Open Theremin v4.5 technical issues/qestions

Hi,
I'm new here, I've just posted an introduction message. (Hi from Paris, France!)
I've made a lot of research but I think I'm drying out and need help from experimented people llike you.

I've two problems with my OT :

First one is the sound. It sounds fine, I like the waveforms. But it is never clean. There's always an annoying noise that causes kind of a light distorsion. It's not the old floor noise from a cranked up gain. It disappears when I play a rest. Looking at the waveform, I dont see a simple sine, as if there was some interference at one point. It sounds like an old radio tuner white noise, instable, and a bit irritating.
I've got this noise with all the different waveforms

I've tried all sorts of connections : jack into a console (tried three different ones), into a guitar amp, into a DI box (tried to ground lift, the sound is very interesting but even more distorted), into an RME interface.
I've tried different cables and chargers. None was premium but they all sounded the same.

My second problem is stability. After calibration, and tuning in (I try one octave in the hand, as in Carolina Eyck's Theremin lessons), the instrument slowly drifts and I have to progressively raise the pitch knob value, until I can't crank it up more and get stuck in a setting that is hard to play.
If it was an analog design I would suspect warming time, but I dont know if this applies here.

Many thanks for your help.
All the best

Francesco

"I've got this noise with all the different waveforms"  - Francesco

Could you provide an MP3 or similar of the noise?

"My second problem is stability."

I'm guessing the tempco of the ferrite coils is the likely cause of this.  If it's in an enclosure you might try to vent it so the heat doesn't build up inside.

Hi dewster, thank you for your reply!

About the temperature, you're probably right, I didn't think about it. I like my 3d printed enclosure very much, but I'll try it stripped and see (hear!) how it goes. Thanks a lot for that.

Here's a clickable image that links to the video of a recording with the spectral view. I must say it's less obvious on headphones than on my monitors. Don't hesitate to push the volume. On airpods, it sounds like a speaker membrane vibrating. 
Hope that helps. Thanks again!!

"On airpods, it sounds like a speaker membrane vibrating."  - Francesco

I pulled it into Audacity and see mains hum of 50Hz and harmonics when you play with louder volume, but not when you play with softer volume.  It also looks like the mains hum harmonics are intermodulating with the pitch you are playing (when played loud).  I've seen this in analog Theremins (mains hum intermodulation) but don't have any ideas why it would be volume dependent.

Thank you dewster for having taken the time to study my issue!

With precise monitor, the noise is audible at any volume. It follows the general tone volume, so when it's quiet, the noise is also quite quiet, but it's there.

I have no knowlegde in electronics but I know sound signals better ; I was still suspecting some interference between several frequencies. So, I got interest in this :

From the Open Theremin website FAQ:

Q: I have a cracking sound on the audio after updating the software of my OpenTheremin . Why?

What you hear is the gate signal on the second channel of the audio. See here for more info on GATE and CV. You can remove the cracking sound by turning off the gate. To do so change the following lines in the file build.h to: #define GATE_ON 128 #define GATE_OFF 128

I did the software mod (even though it's not 'Craking') but with no success. At first I thought it had worked because the noise was pretty quieter. But it came back again. 
Since the noise can come and go, it may have something to do with the environment.

I'm thinking about getting a proper psu, like this one : Raspberry Pi official PSU Is it a good idea?

Another thing, that is maybe useful: I want to make a bigger enclosure and use larger antennas. So, I made a test, wiring the OT antenna connections to the antennas that are still attached to my old and broken Kees Theremin (about the size of an Etherwave). For the purpose, I used wires that are too long. 
It did work, but the problem was largely amplified, no to mention that the calibration was over the place.
Could it indicate that it's a problem of capturing ambiant noise/electomagnetism/something ?

As for my drifting problem, it needs further testing but I feel it's better without the enclosure. Many thanks for having pointed that out!
However, the box is made from the official OT 3D print file ; maybe they should add vents and update it...

If you have any further idea, let me know

All the best

"With precise monitor, the noise is audible at any volume. It follows the general tone volume, so when it's quiet, the noise is also quite quiet, but it's there."  - Francesco

Perhaps the lossy audio compression in the video is removing it at lower volumes?  Or perhaps you have mains hum coming in through the AC adapter, and higher volumes start riding the power rails?

"I'm thinking about getting a proper psu, like this one : Raspberry Pi official PSU Is it a good idea?"

Hard to say unless you try!  What are you powering it with now?  Could you try maybe a phone or tablet charger or similar?

"I made a test, wiring the OT antenna connections to the antennas that are still attached to my old and broken Kees Theremin (about the size of an Etherwave)."

Unless the antennas are electrically disconnected inside the Kees, this experiment isn't exactly definitive.

Grounding can really influence mains hum.  Is it properly grounded?  Try eliminating ground loops too (multiple grounds).  Is the audio output strong enough to drive headphones directly?  If so, that would be one way to avoid amp grounding.

Perhaps the lossy audio compression in the video is removing it at lower volumes

Possibly. Or we're not talking about the same noise ?

What are you powering it with now?

It's a regular phone charger. Nothing premium. I tried with different ones.

Unless the antennas are electrically disconnected inside the Kees, this experiment isn't exactly definitive.

They are. The Kees board is still there but not plugged on connected in any way. I just have the antennas wired to the OT board with long wires. I'll try it with shorter ones.

Grounding can really influence mains hum

Yeah, that's what come in all the discussions. It's true that when connected to the console they're in a system with several devices : the console itself and the two monitors are grounded, the other ones (RME interface, preamp, fader controller...) are not. 
Ground lifting with a DI makes things all over the place (I hear two notes with a stong interpolation)
Plugged to my small portable guitar amp, which is not grounded, I have the same noise issue, it's just worse.

"It's a regular phone charger. Nothing premium. I tried with different ones."  - Francesco

Maybe try one of those lithium battery USB "power packs"?  Some folks who have hum issues use them to power the D-Lev.  Then explicitly ground it via the audio port only.

Good idea. I'll try it this week. 
I had some more tests, it's wierd. I'll tell you about it

Thanks again for your support