Hi there,
I've read some posts there, but still don't have any clear solution. In Europe there is only "two holes" adapter, so it cannot be grounded, if I understand. What should I do, use some another adapter or so? The theremin has a very "buzzy sound" and I think it's because of bad grounding..
Thank you for patience with me :-D
Have a nice day,
Martin
Is there a picture of Safety Ground at the wall. Is this direct earth ground or just the ground side of the two prong plug?
I am looking at a European power cord with two identical male prongs for power, the safety ground appears to be a female third connection.
What do Thereminist do if no safety ground is at the wall?
Ground is so important in my own research I am reluctant to encourage European builders. They may not understand the importance of earth ground and then think their poor results are my design. The beautiful sound begins at the Pitch Antenna if not in the Pitch Field. Only a theremin in harmony with Nature will produce it. Most misunderstand this so go digital which could be fun and give up what is remarkable about the theremin.
A squeaky Analog theremin sound can be bold and beautiful with $5 in parts and a little bit of knowledge. Sample.mp3
A theremin sound needing reverb to sound alive is spiritually dead and misunderstood.
Does your sound seem alive or more like a zombie
Christopher 
"I am looking at a European power cord with two identical male prongs for power, the safety ground appears to be a female third connection."
The two male prongs carry 'zero' and 'AC phase', the side contacts are 'safety earth' and are connected through a third cable to an actual earthing rod via(in the past before widespread use of plastics it was often connected to a tube of the water supply) that ideally reaches the ground water.
"What do Thereminist do if no safety ground is at the wall?"
Use an extention cable from a socket/group that has safety earth side contacts. Officially not recommended I think. In my days of tinkering with germanium diode 'crystal receivers' I clipped a lead on a bare metal part of a central heating radiator for an earth connection.
As to using reverb/delay or not, this is more of a taste question/option. Delay or reverb act a little as help to keep a steady pitch, as you get 'beats' between the direct and delayed signal when the pitch wanders off, so they can act a little like 'training wheels'. Lately I often don't use any delay or reverb and the only effect I may use is an envelope filter effect. Getting a pleasing / fitting sound the is the interplay between the theremin and its waveform and brightness controls, optional effects from pedals/stompboxes, amplification chain (with tone/EQ control) and the speaker system including cabinet characteristics.
And a big part of the sound when actually playing comes from the hands/fingers of the player. A good player can sound better playing a mediocre theremin that doesn't have the most ideal/beautiful sound than a bad player on a theremin that of itself has a beautiful sound.
Paying with reverb can give the illusion of playing in a big reverbant space in a livingroom and the delayed signal can help to keep one pitch steady and also helpful with interval jumps, faintly hearing the trail of the previous pitch. It can mask some shortcomings of both the tone as wel as playing accuracy, playing without any reverb in an acoustically 'dry' room is more revealing and challenging in my experience.
Dreadvox, why are you not sleeping?
That was an excellent response, one I will copy and keep in a separate file for future reference.
It was my crystal radio at 10 years of age that got my electronic journey going. Fortunately I had ham radio operator next door who I never met with a huge 40/80 meter antenna over his house. His signal was so strong I think I could have driven a small speaker directly. Odd the city allowed such things. Then I made a AM Radio transmitter at age 12. Those two circuits reveal all someone needs to know in how a theremin works.
Thanks again for the refreshing response.
Christopher
Christopher, I'm pretty much a night owl type
I often listened to my crystal radio when I was sent to bed but couldn't sleep yet.
I also had a Philips electronic experimenter kit and I remember one of the example circuits being a light theremin with a LDR.
Also remember making few small low power FM transmitters in my early teens.
Does your sound seem alive or more like a zombie.
Not sure, but yours is that of a broken record 
A theremin sound needing reverb to sound alive is spiritually dead and misunderstood
Let me guess how you determined that: https://www.youtube.com/watch?v=20q2AEppa1I&t=3m06s
You might be surprized by how popular of an effect reverb actually is with a lot of instrumentalists and vocalists alike.
They can't all be using zombiefied spiritually dead instruments? Or can they??? 
I have heard even some soul and gospel singers who use reverb. Now they would really be the least suspected of being spiritually dead, eh?
They all seem to think that reverb adds something that's missing in the pure instrument itself.
Opera singers don't need reverb. But they sing in large halls. Apparently it takes a hall to be filled with sound. A reverb pedal is somewhat more economical and available than renting a concert hall whenever you feel like playing a little, though.
His signal was so strong I think I could have driven a small speaker directly. Odd the city allowed such things
Why would that be odd? Roughly 1 kW @ antenna is not uncommon as the legal limit for HAM radio operators. Varying from country to country. here it's 750W on the bands you mentioned.
tinker where I live the city will give you a ticket or fine for leaving a trash can in the front yard over night. Is it a good idea to put up a 40 meter antenna over your house when even a normal TV antenna in site from the street is not allowed. In this age of cable how can you ask Why would that be odd, think about it. Many cities in the States are like this today.
Over the Air analog communication has become a thing of the past..... until a great disaster comes, seems we are getting closer.
Edit: We do not even have power lines as they are all underground. The cell phone towers are dressed up as trees, now that is funny.
Christopher
I do not have ground from a three prong plug, I use batteries.
I don't think people understand GROUND.
There are two plates in a capacitor, one is the antenna on your theremin, the other is ground. That is you instrument cable and the stand if it is designed properly.
Your instrument cable should be long and most of it should laying on the floor. That is your GROUND.
You are also GROUND. If the cable isn't sufficient put the cable in contact with YOU. You take a small section of the cable and stick it into you sock or take you shoes off and step on it. I did this often with the etherwave and if I am near radio broadcasts (big bow sf) I have to do that with the 302.
The longer your cable the better your ground.
If this doesn't work, you have a bigger problem. Try another instrument in the same location or try it somewhere else.
Rupert
Happy theremining! Once you get a long cable.
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