coil tuning

Invisiblejelly,

In all the 'blurb' above, there is a "hidden" simple way to tune your theremin - ok, one will still need to tune the oscillator/s , and for this you can either use adjustable inductors or your magnet method.. But there is no need to make your magnet adjustable from the outside - there is no reason to have any tuning control knob on a theremin!

Oops - in the following, M3 is wrong - I used M6 ... But its not critical - M6 was easy to get and robust - You could use any thickness, thinner requires greater length, so tends to wiggle more.

My H1 pitch-only theremins never had a tuning control - I had a piece of M3 threaded studding about 7" long, and a 7"" piece of aluminium tubing with inner diameter into which the studding could just fit.. I placed the studding into the tubing, leaving a couple of inches protruding, and then squeezed the tubing in a vice to cut a thread in the soft aluminium ..

I then covered the tubing with heat-shrink sleeving, fixed this antenna using M3 nuts / washers / solder tag (to which the antenna / equalizing inductor was connected)..

Tuning was simply a matter of rotating the antenna (tubing) by hand - I had a free M3 nut under the tubing which allowed me to tighten (lock) the tubing in position when the correct length had been achieved.

This is the best way by far to compensate for changing capacitive environments - by altering the antenna length / capacitance one is tuning the EQ circuit without incurring any loss of sensitivity.. The adjustment required is extremely fine - a telescopic antenna does not give anywhere near the required precision - but 1/2 turn of a M3 thread is fine enough for precise control - one can compensate for huge variations in background capacitance and maintain precision adjustment with this method.

Fred.

ps - for extremely simple theremins without equalizing coil, one can probably get enough adjustment from this antenna that one could use fixed value inductors for pitch and reference oscillators - probably get away with doing this even when using an equalizing inductor if one plays with soldering different tank capacitors for these oscillators.

Picture is better:

Tubing needs to have thread for studding - see previous posting.

"Tuning was simply a matter of rotating the antenna (tubing) by hand..." --FredM

Fred, it's almost scary how many excellent ideas you have when it comes to Theremins!

I have long been a promoter of the tuned coil theremin antenna design. This is the future of the theremin but its application has not yet been recognized. Pictured is a simple combination that will develop a wonderful natural theremin voice with a perfectly linear pitch field right up next to the antenna. Drift is a non-issue due to balanced circuit simplicity. This antenna method also works perfectly with a tube/valve pitch oscillator. The antenna is composed of a tightly coiled spring pulled slightly apart by the tuning jewel on top. This is a broad tuned phenomenon so there is no need for linearizing inline antenna inductors. "No brag, just facts!"

Christopher

"Can we all get along?" In Memory of Rodney King 1965 - 2012

Fred said "My H1 pitch-only theremins never had a tuning control - I had a piece of M6 threaded studding about 7" long, and a 7"" piece of aluminium tubing with inner diameter into which the studding could just fit.. I placed the studding into the tubing, leaving a couple of inches protruding, and then squeezed the tubing in a vice to cut a thread in the soft aluminium .."

"Well that's a really nifty way to tune a theremin.I did some messing around with magnets on top of me EW standard box and I could get the playing field to shrink to about 6 inches from the antenna from a playing field of around 20 inches...just by putting a little ceramic magnet  1 cm.dia and 1/2 cm thick on top of the box above one of the coils...stacking some more magnets about 8 only gained an extra inch towards the antenna.I tuned the magnet stack upside down reversing it's polarity and the playing field went back to about 24 inches.So polarity affects the coils.. does that prove that there is a DC component in the coils on the EW standard?...I think you did say that somewhere.I also put a tiny neodymium magnet directly on one of the coils ie stuck it on the outside metal on the side..this radically altered the tuning so much so that I couldn't retune it with the tuning knob.This was surprising as I thought the metal box containing the coils would act as shielding.I put this little neodymium on top of the coil touching the ferrite but with polarity sideways  this put the null point right next to the antenna.

