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I have just recently assembled, calibrated, and test-played my OpenTheremin V3. With what little I know, I believe the instrument to be performing as expected. It sounds reasonable to my ears.

Furthermore, according to the lone entry about grounding in the FAQ, I don't seem to have a grounding issue:

Q: The volume antenna does not seem to work
Make sure the theremin is grounded well. This can be a reason for the volume antenna not working well. See the instructions for different grounding methods.

There is a chassis ground on the OpenTheremin V3 that I have yet to connect. It is recommended in the assembly instructions to ground the power, audio, and/or the chassis. enter image description here OpenTheremin V3 - Circuit Board PCB:

My power (USB -> 120v AC) is NOT grounded, and I'm using a fairly typical 3.5mm to speaker system, which I believe has some grounding in the cable sleeve only.
enter image description here

Do I need to chassis-ground my Theremin?

What should I be experiencing without the ground, and what should I experience with the ground?

Here is a quick video testing my Volume Antenna on the latest V3.1 firmware with my current chassis-ground which is a twisted-copper speaker wire tied tightly around the ground cutout, and grounded to an exposed metal pivot on my tripod. To me, the volume antenna seems a bit non-linear, but I also have the volume pot turned all the way to the right to get better line-out volume on my computer.

Saijin_Naib
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    Hi, when asking electrical questions, please post a full connection diagram. The USB power feeding your theremin is isolated from the 120 source,for example. Most likely you can reduce background 'hum' by connecting chassis ground to the shield on the audio out cable. Grounding directly to earth leads to ground-loops, which usually is more noisy. – Carl Witthoft Mar 31 '20 at 14:58

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