Is a LOOP essential ?

Posted: 4/8/2013 11:19:13 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

I am looking at the mechanical details of my latest theremin - the practical stuff like what available enclosures I can get, antenna construction, connectors etc - all the sort of things to turn a prototype into a manufacturable / sellable unit.

Not going to go into too much detail about this theremin - It may never get to production.. But the antennas are a major feature.. They are directional - one side is completely insensitive to capacitive change, and the other (player facing) side has a uniform sensitivity in a field of about 100 degrees, reducing as one approaches 180 degrees. These antennas are quite complex to make - they have 3 layers.. One sensing layer which faces the player, a shielding layer behind it, and a ground layer facing away from the player - A flexible PCB inserted inside a perspex tube facilitates the sensing / shielding layers (and has some active electronics on it), and foil on the outer layer of the tube facilitates the earth layer.

Implementing a volume loop in this technology requires (too) expensive moulding - (Implementing the volume antenna with the same directional nature is almost essential, as the shielding is an integral part of how this theremin works - Even though directionality for this antenna could be seen as a waste of effort, it would be more difficult to make this antenna "normal", and has the advantage that the theremin can be placed on a metal grounded table, or on a synth rack for example, and still work without problem)

 I also want easy left-right hand playing options, so I have devised a scheme which does everything I want.. But loses the loop.

Below are diagrams.. There are (or will be) 3 stereo 1/4" jack sockets on the theremin, one on the top, one on the LHS and one on the RHS. The antennas are encoded so the theremin knows whether a volume or pitch antenna is plugged into any socket, and sets itself up appropriately..

Red shows the sensing area facing the player .. The antennas can be rotated so the player can focus the field to whatever their preferred angle.

I would like feedback, positive or negetive - Would YOU be happy to replace a loop with a rod ?


The above are not to scale! Actual theremin enclosure has not been defined yet. Antennas are 50cm in length, 18mm Diameter perspex tubes with 1/4" jack plugs on the end.. the "70cm" shown will depend on the enclosure chosen - it will be this enclosures width / 2 + 50cm.. Most likely the enclosure will be 22cm width, so the distance from its centre to the volume antenna outer tip will be about 61cm.

The volume antenna is only sensitive from about 20cm from its end - volume can be changed by hand movement both in the horizontal area (louder as the hand approaches the theremin and moves away from the sensitive area) or in the normal way when the hand is above its sensitive zone.. At a future time I hope to have dual sensors on the volume antenna to facilitate other functions.


 Some case styles I am looking at: (these are combined metal + plastic - If anyone knows of a nice sloping panel plastic enclosure with a flat top section, a panel area of about 200mm x 100mm and depth of >= 30mm at the shallow end and perhaps 100mm at the deep end - Or any thing really, with a flat top for mounting the vertical antenna socket, please let me know!. is the supplier am am most interested in at present.


Posted: 4/8/2013 1:22:04 PM

From: Colmar, France

Joined: 12/31/2007

There have been several theremin models without loop on the market. In most cases they had plate antennae but none of them sold as much as the "classic" instruments with loop.

From my player's perspective I'd for example never buy a Flute or Clarinet which has not the Boehm key mechanics because I'm used to it and it is widespread. For the same reasons I'd clearly prefer a theremin with a loop in the same way I'd have difficulties in playing a violin which has no scroll in top of the pegbox. There is absolutely no technical reason for that, it's rather a personal/intuitive/emotional/traditional thing.

Posted: 4/8/2013 1:42:35 PM

From: Toronto, Ontario

Joined: 3/6/2013

I tend to agree with Thierry. Although your design has interesting advanced new features, people may still want the instrument made the old-fashioned way...

Another example of failed "improved" designs was the Janko piano keyboard, way advanced and more practical than the traditional and linear 88-key design, yet nobody wanted to learn (or re-learn) to play a new instrument, or buy a piano that did not look like a piano!



Posted: 4/8/2013 2:14:51 PM

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

"There is absolutely no technical reason for that, it's rather personal/intuitive/emotional/traditional thing." - Thierry

Thanks for that honest answer Thierry - I had suspected this.. I have actually found that a rod volume antenna was as easy as a loop - perhaps easier.. But my judgment on this is meaningless.. Because I find playing volume and pitch simultaneously absolutely impossible whatever I do!  ... I do hate plate antennas ... I find them even more than impossible - A rod feels, to me, the same as a loop.. But, as I say, I cannot be sure of my judgement.

