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

Lookin' sweet André!  My preference is the "simple box on an angle" look.  Very neat layout inside too, placing the opening on the bottom rather than the top precludes stress on the cabling when servicing it, and unwanted cable interaction when closed.

You might experiment a bit with the mounting of your coils.  The windings being placed immediately next to a lot of wood might be hurting their Q a bit?  Even a cm or so of spacing might make a difference.  You can check this with the librarian, or with a frequency counter placed somewhat near either antenna.  It might be nothing too, I don't mean to alarm you over this.

The LED brightness is dependent on their efficiency - you might be able to lower it by increasing the resistance of three current setting resistors on the tuner driver board.  The analog brightness range provided by these ICs isn't the widest, and there is a weird overlap between the two current modes.

André: would it be OK if I post your pictures on the D-Lev web site?

Hi Eric,
Thank you for your kind words!
I didn't realize that the wood could have an influence on Q.
I tried to measure the frequencies but my old frequency counter is not sesitive enough or I should use a different coil.

Anyhow, moving the coils is not difficult. The inside height of the box is 60 mm, so I can space the coil equally 1 cm from top, bottom and side.
Could there be a problem if the pitch coil is too close to the Prozor DACs? I will be at 1 cm also, instead of 2 cm now.

For the LED brightness I will try 6.8k resistor value, which is the maximum specified on the data sheet (page 14).
6.8k gives 3 mA
3.9k gives 5 mA
So, with 4.7k, we should have around 4 mA.
Since the current is given by the resistor value, what is the adjustment principle by the LED parameter on the Diplay screen? PWM?

Of course, I will be honoured if you post my pictures on the D-Lev site!
These pics are not of very good quality.
I will post better ones when I move the coils... next year!

"I didn't realize that the wood could have an influence on Q." - André

I haven't done an exhaustive analysis of this, but noticed years ago when performing my hand capacitance experiments that a wooden stick against the antenna influenced the frequency in a way that a plastic stick didn't at all.  

"Anyhow, moving the coils is not difficult. The inside height of the box is 60 mm, so I can space the coil equally 1 cm from top, bottom and side."

Sounds good.

"Could there be a problem if the pitch coil is too close to the Prozor DACs? I will be at 1 cm also, instead of 2 cm now."

I would worry more about placing the coils close to significant metal and / or operating circuitry than wood.  But it is all relative, and experimentation here gives the final answers.

Since fields drop off as the inverse of the square of the distance, even small separations can help.  The moisture content of the wood is likely a factor.  Not hollering at you at all, and it's hard to know how much the wood might be influencing things (most likely not a lot) but I try to discourage folks from disassembling the coil / AFE boxes in the kit unless there's an overriding reason to do so, as the boxes guarantee some minimal distancing for the coil, and the AFE is placed off to the side where it services both the hot and cold ends of the coil (something of a compromise position).

I just noticed that both coil wires seem to be coming out of one end of the coil?  That is likely more of a problem than any proximity to wood.  It would be best to run the low impedance drive wire outside the coil and away from the high impedance hot sense end of the coil.  Think of the coil itself as an antenna, with 0% sensitivity the cold end and 100% at the hot end, and with a gradient along the winding length.  You can run your finger along the coil and see this effect on the tuner.  Running the cold wire so close to the hot end of the coil is almost certainly reducing some of the absolute sensitivity.

"For the LED brightness I will try 6.8k resistor value, which is the maximum specified on the data sheet (page 14)."

Let me know how this works for you.  The current value tries to balance dark room viewing vs. bright stage viewing, but the dynamic range isn't the best.

"Since the current is given by the resistor value, what is the adjustment principle by the LED parameter on the Diplay screen? PWM?"

The brightness knob sends codes to the analog drive circuitry in the driver ICs to control the overall brightness.  PWM is used for the grayscaling between LEDs.

"I will post better ones when I move the coils... next year!"

OK, I'll wait until then, thanks!

"I just noticed that both coil wires seem to be coming out of one end of the coil? ... It would be best to run the low impedance drive wire outside the coil and away from the high impedance hot sense end of the coil."

The drive wire goes to the end of the coil which is close to the AFE.
The sense wire comes from the farther end of the coil, through the coil tube to the sense input.
So the drive wire is inside the tube but not inside the coil.
The sense wire does go through the coil.

If I understand well, the drive wire should go to the farther end of the coil, outside of the coil and at some distance (2 cm?)
The sense wire should come from the closest end a go directly to the AFE.

Am I right ?

"Am I right ?"  - André

Yes!  

It goes like this (very sorry for not providing any guidance here earlier):

1. Orient the hot sense end of the coil so it is closest to the antenna, the cold drive end farthest away.
2. Keep the drive wire outside of and at least 1cm away from the coil; this spacing matters more the closer it runs near the hot end of the coil due to the increased sensitivity there.
3. Locate and orient the AFE so it accommodates the above, and isn't super close nor too far away from the hot end of the coil.

Thanks Eric, this is very precise and clear.
I will position the coils about 10mm away fom the wood and I will run the drive wire in the corner between the top plate and the side, i.e. about 14 mm from the coil.

Photos and first results, beginning of next year.

Is there any open build project with D-Lev cabinet with pipe antennas based on standard D-Lev kit?

"I will position the coils about 10mm away fom the wood and I will run the drive wire in the corner between the top plate and the side, i.e. about 14 mm from the coil." -  André

Sounds good!  I must say that my advice is based less on precise experimentation and more on the intuitive sense one develops after dealing with this stuff for more than a decade + an abundance of caution.  Anything that loads the antenna with C without contributing to the hand signal will reduce the sensitivity.  Can the D-Lev function OK with reduced SNR?  Generally yes, but Theremins are accidents waiting to happen, so if some degradations aren't too difficult to fix, it makes sense to address them.

"Is there any open build project with D-Lev cabinet with pipe antennas based on standard D-Lev kit?" - Buggins

Not that I know of.  Perhaps someone will take this on.  I'm due for constructing another pipe type cabinet early this year and will post the results.

Eric, I have equal confidence in your experience and in your intuition !