Theremini - quavering sound

Hi!

I recently got a Theremini as a birthday present, and started to learn to play it as a "real" Theremin (pitch correction at 0, and response time at the "fast" setting). However, in the lower registers the pitch is noticeably and unpleasantly quavering, so what should be a steady note sounds like a trapped fly. It's not that my hand is trembling, because in the upper range (about 2 octaves) the pitch is stable. Setting response time to "medium" helps somewhat - there's still some quavering, but it's less prominent.

Question to those who have experience with Theremini - is this normal? Is there anything I can do about it?

I realize that this is not a professional instrument, but still, is it supposed to be that bad?

Thremini are very sensitive to external electric fields. You can try to turn off all unnecessary devices, ground the theremini with a wire to a large metal mass or water pipes, try turning it on in another room.

Thanks Valery.

I have a laptop and WiFi router in the same room - not sure if Theremini is sensitive to WiFi, but I tried moving to the corridor just to rule it out.
Also tried grounding it to the heating battery. Alas, the quavering is still there. Maybe my ad-hoc grounding didn't work?

I hope you're right and it's some external interference, but I don't know what else I can do.

The Theremini has really low voltage fields, so it's no surprise if the low notes / far field are erratic.  I was able to substantially perturb it just by laying a test lead from my function generator near the antenna.  Lowering the response time increases averaging / filtering, which statistically removes some of the noise, but at the expense of faithfully tracking your hand movements in real-time (i.e. you may notice a lag / the amplitude of fast vibrato may be diminished).

Maybe line the walls, floor, and ceiling of your practice room with grounded chicken wire?  (DIY Faraday cage.)

Or you might try better grounding your body, perhaps by sitting on a grounded metal plate (not nude!), perhaps connecting that plate directly to the Theremini ground.  You also might try lifting the Theremini ground to see if the noise gets better, and if so concentrate more on the return path between you and it.  I'm amazed that Theremins work at all, but low voltage fields are kinda asking for it.

Getting another 12V DC power supply, and taking care of grounding through either connecting it to an amplifier that is plugged into an electrical outlet that provides safety ground, or connecting the grounding screw on the back to something providing earthing took care of a warbly noise, that was caused by the power supply radiating strong disturbing EMI around. Two different Thereminis and three power supply units had this issue, and it went away when I started using a third party power supply for keyboard instruments. I don't know if these Theremini power adapters work better on USA 110 V and give issues with European 230 V, but with 230 V they turned out to be unacceptbly noisy and create a field of interference around them, which is probably also conducted by the DC cord as ripple (the cord doesn't have a ferrite RF f), but I had it even induced a noise/squeal (around 2-3 kHz) in a small tube amplifier that had nothing plugged in to its input. The low voltage fields and the sensitivity to disturbances makes a theremini unplayable when its closer that about 4-5 meters to an Etherwave, but playing the etherwave makes the Theremini play along (out of tune), and the Theremini can detect lightning when you play it while a thunderstorm is approaching, at the moment lightning occurs, even when it's still rather far off and hardly visible, it makes a dip/glitch in the Theremini's sound. 

@dewster, @DreadVox - thanks for the tips!

Don't think I'm going to build a Faraday cage - that's way too much effort for my amateurish needs ))
I'm going to try and find a metal plate or a new power supply, that seems more realistic.

@DreadVox, what was the power supply that made the noise go away? Or, what spec should I look for?

@boreg, it was sold as a replacement power supply for Yamaha keyboards, 12V DC, center positive.