My homebuilt theremin

Hi! I recently found out about this forum so I thought I just might post a little something about my home-custombuilt theremin here. I'm electronics engineer and also musician so when i found out about the theremin a few years ago i really wanted to build one! Took about 9months to finish from idea to product.

Here's some pictures:

link (http://img136.imageshack.us/img136/665/77114908.jpg)

Together with the Vacuum Tube VCA

link (http://img413.imageshack.us/img413/1765/18265635.jpg)

It's built around a XR2206 VCO, controlled by a traditional theremin pitch circuit operating on about 80khz. The volume controller operates at about 1Mhz, this prevents interference between the two.

This is what it looks like on the inside:

link (http://img213.imageshack.us/img213/15/insida.jpg)

The connectors is located on the backside. There's a switch between triangle and sinewave operation and a timbre-potentiometer.

The Vacuumtube VCA circuit works quite the same as regular ones but because of the size, it had to be in a separate box. It also produces a much more intrestning and nice sound, just lite the old ones. A controlvoltage is sent out from the velocity/volume controller wich is operated by the right antenna and sent to the VCA box (voltage controlled amplifier). The pitch antennas is to the left, this is because i'm lefthanded.

Here's a picture from the development and laboration of the VCO circuit

link (http://img21.imageshack.us/img21/4049/labn.jpg)

I've done many recordings with it and it works very well. The theremin is a very nice instument! I think it comes to it's very right just with a soft piano accompaniment, however just to show something more fun, here's on my latest recording with my oneman band :)

connie.mp3 (http://goto.glocalnet.net/schelinstudios/connie.mp3)

edit: Two more songs

When you wish upon a star (http://electricman.homeip.net/fritzell/when_you_wish.mp3)

alskamig.mp3 (http://electricman.homeip.net/fritzell/alskamig.mp3)

Thanks

Nice job! ... and very good playing for just starting out on theremin (I'm guessing there).

It looks like you have dispelled a little of the myth that a function generator chip won't work well for theremin.

It's interesting that you decided to do a vacuum tube VCA. That's the part people usually find optoelectronic or analog multiplier chips to be the best choice.

Nice cabinet work, too!

Don

Hi Andreas,
That is a superb Theremin.. Congratulations!
I am extremely interested in what you have done, technically - The circuitry looks quite simple, and I would be particularly interested in knowing what the range and linearity is.. You are producing a control voltage to drive the function generator.. is this based on a variable frequency oscillator sensing the pitch antenna, or based on a fixed frequency oscillator whose amplitude/phase is a function of sensed capacitance?
I would love to see your schematics for this Theremin! - I also think that if you have developed a simple way to generate a voltage proportional to capacitance, and this has good linearity and range, you could make money from such a design.. I have developed such a circuit, but it is much more complex than yours appears to be!
I am most impressed by what I have seen and heard here!

Me too. Very impressed with the instrument's looks and timbre and your playing.

But, I'm looking for a pitch antenna, and I am seeing two coils? I don't understand.

Theremin with organ accompaniment! I thought I was the only one who thought of that (recently:)

Pretty sound, and an unusual looking pitch control. Are those 2 hand wound aircoils on the arm, instead of the traditional rod?

Thanks all! :D

djpb_designs: It took quite much time to get to a working VCA-circuit. I laborated with the optocouplers, but didn't quite get it to work satisfactory. Also the principle to control a light-bulb or a LED to light on a LDR-resistor which is included in a amplifying circuit. But this never made the volume go down to bottom zero, and I always thought the tube-theremins had a nicer sound :).

FredM: The range is pretty good, it was a long time ago since i checked but at least 2 octave that is useful. The linearity also turned out very good. This is much because of the double pitch-antenna coils which is simply regular solder tin/lead. I discovered that this has very much "mass" picking up the hand while playing.

With just one, the closer i get, there will be a sort of unlinearity of the hand getting larger that the coil when coming closer, but when using two parallellcoupled, making sort of a triangle it gets very good. It is about 1/4 inch per semitone and with the hand about 6-8 inch away from the pitch-antenna it produces no sound (0Hz) while playing. This can also be trimmed in with a pot.