The big advantage for me of a rod volume antenna is its simplicity - It allows a smaller theremin box to be extended by an insensitive area of the antenna, which allows the pitch antenna to be simple and positioned in the most stable place, directly above the theremin stand ... I feel that stability is far more important for the pitch antenna than for the volume antenna.

I know that many people will be put off by looplessness, I suppose I need to know if everyone will be put off by it..


Posted: 4/8/2013 3:44:47 PM

From: In between the Pitch and Volume hand ~ New England

Joined: 12/17/2010

The melodia doesn't have a volume anything... Rob loves it :)

Posted: 4/8/2013 6:02:29 PM

From: Northern NJ, USA

Joined: 2/17/2012

Not a player (hate the game, not the player!) but still have unfounded opinions I can't seem to keep to myself! :-)

Shielding has the potential to make this a real table top instrument!

I like the way you are implementing a "dead zone" near the case on the volume antenna, which allows you to use a more compact case structure.

I'm wondering if earth grounding in addition to shielding is necessary on the antennas?  Please don't reveal anything you aren't comfortable with when discussing this, I'm mainly just curious.  Without grounding you could use UHF or similar connectors, including a 90 joint for the pitch antenna so both could plug into the ends and the case wouldn't need a flat top.  But I suppose if there are active electronics in there as well that's not an option because of powering issues.

As for the loop, I'd say go with your gut, do what makes the most sense from all angles, and don't worry too much about how it might be accepted by traditionalists.  Unlike woodwinds and such, I'm not sure there is enough of a critical mass of players and builders our there to really define what a genuine Theremin is in the first place.  The fate of your design will likely be determined more by other factors (cost, availability, features, stability, etc).  Also, when introducing something radically new like capacitive shielding to the mix, you're allowed, nay required, to alter some of the physical basics.  This new design perhaps should stand out from the crowd somewhat.

I'm quite partial to the subscope layout, and am aiming for all controls on a horizontal top to facilitate the easy switching of handedness.  Makes much more sense than the orientation of EWS controls.

Best of luck on this, if anyone should be designing and marketing Theremins it's you.

Posted: 4/8/2013 7:26:15 PM

From: Colmar, France

Joined: 12/31/2007

I think that there are also technical reasons to prefer a loop: the loop itself is already directional - much more sensitive above (and below) than besides. Especially on theremins which have a wide pitch field (tVox tour, Etherwave with ESPE01), the (right handed) player tends to position himself rather towards the left when he/she wants to play in the lowest register. A non-loop theremin without other directional technologies for the however formed volume antenna would risk to be muted by the approaching paunch...

BTW: Fred, a loop does not forcibly mean that the cabinet has to be larger, because it is nowhere written that both ends of the loop have to be fixed (cf. tVox tour, where only one end enters the cabinet while the other is in the air).

N.B.: The Moog Melodia has a volume plate antenna integrated in the left side of the cabinet, AFAIK.

Posted: 4/8/2013 7:29:49 PM

From: portland

Joined: 11/30/2011

Well there are small considerations besides theirry's comments which i agree with. One is muting, It has to be stable enough to hold the instrument cable over it, and if it is directional, If I stop briefly to talk to someone, I usually mute the theremin with my hip, which wouldnt work. Ill assume youve have some more elegant method of muting anyway.

It feels natural  to rest the hand in the loop, I suppose if the rod was larger in diameter, it would work, but I dont want to grip the volume rod/or have any tension in that hand at rest. 

Largely though I think its aesthetics, the loop is going to look better. 

