Construction help on a 2 person theremin

Posted: 1/20/2010 3:15:38 PM

Joined: 1/20/2010

I desperately need some advice from the mod community. I'm building a theremin designed to be played by two people, to accomplish this I have built a 40 inch case for a Burns B3 instrument. The problem is when I try to extent the volume antenna by attaching 20 inches of wire to move the volume antenna to the end of the case the antenna stops working. It seems to cut out whenever I try to extend beyond 8 inches. I've looked through numerous modifications and notice that often only the pitch antenna is moved outward from the "guts" of the instrument. Can anyone tell me if this is purely an aesthetic choice or if there is some technical limitation. Also when performing tests with different types of materials for new antennas I've found size to be very limited. Can the size of the field produced by the antennas be increased by increasing the amount of power supplied to the unit? I'm sorry if these questions seem amateurish, I'm far from an expert on electronics. I appreciate any help you can provide to a girl in need...
Posted: 1/20/2010 4:46:28 PM

From: Croxley Green, Hertfordshire, UK

Joined: 10/5/2005

Welcome to Theremin World.

I expect there'll be an expert along soon, but in the meantime I'll mention a couple of things.

First of all it's going to help if you don't think of the "antennas" as antennas. They are not. That is a misnomer. Each antenna is half of a capacitor (the player's body generally and his/her hands in particular, constitute the other halves.)

The second thing is that the wire connecting the volume loop to the rest of the circuit (equally, the pitch rod) is part of the "half a capacitor" - by adding extra wire you are effectively increasing the size of the volume loop, and that changes its electrical properties.

I think it would help if you could talk a little about your plans for this most unusual theremin, so that people can make suggestions that are appropriate to your requirements.

Posted: 1/20/2010 11:02:11 PM
Jeff S

From: N.E. Ohio

Joined: 2/14/2005

A schematic would help, but I don't think we'll be seeing that anytime soon.
Posted: 1/21/2010 1:57:12 AM

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 . ...................................

Joined: 12/7/2007

ok -
20" wire constitutes quite a big capacitance! - as this wire is effectively an antenna, it will see any ground presense - and if a volume antenna 'sees' a lot of ground, it silences the Theremin.

Possible solution:
There is (hopefully!) a coil (Equalizing coil) in series with the antenna lead.. The side of this coil which is connected to the antenna is most sensitive to capacitance - the other side (connected to the circuit) can be a lot less sensitive.

Remove this coil (carefully) from the circuit board, connect the extention wire to the circuit, connect one end of the removed coil to the other end of the wire, and the other end of the [b]EDIT>>[wire] should read COIL [/b] , via a short lead, to the antenna.

You should also do this procedure on the pitch antenna if you are extending the length of the wire - whilst the pitch wire will not cause the Theremin to go silent, increasing the capacitance (by adding any wire to the antenna) will cause the Theremins linearity and sensitivity to decrease dramatically.

I do not have a burns schematic - but have sketched a quick diagram to show what I suggest .. see : Longwire.pdf at Element-14 theremin-general-resources (

If there is no equalizing coil on the volume antenna, you might have a real problem.. You could try fitting a small capacitance in series with the wire (fit this capacitance close to the circuit board).. this will reduce total capacitance 'seen' by the circuit - but will also proportionally reduce sensitivity.. You would need to experiment with values - I would expect 10pF to be about right - but you may need anything from 5pF to 22pF.

I have jumped in here assuming you really just want to extend the wire, and know what you are doing! .. But Gordon is right - we really need as much information about your objectives as we can get.

[i]>>>Can the size of the field produced by the antennas be increased by increasing the amount of power supplied to the unit? [/i]

NO! NO! [b]NO!!![/b]

Field Size (sensitivity) is a function of capacitance.. To increase sensitivity, you must effectively Reduce circuit / wiring capacitance.. When you move the coils as described in my .pdf, you should be reducing capacitance on the active antenna leads, and the field size (sensitivity) should increase substantially!

BUT - You do need to appreciate that messing with a Theremin (particularly the sensitive bits, like the antenna circuits) is not anything like extending (say) the wires going to a loudspeaker.. Everything in this area is (or should be) carefully designed - wire lengths, track lengths, coil inductances - All are tuned for optimum performance... Even putting a metal screw too close to something can adversely affect performance!

It is extremely unlikely that your enlarged Theremin will work as well as the unmodified Theremin did.. best you can hope for is that it works acceptably... But there might be other things on the circuit board (trimming / loading capacitors for example) which mess up your plans even if you do move the coils.

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