Here's a picture from the actual making of the "coil". It's wound around a wooden cylinder.
link (http://img254.imageshack.us/img254/5903/75262424.jpg). However, since the wire is unisolated, it's not really a coil, more of a heavy thing of metal :). There is no LC, RC or other filters in series with the antenna, it just rides directly into the circuit.

The pitch-circuit (the board to the left), senses capacitance and actually produces a very rough audiosignal, something like 0Hz-10kHz. This is made into squarewave and goes to the middleboard into a integrated circuit (LM2907), a frequency to voltage-conversion IC. This voltage is then filtered and inverted and put to the XR2206 which is configurated as "Frequency Sweep mode" or VCO mode which can be seen in the datasheet. The VCO could be disconnected and run separately with something like 3V to 6V to produce the same range as playing the antenna.

The right antenna is made of a PVC-tube (is it called that?) that is painted. Inside runs 1mm wire connected to the volume-controllerboard.

All of the circuitboards could rellay be just a large one, but since I built this theremin in small stages and the boards was tests it turned out this way.

I shall see if i can host some more recordings..

Andreas,
If I understand correctly, you have a something like a 80kHz - 90kHz capacitance controlled (pitch) oscillator, probably a reference oscillator running at 90kHz, and you are heterodyning these to produce a 0 to 10kHz waveform which you do a frequency to voltage-conversion on with a LM2907.. (?) You are effectively using 2 capacitance plates for the pitch oscillator (the 2 rolls of solder) and using geometric triangulation to compensate for the non-linearity (caused by the square law) as your hand approaches these antenna - this gives a bias towards the exponential law voltage required to drive the linear function generator musically.
Having large 'lumpy' antennas, and a smaller than usual sensing distance probably helps to keep things linear, without requiring equalising coils or other circuitry (Log or Exponential amplifiers for example).
I am interested that you feel the antennas mass is important - my understanding is that it is only the capacitive area which matters, and you could have got the same effect with aluminium foil wrapped over a toilet roll.. I would be most interested if you have played with various antennas and found that mass does make a difference!
If someone had suggested the design you have presented, I would have been doubtful (to say the least!) about its potential.. But having heard what you have produced with it, I feel that you must either be exceptionally musical, or your instrument has extremely good linearity, or both!

That is part of the fun of theremin construction ... sometimes wacky things work well! Solder used as antennas ... who knew? Of course those would be about 20 times more expensive than wirewound coils. Tin/lead solder here is going for about $20 a pound!

I am using a vactrol device in my volume loop circuit. This is a LDR + yellow LED packaged into a single part. They are very linear and were specifically designed for audio use. I have never had the pleasure of working with a reactance tube modulator type of VCA, but they are very popular with analog recording fanatics.

Don

FredM: Yes, that is all correct. It's nice to see that you know much about the electronics :-).

[i]...and you could have got the same effect with aluminium foil wrapped over a toilet roll.. [/i]

Yes.. I actually really tested many forms of antennas. Big coils of both thick and small wire, rods of iron and aluminium, springs, tools of steel or iron - just everything that i could find in the workshop. Stuff with more mass seemed to work better and i spotted the solderlead and thought, hey why not? And this worked very well, so i stuck with the idea. I guess it can work also very well with small antennas, but i guess it's needed more filtering and sensible circuitry.

It's very intresting that you say so, I never thought of it that way but I realize that the idea of using solderlead is quite unusual.

djpb_designs: Ah, sounds intresting. Is it a home-made theremin too?

Here's two more songs with the theremin.

When you wish upon a star (http://electricman.homeip.net/fritzell/when_you_wish.mp3) A standard, but quite intrestning song (and hard!). Here in sinewave-mode. The piano was pre-recorded (by myself).

alskamig.mp3 (http://electricman.homeip.net/fritzell/alskamig.mp3) An unknown (however wellknown in Sweden) song, written by Benny Andersson, member of ABBA. A little out of tune here and there, also quite long and boring but just something to show what it sounds like in triangle-mode.

Hope you like it!