2 theremins next to eachother

Posted: 6/10/2013 11:00:01 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 WaveCrafter.com . ...................................

Joined: 12/7/2007

Hi QM,

You have an ability to push my buttons ! ;-) .. I was fortunate to have been sent for 3 years of my childhood to one good school - the Waldorf School in Constantia, Cape town.. For these 3 years my class teacher was Edward John Dawes - a man to whom I owe more than to any other person on this planet... He saved me, quite litteraly - I had been written off by the education establishment and child psychologists.. Some sort of "Autistic spectrum disorder" - But after a year in his class I was more capable and knowledgable than those of higher classes in schools I had been dumped from... And more important - I was happy - I was enjoying my learning and discoveries, and was not being swamped with dogma, not being bullied..

John was remarkable because he was a friend - no "sir" nonsense - we called him John.. We enjoyed every moment of lessons - and often if we got bored we were free to leave the class and go play outside.. He believed that there was no point in caging children (or any person) when they were not in a receptive state - far better to let them do something else.

The reason I mention John is because he never (or extremely rarely) answered questions - ask a question, and he gave it back to you - often re-phrased, and sometimes with reminders about related facts.. then let you go through the process (sometimes with gentle help) of working out the answer for yourself.

John was rare - even within the Steiner / Waldorf model - he had patience and devotion I have never witnessed from anyone else, and I could never hope to attain.. But I know that his method was right - Certainly extremely right for me, but also best for most of the other students.

The reason I detailed the above is because I think you can answer the questions you have asked - I will try to put some "flesh on the bones" - But telling you "yes you can" or "no you cant" would be unhelpfull .. The other reason I said the above is that, for some reason, John has been in my mind a lot lately ..

So -

First - Forget "antennas" - Primarily, theremin "antennas" are best regarded purely as "plates" for a variable capacitor - they DO NOT function (or at least not intentionally) in the same way as antennas designed for radio transmitters and recievers.. With radio, the capacitance of the antenna to its surroundings is more of a bother - Electrostatics is not how radio primarily works.. Theremins, on the other hand, work as a result of capacitance / electrostatics, NOT in the way long range radio works - Yes, there are "radio" effects, but for theremins these are a bother...

When you say "Is it possible to mount the antenna of this radio on a theremin? I assume it will change the pitch, because the electric field around it will also change due to the different diameter of the antenna."

Re-phrase that / think about it differently.. forget "field" - think capacitance - think AREA - The capacitance of any conductive objects with respect to each other is a function of 3 things..

1.) The AREA of the "plates" (or more particularly, the "overlapping / in-focus" area of the plates)

2.) The DISTANCE (or in the event of multiple "plates" the combined distances and reletive angles) of the plates to each other

3.) The DIELECTRIC seperating the plates (can be assumed to have a value close to 1, but this does change with humidity temperature etc, so does affect the capacitance - particularly noticable with theremins where a tiny change in capacitance can cause a pitch shift)

Think about the relationships which make a theremin work - You have a fixed frequency oscillator, and a variable frequency oscillator - these need to be at about the same frequency when the hand is say 60cm from the antenna... (at the "null" distance - say 60cm, the variable oscillator will be at the same frequency as the fixed oscillator - so there will be no difference frequency.. As one moves the hand towards the antenna, it sees more capacitance, and its oscillator frequency will go lower - this causes the difference frequency to increase - so pitch rises as the hand approaches the antenna) Increase the antenna capacitance, and you lower the variable frequency oscillator.. Reduce the area of the antenna, and you reduce its capacitance, increase its area and you increase its capacitance.. move something "grounded" closer, and you increase its capacitance .. etcetera etcetera etcetera ..

" if I touch the theremin, sometimes it makes no sound, sometimes it does. I thought it did not make sound when I was not grounded (standing on thick rubber doormat). What I saw was that me or my friend touching the theremin had the same effect: there came no sound. When i touched the theremin and my friend touched me, than it DID make sound. "

If you are insulated from ground, and touch a pitch antenna, you massively increase the area of that antenna, therefore greatly increase its effective capacitive coupling to the "other" plate of the capacitor which sets the theremins oscillator frequency - this other "plate" is GROUND (or the theremins internal "ground") (see postings related to ground on this thread)

Increasing the capacitance lowers the oscillator frequency, which (on a correctly tuned theremin) should increase the audio difference frequency - possibly beyond audible frequency.

