martel

I would like to know more about this too as I haven't researched it myself.

Maybe start from the beginning? From a careful consideration (study even) of the theremin waveform formed by mixing two high-frequency oscillators?
Maybe the oscilloscope screen does not show those "anti-timbre" features of the theremin signal, which exclude the operation of the usual timbre-forming circuits?
(Keep in mind that any simple CMOS sound generator can be modified by timbres and filters very effectively.)
But here you probably need a specialist not only with an oscilloscope, but also a spectrometer))
Maybe this photo is a clue? and it is necessary to solve the riddle in the given circumstances?

The designer of theremins Korolev declared wide possibilities in his models. Sounds of violin, oboe, horn, flute, trumpet. And these schemes are known, and the approach for obtaining these timbres is known. But (!!) there are no demonstrations of this. There are no sound samples specifically for timbres. 
(I collected some of his models, but did not find timbre richness)
On all videos of Korolev theremin I hear a standard, dry, flat sound.

Martel, 

I had all the same thoughts about timbre control early on. Eventually I figured out the classic theremin sound occurs by distorting the simple sine wave. The throaty vocal happens with a lazy wave shape before the 1N914 mixer diode and the upper harmonics for brightness occurs after the mixer diode by using an audio transformer to skew the opposite side of the wave form. Controlling the sine wave shape is why my theremin sounds were always different.

oldtemecula - I didn't understand your verbal description of the tone shaping method, but these two audios sound very appealing. There is volume, airiness, velvet. Musicality. This is a success!

I am happy you got the inspiration to explore this subject as it was a gift for the 2020 - 100 year anniversary.

My sound begins with a very relax sine wave as seen here. This audio wave shape is a reflection of the RF wave shape after detection. I use a special pitch antenna (as seen in video) which also enhance the RF wave shape. My sound needs a sine wave with a narrow top peak and a fat bottom peak. “Some” vacuum tubes can create a better fat bottom peak as they create a slower roll over of the sine wave.

The brightness of the sound is created by passing the sound through a small audio transformer to skew or fold the top peak which adds even harmonics. I wanted to study this more for a EWS add-on but father time has taken his toll.

A good Earth ground is necessary for proper sound wave shape as we are working with a radio wave that is about 1000 meters in length and the energy needs to go somewhere or it will distort. Digital does not use the RF wave shape for sound, why they sound different, just for response. Go outside and pound in a two meter rod into the ground and run a wire through a window or connect to a metal water pipe.

[center]- None of this is easy -

For me transistor oscillators are too snappy for beautiful natural sound. The video is using transistors as I use a bit of gimmickry to slow the RF wave. Visit my website where I go into more details.  www.Hwy79.com 

(I read and write through Google translator and hope for his skill))) Thank you for participating in the topic.
I'm much more of a musician than an electronics guy, and I don't understand a lot about electronics. And usually I wade through to its understanding with a small set of knowledge.
You present pictures of the theremin signal, which are very conditional and do not reflect the real picture, and this confuses and stops me, but I understand that you explain the importance of the shape of the sinusoid at the top and bottom, which of course is also very significant. But the question is HOW to most effectively change this form. 

(Distraction. - I recall the peculiarity of the sound of a simple musical synthesizer Stylophone, its melodiousness and recognition. And there it is due to the type of generator that produces such a non-standard signal))). Far from sinusoid))

I have audio transformers, and often wind them myself, but this is the first time I hear that they can change the signal shape at all. (well, maybe in combination with other elements).

Grounding! I’ll have to immerse myself in this aspect and study how it affects the sound ((I myself am an electrician and I thought about putting a real ground in my apartment, but I didn’t find the opportunity (this is a multi-storey building without drainpipes).

Can you be more specific about the problem you are having?  It seems pretty simple to me -  if you don't have enough harmonic content in your waveform, clip it; if you have too much harmonic content, use a lowpass filter.

I also don't understand the cult of grounding.  I have never seen it have a significant effect on theremin operation, and especially not on mixer output waveforms.

edavid the answer is found in the sound, post your best sound sample, it will reveal to everyone what we are talking about.

Martel,  My approach shows what is possible, smarter minds will refine it.

I am hoping that bright eyed creative kid comes along and reads this, to duplicate my results is a reward just waiting to happen. Let us not lose touch with the Analog theremin.

I am not a musician and have never worked in electronics yet one day the theremin invited me. I was making my daily 1.5 hour drive to my job and I had a wonderful rare moment of inspiration, a kind of happiness. From then on I let the theremin show me what was going on circuit by circuit as i was learning as I went along.

With analog theremins there is no way around not using an ideal earth ground or it would be a violation of Nature and the oscillators will fight with one another creating what I see in “your” wave shape. That is the exact wave shape I use for my volume side control not beautiful sound.

Here in the states I wonder how many theremins were sent back to the manufacturer due to poor earth ground. Our houses have 3-prong power plugs but that does not mean all receptacles that are wired correctly have a quality earth ground.

I believe using a hand held oscilloscope and a special circuit someone could design a gadget to determine the quality of earth ground by the signal wave shape of a 300 kHz sine wave. This would eventually be treasured by all Thereminist.

I do have a second year MIT student and neighbor I am hoping gets the theremin bug, the amazing Tanner Packham

My research is somewhat disorganized after 20 years but all my knowledge is here. A Theremin by Old Temecula

I do have a second year MIT student and neighbor I am hoping gets the theremin bug, the amazing Tanner Packham

If Tanner Tech is one of his YT channels, it looks like he already got the theremin bug, built his own tube theremin and has an Open Theremin as well.

https//www.youtube.com/@TannerTech