EW/EM theremin build report

Posted: 4/8/2020 7:23:16 PM
IsawU

Joined: 4/8/2020

Hello there,
About three years ago I was building a theremin and the community here helped me with the project.
I specifically remember something about people rarely reporting when finished. (Honestly I have no idea, what that was about *wink*)

Here's a video I made in the summer of 2017 as a convenient report form.


Short audio example starts at 0:52. (If you have perfect pitch, I recommend listening to it with audio muted)
I have some waveforms, that it produces on different settings, shown in the video.

The tuning of this thing was kind of nightmarish and the volume response could be a little better.
There is a little quirk, when you touch the pitch antenna, the pitch is so high, that it probably locks on to some other oscillations, making the pitch higher, which can be resolved by moving the hand far from the antenna, therefore introducing enough "force" to "rip" the audio signal away from this oscillation, returning to standard operation.
The video mentions high pitch which is audible in certain settings of the volume tuning knob.

If you have questions with your potential build, ask away.
I can't help that much with troubleshooting (I do have my tuning process written down) and how it works (for me, there is still some magic in it), but I might be able to help with the inductors I used, the hardware and should be able to provide my schematics, layouts and drawings.

Edit:
KiCAD files + some weird bottom and top layer of PCB hybrid + power supply and theremin schematic (the power supply has a ground lift switch, not shown in the schematic, should be possible to omit if recording through a DI box with ground lift, or not recording at all + using a non-grounded amp)
https://drive.google.com/open?id=1qAOFzKHJ1RQBQ26bM-Xo-V98v966E96r

Posted: 4/8/2020 7:45:47 PM
oldtemecula

From: 60 Miles North of San Diego, CA

Joined: 10/1/2014

Excellent presentation, woodwork and build.

Are you in the States?

Christopher  

Posted: 4/8/2020 9:02:16 PM
IsawU

Joined: 4/8/2020

Excellent presentation, woodwork and build.Are you in the States?Christopher

No, I'm from Europe.

The woodwork was quite a bit assisted, I don't have a well equiped workshop, nor the skills, so I had a relative cut the pieces out for me and after I glued it together, he rounded the edges and beltsanded it, so all that was left for me was to put on a finish.

Posted: 4/8/2020 9:17:53 PM
dewster

From: Northern NJ, USA

Joined: 2/17/2012

"The tuning of this thing was kind of nightmarish and the volume response could be a little better." - IsawU

That's pretty much my experience with the EW too.  The Big Briar EW volume response seemed lamer to me, but the pitch side was less squirrely.

"There is a little quirk, when you touch the pitch antenna, the pitch is so high, that it probably locks on to some other oscillations, making the pitch higher, which can be resolved by moving the hand far from the antenna, therefore introducing enough "force" to "rip" the audio signal away from this oscillation, returning to standard operation."

An inherent danger, I think, of the EQ inductor approach to somewhat linearizing the near field.  I wonder if locating the EQ inductors on the main PWB is the best approach?  Perhaps stringing whatever is required out on some other substrate or mounting situation might work better, as it would allow you to mix and match whatever might be needed there.  It all seems rather touchy.

Thanks for the very nice video!  I prefer the way you have attached the pitch rod to the plumbing elbow crap on the Moog EW (just waiting to fall on the floor and break).  And your volume loop only having one attachment makes more sense too.

Posted: 4/9/2020 8:41:51 AM
IsawU

Joined: 4/8/2020


I prefer the way you have attached the pitch rod to the plumbing elbow crap on the Moog EW (just waiting to fall on the floor and break).  And your volume loop only having one attachment makes more sense too.

There is also one, thing that can be somewhat visible on the test chassis, regarding the conductive connection of the antennas, that I kind of like.

I use this kind of fitting for both antennas

and while the outside nut holds only the antenna, the nut on the inside holds the fitting in place and this little thing

that connects to the board itself and solder on the copper tubing works perfectly. The wire is not very flexible, but you don't have to move it once finished - I believe that original EW has a more flexible wire.


Perhaps stringing whatever is required out on some other substrate or mounting situation might work better, as it would allow you to mix and match whatever might be needed there.  It all seems rather touchy.

Perhaps, but I tend to dislike modularity in a device that can be sensitive to every minor change. If I ever find myself with surplus of free time on my hands I might make my own theremin (which I would understand more completely) and then I can get into experimentation.

Posted: 4/9/2020 12:12:16 PM
dewster

From: Northern NJ, USA

Joined: 2/17/2012

"I use this kind of fitting for both antennas"  - IsawU

It looks like the brass ferrule is split, and mounted in rubber ring perhaps?  I imagine this makes it so you can tighten it pretty well with just your fingers?

Posted: 4/9/2020 3:21:34 PM
IsawU

Joined: 4/8/2020


It looks like the brass ferrule is split, and mounted in rubber ring perhaps?  I imagine this makes it so you can tighten it pretty well with just your fingers?

That is correct. The inside nut is tightened by a tool, since it rarely needs to be taken apart, and the antennas, which I demount for travel and storage, can be tightened by hand. It's not holding super tight - you can pull out the antennas with no special effort, but they won't fall off if you flip the theremin upside down and shake it in the air. The connection is properly conductive, even though I did not entirely expect it at first.


One thing, that I did not mention about having the small pieces of tubes in the fittings at all time, is that they also act as an end stop when inserting the antennas, resulting in (ideally exactly) the same antenna setup every time.

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