"So polarity affects the coils.. does that prove that there is a DC component in the coils on the EW standard?" - IJ

If one had no schematic, and performed this experiment, one cound not take these results as "proof" that there was a DC current through the inductor, but one could certainly suspect that there might be.. The reason that one could not take it as proof is that the inductors former could be magnetised (have a permanent magnetic field not induced by DC current) and one would then get a different result depending on theb polarity of any applied field.

As we have the schematic, these results are what one would expect - We can see that there is a dc current through the inductor by analysing the circuit, and this is confirmed by simulation.

There is nothing "wrong" about having a DC current through the inductor - it only becomes "wrong" if this current (a) varies, causing drift and/or (b) is sufficient to cause the inductor to become innefficient due to it operating at or close to its saturation and/or (c) the current is sufficient to cause heating of the inductor, which could cause drift.

Fred.

Ps.. This may just be alarmist - but I would be careful about putting powerful magnets close to your inductors.. It is possible to  magnetise ferrous materials permanently by applying strong magnetic fields to them - I do not know how susceptable ferrites are to this (being as they consist of individual particles bound together), but I would be cautious.

"Fred, it's almost scary how many excellent ideas you have when it comes to Theremins!" - Dewster

Thanks Dewster -

But its more about being forced to find solutions to tricky situations.. The H1 was a real challenge - having 16 of these instruments operating simultaneously in a public place with people playing them all day, and with any knob being an invitation for abuse and vandalism, a tuning knob simply wasnt an option (there were no knobs - all adjustments were via a closed panel at the back of the instrument and  hidden in the speaker port).

But I had to be able to re-tune the theremins regularly, and the changes in "background" capacitance were too huge and unpredictable to make oscillator tuning an option - There were 2 modes of operation - the instruments were set to loudspeaker operation when there were no other activities being performed, and set to headphone operation at other times - I did not supply the headphones, they had heavy curled leads which were not long enough and added a large capacitive influence when people wore them.. Also, there were all sorts of contraptions being moved into and out of the pitch field at regular intervals (movable scafolding to adjust the overhead lighting, music stands, large racks of synthesizers, even a large petrol powered "musical" penguin! LOL)  So every theremin had to be re-tuned at frequent intervals.

I did reasonably well catering for the demands - the antennas were certainly the best innovation (combined with visual null-point indication and wrong-side-of-null muting, which automatically silenced the theremins when no one was playing them) - I dont think I would have managed to pull the event off if I had not had these - And I never imagined how stupid and destructive people could be.. Two antenna had to be replaced because people grabbed them and trued to use them in "joystick" mode.. they were 6mm thick stainless steel studdung with a 10mm thick aluminium tube over them - and people bent the bloody things!

Fred.

" This is the future of the theremin but its application has not yet been recognized. " - Christopher

Christopher,

I know that you have seen this antenna work, and I trust your observations - the problem is that I have not been able to replicate your results yet. I have not spent a lot of time playing with 'your' antenna idea, but hope to be able to seriously get back to my lab some day and buy your specified spring (past experiments I did, I used wire round a pvc tube).

I have also been thinking about possible ways in which your antenna might work - I have proved to myself that the primary mechanism by which theremins work is capacitive, so completely disagree with your hypothesis on the mode of operation - but this is irrelevant - you have seen your antenna work, that is enough for me.

My thinking has been on the following lines.. Imagine one had a tapped inductor, each tapping connected to a small 'antenna' and these antennas being connected in a spiral round a dielectric tube.. This is 'effectively' what your spring 'is' (from an analysis perspective).. I have been thinking about the different capacitive loadings on these progressively coupled 'antennas' and wondering about the possible phase / frequency relationships as the (hand)capacitance changes and the VFO frequency changes, and how this could influence linearity.

The major problem in terms of finding an explanation, is the extremely small inductance posessed by the spring - even at your higher operating frequency, this inductance seems insignificant.

