GT Theremin

Posted: 4/25/2008 12:46:18 AM
matt.britt

From: Atlanta, Georgia

Joined: 4/24/2008

Hey folks, I thought I'd share a recent Theremin project completed by myself and a friend. It was built as my senior design project, but it's something I've thought about doing for awhile.

Let's begin with the end result:
* The GT Theremin (http://mattb.alwayssleeping.com/images/theremin/theremin_32.jpg)
* Close up (http://mattb.alwayssleeping.com/images/theremin/theremin_30.jpg)
* Closer up (http://mattb.alwayssleeping.com/images/theremin/theremin_31.jpg)
* Inside (http://mattb.alwayssleeping.com/images/theremin/theremin_33.jpg)
* Filament transformer, tuning caps, internal supports for antennas, etc. (http://mattb.alwayssleeping.com/images/theremin/theremin_34.jpg)
* Detail on sliding contact for the removable antenna (http://mattb.alwayssleeping.com/images/theremin/theremin_35.jpg)
* Detail on one of the two stacks of circuit boards separated by a copper ground plane (http://mattb.alwayssleeping.com/images/theremin/theremin_17.jpg)

First of all, yes the blue glow comes from LEDs... Blackbody radiators have to be pretty darn hot to emit in the 470 nm band. :P The physical design of this Theremin was intended to create an eye-catching aesthetic to go along with such an interesting instrument. Thus the metal look, the LEDs, and the somewhat unnecessary use of a glow-discharge tube (the big tube glowing neon orange).

I wanted to build it with vacuum tubes because I had never worked with them before, they add to the aesthetic (sorry, but an op-amp design in a plastic box isn't very interesting to look at!), and I think that doing so somehow pays homage to Termen's original design and the instrument's popularity in sci-fi movie sound tracks. The design uses only vacuum tubes as active devices, though I did resort to using a single semiconductor diode in the power supply to avoid me having to build a (complicated) error-feedback voltage regulator late in the design stages. Obviously the construction methods are modern, with molded (rather than wound) inductors in the RF oscillators, protoboard and heat-shrink-wrapped header pins, lovely compact electrolytic decoupling and filtering capacitors, etc. Again, I wasn't necessarily aiming to create an authentic early-twentieth-century design.

Here is the full circuit schematic (http://mattb.alwayssleeping.com/images/theremin/theremin_full_schematic.pdf) if you're interested. It's entirely original, though I was influenced by Doug Forbes' and Art Harrison's tube theremin designs (kudos to them). I prototyped all the circuitry on several breadboards (http://mattb.alwayssleeping.com/images/theremin/theremin_10.jpg), but RF coupling issues were fairly severe with that setup so it still required some tweaking once it was fully assembled and in its present form.

The Theremin as shown was assembled over the past two and a half weeks or so, with the physical design being mostly dreamed up as we went, laid out in CAD in the evenings, and built during the day. The circuit design has taken me the past couple of months to conceive, simulate, and test. Some crucial components like the oscillators were simulated in EWB Multisim (http://mattb.alwayssleeping.com/images/theremin/12au7_colpitts_cp_v3.png) with some custom tube models that I found online and tweaked for my purposes. My friend is responsible for a lot of the good construction ideas, including the stacked circuit boards and the arrangement of the top.

So how does it play?

Pretty well! The pitch range is roughly 100 Hz - 3.4 kHz over a sense distance of around 12" (just guesstimating on the distance; I haven't measured in awhile). Volume dynamic range is about 28 dB, which is sufficient to bring the output levels from 1.5 Vpp max down to a few millivolts over the noise floor. There is a lot of harmonic content at the lowest frequencies, and the highest frequencies are obviously too closely-spaced to be very useful. I'd say the useful playable range is about
Posted: 4/25/2008 1:40:39 AM
Alan_in_CA

From: Fresno, California USA

Joined: 3/26/2006

Nice look!
Question: Does "GT" stand for Grand Turismo?
Posted: 4/25/2008 1:45:25 AM
matt.britt

From: Atlanta, Georgia

Joined: 4/24/2008

> Question: Does "GT" stand for Grand Turismo?

Georgia Tech (http://www.ece.gatech.edu/), my alma mater.

Admittedly, the name is fairly uninspired. I wanted to call it the Electrotone 5000 (http://mattb.alwayssleeping.com/images/theremin/electrotone-mod.png), but I couldn't think of a cheap and straightforward way to etch that lettering into the case, so half of the impact is lost... Consider "GT Theremin" a placeholder name for now.
Posted: 4/25/2008 8:33:31 AM
Brian R

From: Somerville, MA

Joined: 10/7/2005

Looks great--I LOVE the exposed tubes. Thanks for sharing!
Posted: 4/25/2008 11:35:46 AM
FallsAStar

From: Saint Charles, MO

Joined: 4/3/2008

nice!
Posted: 4/25/2008 2:10:28 PM
djpb_designs

From: Escondido, CA

Joined: 2/6/2008

That is an awesome idea ... inserting blue LEDs into the tube sockets! I LIKE IT!

Congratulations on creating a great theremin design!

Don
Posted: 4/25/2008 2:57:43 PM
djpb_designs

From: Escondido, CA

Joined: 2/6/2008

Ok, here's an idea for your next project ...

Use one of those RGB LEDs inserted into the pitch oscillator tube socket and modulate its color in accordance with pitch.

It might be the most accurate pitch indicator, but it would look very cool!

Don
Posted: 4/25/2008 3:41:48 PM
matt.britt

From: Atlanta, Georgia

Joined: 4/24/2008

The space in the center of the tube sockets is very small. It's quite difficult to even fit a 3 mm LED in there. Anyway, I did originally want to modulate the LED brightness with the sound, but due to limitations of the high voltage power supply and the additional complexity it would incur, I left it out. Maybe I can do something like it in the future, though. Currently, the LEDs ramp up to full brightness as the power supply warms up since they draw current from the high voltage rail.
Posted: 4/25/2008 8:01:34 PM
FredM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as WaveCrafter.com . ...................................

Joined: 12/7/2007

This is Most Impressive! - Particularly the speed at which it was designed and constructed - I am also impressed that you get EWB to simulate the design.
Once again, the simplicity of the original Theremin concept shines through. You have also paid great attention to construction tydiness.
Great project!

I notice on the simulation a figure of 502kHz.. You may find that lowering this frequency (increase L) to about 200kHz will improve linearity and extend the sensing field - This is an area which has surprised me with my experiments, and I am probably being stupid and missing something obvious, but I cannot explain.. I found that sensing field on simulation versus sensing field in reality are quite different - and that lower frequency (150 to 300k) seems to give the best range + linearity - at least with my circuits.
Posted: 4/25/2008 10:46:10 PM
Jason

From: Sammamish, Washington

Joined: 2/13/2005

Beautiful project!! I especially love the RCA tube right up front. It's like a nod to the original RCA theremins of yesteryear. Fantastic!!

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