Let's Design and Build a (mostly) Digital Theremin!

The squeak noise has disappeared some time ago when I changed the 5V power supply.
I still have a light 50 Hz 100Hz hum.
I will build a good linear power supply with a switch to connect, or not, the negative output to earth.
It's always good to have the choice, for example to avoid ground loops if the amp is already grounded.

"I still have a light 50 Hz hum." - André

I may have asked this already, but on the SYSTEM page is the "50Hz" knob set to 1?  If not, it should be.

You might experiment with grounding via a 0.01uF capacitor and / or a resistor.  Some have resorted to USB power pack powering to eliminate hum, I imagine that many older European homes aren't wired and grounded all the best.

Yes, the "50Hz" is set to 1.

For the moment, I use a USB charger with no grounding.
I have a separate grouding wire that goes to a plug used for groung only.
The grounding of my house is excellent (typically 7 ohms) because I have two electric cars that require good grounding for charging.

I will try the resistor or capacitor grounding.

Maybe something with Earthing system, if I'm not mistaken in USA you have TN-C and we have TT in France (for homes). A consequence of TT could be that, due to wire resistance, neutral may not be at 0V. Not so sure about impact on 50Hz hum...

Yes, the earthing system is different here in France.
The neutral is connected to ground at the local transformer (20kV to 230/400V).
Each house has a local ground, and neutral is NOT connected to this ground.

Depending on the resistance of the cables and on the electrical load of each house, the voltage between neutral and ground may vary in amplitude and phase.
The maximum acceptable value is 10V. I have about 1.5V.

My D-Lev uses a 5V USB switching charger. The output is supposed to be isolated from neutral and phase and is not connected to ground.
The negative output is connected to ground via a separate wire. The ground connection is located at the I/O board.

In fact, this is not 50 ou 60Hz hum. It's a higher note.

The note is close to a G# (100 Hz) when "50Hz" is set to 1.
It changes up close to a B (120 Hz) when "50Hz" is set to 0.

I tried to visualize it on my scope connected to the output of my mixer, but the signal I get is... 1.37 MHz, the pitch oscillator frequency, and I can't see the hum!

I'm wondering if it could be generated by the D-Lev circuitry itself.
I must say that it's very weak and not audible when playing, but it's audible during the silences.
And since the silence after Mozart is still Mozart...

Video of the hum frequency change when pressing "50Hz" knob.

https://u.pcloud.link/publink/show?code=XZQIsJ5ZiMUeQ1DpcPQeW0jJkmJz14VyEX70

Grounding with a 10 nF capacitor does not make a difference.

"In fact, this is not 50 ou 60Hz hum. It's a higher note." - André

As interference, the harmonics of mains frequency tend to be stronger than the fundamental, particularly the second harmonic.

"The note is close to a G# (100 Hz) when "50Hz" is set to 1."

In Audacity I'm seeing 50, 100, and 200Hz.

"It changes up close to a B (120 Hz) when "50Hz" is set to 0."

I'm seeing 50, 100, and 240Hz.

It's quite strange.  It almost seems like noise of some sort is stimulating the mains hum notch filters.

Does the "Mon" knob control the volume of the tone?  Does the volume hand control the volume of the tone?

Do you have a USB power bank handy to test powering it with that?  This was the solution for one French owner.

Another test is to bring the TOSLINK out to an external and separately powered DAC box to see if the problem persists.