The other area of investigation is the mechanical distribution - unlike conventional antennas which have one prominent sensing 'plane', your antenna has a 'spacial' plane of the thickness of the antenna - about 1/2 of the sensing is performed (for a 1/2" former) 1/2" further from the hand than the other (front of coil) half - and there is a degree (cosine?) of sensing for the full curvature of the spring.. Then there are the complex field interactions (edge effect etc) occurring at every rotation, and across the z plane.. there is also the dielectric 'shielding' between the 'front' sections of the spring and the 'back' sections (which, I think, will effectively 'widen' the 'distance' between the front and back sensing 'planes') to weigh into the analysis.... This issue is on the edge of my ability to devise a computer simulation - but I have time, so am trying to construct realistic models to at least start the process.

It may be that the reason my experiments were fruitless is because the wire I was using was circular - perhaps the spring wire has a square or distorted shape? When designing (short range) capacitive sensors in my past life, the edge effects were critical - I wonder if there is something going on which would make the 'reason' obvious, if we simply were aware of this hidden 'something'.

Fred.

Hello Fred and everyone,

It is good to revisit this antenna subject once in a while. I once tried to construct the antenna after discovering the effect while using the spring by winding 22 gauge enamel coated wire on a wood form and could not duplicate the result. The key is the frequency used with the dimensions of the  spring, ~900 kHz.

A new observation made about this antenna is perfect linearity happens when the spring coil is suspended but when I insert a wooden dowel inside the spring as seen in the photo (more visually acceptable)  I lose the inside 1" (25 mm) of the pitch field while the rest of the field remains linear. This inside area is too high in pitch to find useful so it is a negligible trade off.

I do have someone constructing my work currently and they have made a nice design of the suspended coiled antenna. They own several classic theremin instruments so can make an honest comparison. I can find out if my original theremin design I called the "Illusion" was just that or something much more.

I have experimented with external magnets and coils. I used alnico magnets as adjustable cores in my early day air coils. What I found interesting is alnico increased the inductance while a simple brass screw in the core would lower the inductance which is another simple method of theremin tuning.

In another experiment I made the observation that when a powerful magnet was brought near an operating theremin pitch oscillator's unshielded ferrite cores its operating frequency would "momentarily" be pulled of center. The soft ferrite seemed to magnetize briefly while after 15 seconds the oscillator cycles brought the original frequency back as if degaussing the core.

The strength of a powerful palm size external magnet had a reach of up to 18" (45 mm), very sensitive and possibly usable for triggering an effect in some theremin application with a magnet mounted in the volume hand.

I use this magnet

I think what got my earlier theories into trouble "is not" that I lack the ability to break them down mathematically to understand how they work but I expressed a belief that my ideas were not purely mine and gave credit to the fact that we are not alone in this universe?

Did you know some people still celebrate that the earth is flat (England) or even the center of the universe?

Invisiblejelly, I admire your curiosity and exploration. I also have a simple and adjustable method to enhance the Bass response of the EWS, costs less than a $1. I will keep that to myself for now as my .mp3 sample has been posted at TW several times before.

http://magnets-cow.com/

Cow magnet??? Live and learn, LOL -

What makes me laugh is the claims about non-toxicity - eating magnets is a dangerous idea for any animal, sod the "toxic elements"..

When I worked in a hospital (Medical Physics) there was an incident where a child had swallowed two small neodymium magnets - they had pulled towards each other in the intestine causing severe damage.. No-one knew what the problem was at first (the child had not told anyone that they had eaten magnets) but following an ultrasound and x-ray, suspicion was aroused (by this time the child was unconsious in ITU).

The suspicion was verified after I provided the doctor with a gauss meter - the field was so strong that a small compass would have picked it up.

So the idea of cows eating magnets, and farmers being worried about the toxic effect of the encapsulation plastic etc - well, kind of reminds me of swallowing camels..

Fred.