Your friend touching you should lower the oscillators frequency even more - so there is no simple explanation for pitch coming back to audio.... But you are pushing the operating point well beyond the zone of predictability - you could be hearing interactions with other oscillators in the theremin, or even other radiations from lower frequenct domestic appliances.

To understand and be able to answer these kind of questions yourself, study CAPACITANCE and the operation of conventional theremins such as the EW.. If you understand these, you will be able to predict that, if you connect a bird-cage to your pitch antenna, you will probably lose any linearity your theremin may have (unless you REALLY know what you are doing, and match the antenna circuits and oscillator frequencies) .. And you will need to seriously re-tune your theremin to bring the VFO frequency up, and the reference frequency down..



Posted: 6/11/2013 12:20:53 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 WaveCrafter.com . ...................................

Joined: 12/7/2007

" What owners of Etherwave theremins discovered to their dismay when they set up their instruments near RCA's, is that the presence of an RCA once it was turned on, muted all the other theremins around it." - Coalport


If you had said "upset all the other theremins around it." it would have been easier for me to try to guess at a mechanism - But I do not understand why the RCA should mute the EW.. My only guess is that it over-drives the EW volume antenna circuit.

- Or it may just be that the little Lev's inside the RCA never liked the sound of the EW, so they told the little Bob's in them to shut up! ;-)


Posted: 6/11/2013 10:15:13 AM

From: Canada

Joined: 8/1/2008

Fred, I don't know why the RCA has a muting effect on other theremins but it is a recognized phenomenon in the world of the theremin. RCA's are very powerful and they do not have the shielding that modern theremins have. It just seems to overwhelm other theremins.


I have done all sorts of experiments with multiple theremins in close proximity and some of them behave very strangely. 


There is one theremin (I think it may be the Ethervox but I can't remember) that can be played by the RCA. First you place the two instruments next to one another, turn them both on but mute the RCA, and then you can play the Ethervox using the RCA as a trigger.


A trigga? Go figga!

Posted: 6/11/2013 2:55:18 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 WaveCrafter.com . ...................................

Joined: 12/7/2007

"and they do not have the shielding that modern theremins have" - Coalport

Shielding ?? ;-)

"I have done all sorts of experiments with multiple theremins in close proximity and some of them behave very strangely. "

Oh, they most certainly can! .. Most of my experiments are with designs / constructions I have put together - Not from a museum of theremin technology ;-) .. And the factors influencing how well theremins get on with each other are many - My H1's were 'honed' for minimum interaction when I built them and tested them at home.. but moved to the Festival Hall, they behaved completely differently, and needed alterations and re-tuning to get them to tolerate each other..

Its one of these "dark arts" where it seems easier just to fiddle about and get them to work.. once they work, it is easier to go back and determine why they work - But trying to design compatible non-interacting theremins by calculating the frequencies required to avoid the oscillators or their harmonics from interacting to produce audio or other bothersome mixtures - well, in practice, this seems to be a bit futile.

The key to the H1's seemed to be the grounding - Each theremin needed to be completely independant - no common ground... Each had its own "ground antenna".. And these "antennas" needed to be oriented so there was minimum capacitive coupling between them.. I could get away with this at the show - Players standing above (or in the strongest "field" of)  a ground antenna got best linearity - but only a couple of players found this optimum orientation, most were just there to make woo-woo sounds.

I also think that there was a massive "bulk" "real" ground capacitance in the venue - and that the capacitive coupling of the "ground antennas" to this made life easier.. It made life a lot harder at first though.

I think that grounding is of major importance.. At home I had a central low Z grounding plug board into which I plugged 4 H1's (I only ever tested 4 simultaneously - not enough space even for these..)

At the Festival hall there were 17 extension power strips chained one to the other.. Each ground connected to the ground of the theremin closer to the mains - the theremin at the end of the chain was a long way and had a long cable to "real" ground, and along the way a complex network of inductances was created by the cable... The client had paid absolutely no attention to my instructions. The theremins which behaved wost and interacted most were the ones furthest from the main grounding point.. And disconnecting the grounds gave an instant dramatic improvement, but also reduced the theremins sensitivity and linearity... Creating seperate "ground antennas" under the theremins solved these problems.


You must be logged in to post a reply. Please log in or register for a new account.