Posted: 4/8/2013 7:45:55 PM

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

*Brown text is technical

"I'm wondering if earth grounding in addition to shielding is necessary on the antennas? " - Dewster

No, not strictly required.. Without giving much away, I can say that the reason I am including ground is to limit radiated signals - and to keep the loading on the shield drive more constant.. I found with experimenting that, for reasons I cannot see, having a large constant load was easier to drive accurately than having a (much) smaller variable load (the load being the capacitance on the shielded side - as in any conductive / capacitance influencing / moving entities behind the theremin)

The radiated signals from the shield are also a possible "problem" from an interference / EMC perspective - The shield drive is low Z, and my operating frequency is above 1MHz (I am using active equalization - got fed up with inductors! )

Another reason for ground over the shield is that, surprisingly, even though low Z, I found that interference from noisy stuff (bad Eco lamps, mains cables etc) was discernable - certainly discernable with test equipment, but if I had a Eco lamp real close to the shield, I could hear it.. And if I drove a seperate test antenna close to the shield with another shield drive (emulating having 2 of my theremins back-to-back) I did get some "crosstalk" .. I would like it to be possible for a crowd of thereminists to be able to play together without my theremins interfering with anyone elses, and without anyone elses interfering with mine.

I dont want anything to turn up as a problem in future - IF I do manage to get this into production (and its a BIG IF - Everything about my life is being ripped apart and examined by divorce "mediators" and solicitors and accountants and other assorted vultures - and the chances of me getting through this with even enough money to buy toilet paper, let alone manufacture anything - well - Not quite infinite improbability, but close ;-).. I know that adding a ground screen gets rid of potential problems I have seen - And I do not fully understand why.. My simulations and such say that it should work just as well without ground. Radiated EMI reduction however can only be achieved with a ground screen, and this EMI could be a problem for other theremins nearbye - so the ground stays.

I want a product which will work on a rack full of synth and audio equipment, but is still attractive to more traditional thereminists - To make any money, I think I need to target a whole new group of users - the synth crowd! - But I cannot risk alienating those who are more into theremins than synthesisers. I also think that the directional technology opens a whole area of wearable playable theremins - So I am hoping to get an enclosure which is suitable all the requirements - And hoping to get this without having to invest in manufacturing it - Something I can buy off the shelf.. The wearable theremin I am not too bothered about at present - I could always have different enclosures for different sillynesses ! ;-) .

But for the main unit, I want something that pleases as many potential buyers as possible, and believe that a unit not at all bothered by the environment behind it or under it (and less bothered by anything above it than most) would give a big advantage to synthesists and "traditionalists" - And particularly to performing thereminists!

But please - dont get your hopes up!  .. I may not be able to do more than, once again, float an exciting idea which comes to nothing.


Posted: 4/8/2013 8:27:48 PM

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

" One is muting, It has to be stable enough to hold the instrument cable over it, and if it is directional,"

Hi nieradka,

Yes, muting has been catered for - as per my H1 theremins, when the reference oscillator is equal or lower in frequency than the VFO, the instrument mutes automatically (and shines a big red LED to let you know!) .. My prototype* has a 3 position mini-toggle, up is "No muting", centre is "auto muting" and down is "mute". In auto, move out of the range of the pitch antenna and theb instrument mutes.

"It feels natural  to rest the hand in the loop".

A big NO to that with my design, I am afraid.. Even if I did put a loop of some kind on the end, the leverage available over a length of 50cm is substantial - down would come thereminist, theremin and all! ... This is my major worry about my design... If the theremin and stand were of substantial weight, less of a problem - But I am looking at really lightweight enclosures etc.

This is an issue which has bothered me a lot - I had devised schemes where the horizontal antenna was attached with magnets (one can buy magnets specifically for electrical connection) so that enough downward force caused the antenna to detach and fall to the ground rather than the whole theremin tipping over - But thought I was being OTT, and went for jack plugs and sockets which are low cost for high quality, robust, and easy to fit and rotate..

Ah well - I suppose if I had large warnings in the instruction manual, and perhaps a horrible warning label (ugh! )-: on the antennas, then if people do crash their theremin they only have themselves to blame.. But I have seen manufacturers taken to the cleaners over health and safety issues like this - apart from which, causing injury for want of careful design is not where I want to go!

"Largely though I think its aesthetics, the loop is going to look better."

Yeah - And I dont think that putting a loop on the end of a 30cm plastic tube will look any better, even if it was easy to do and didnt add cost - which it isnt, and it will!


*my prototype is not a single entity - lots of seperate prototypes or parts thereof..

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