NEW UK DESIGNED & BUILT THEREMIN with volume loop & pitch rod etc

I've always seen that as a good thing (in the EW). It's what allows one to do a slight vibrato by moving the arm in a vertical axis, isn't it?

"Just tried the hand-parallel-to-the-pitch-rod movement on my modified Moog EW." - Chris

Are you saying that the pitch DECREASES (goes lower) as the hand approaches the case? .. You see, its this that I cannot follow.. I expect the pitch to increase!

Fred.

I'm going to put this one to bed here, and now. ;)

The field around the rod follows the shape of the rod. However, near the ends of the rod, the field starts becoming narower, until it rounds the ends of the rods. So, the shape of the field is roughly like that of a cucumber. Naturally, when you move your hand down from the center of the rod, toward the lower end of the rod, the pitch is going to go down a bit, until get closer to the pitch antenna's lead which will cause a slight increase in pitch, unless your hand is located on the outside area of the field. When you move your hand up from the center of the field torard the top of the rod, you will notice a decrdase in pitch as well, as your hand encounters the narrower diameter areas of the field.

Hi All,

Thomas said: “I'm going to put this one to bed here, and now. ;)”

Or open a can of worms. Oh…IMHO the cucumber is a good analogy around the upper two thirds of the pitch antenna but approaching the antenna base introduces unavoidable circuitry that hand proximity can affect. The dominant one is the affects of the L2 coil not connected to the antenna but by proximity has influence on the inside linearity of the pitch field.

Have the antenna connected to L1 tuned properly and the pitch rises as your hand approaches L1. Approach L2 and the pitch will go lower. Each coil has its own influence on the theremin pitch field and this balance is what gets upset when you tune for perfection and then place them together in the same enclosure, it becomes a cat fight!

Those that have built their own theremins will have experienced some of these interesting theremin phenomenon’s and will be able to keep their personal theremin in tune for the rest of their lives.

Christopher

Edit: This vertical antenna response has been in the back of my mind ever since KevinK brought it up years ago. In my own pitch field this effect is not noticeable.

OK, LOL, :) So, yeh, I should have said the field resembles more of a cucumber in a boot then. Yeh, circuit board, plus lead, ect. ;)

OK, LOL, :) So, yeh, I should have said the field resembles more of a cucumber in a boot then. Yeh, circuit board, plus lead, ect. ;)

So it's a fruit in a boot? All clear now......................and before anyone says it, a cucumber is scientifically a fruit (but we just regard the as vegetables).

And none of this gets me any closer to explaining how the pitch decreases as the hand approaches a metal cased theremin (and its something of a stretch for me to accept enough coupling from the reference oscillator to the hand to give this effect even with a wooden cabinet)

Yes, at the top of the antenna, as one moves such that the hand to antenna coupling reduces, one will get decrease in capacitance, and therefore lowering of pitch - I can (sort of) see "cucumber" analogy here..

Ok, I have not played with a real EW enough to say what happens, let alone why whatever happens happens... But I can say that with every theremin circuit I have built, pitch increases* when there is any increase in body-to-ground coupling.. And that every theremin or theremin front-end I have played with has increased in pitch (certainly to a degree observable with test equipment even if not audible) as the hand (or test hand)  got closer to the theremin and / or to the pitch antenna..

So I am in a state, once again, of wondering if I have some fundamental misunderstanding of some essential mechanism, or if this entire theremin venture is some bizzare dream I will wake up from, covered in cold sweat.. Alas, I doubt that I will be so lucky! ;-)

So, assuming this is not a long nightmare - As I will detail below, I believe there are two reasons for severe pitch shifting due to vertical hand movement close to the bottem end of the antenna.. (1) Bad design of the theremin and/or (2) Poor ground coupling of the player.

    *Assumes tuning on the correct side of the null point - As in VFO frequency is below REF frequency

Ok, what follows is completely unrelated to cucumbers (or anything else relevant to this discussion ;-) .. Came about because I was looking over some notes I have for my book, hoping to find some clues.. I didnt.. but after doing all that I just had to make this another long boring posting, so I cut some of my notes, added some bits, subtracted some bits, and multiplied the rest by the log of pi..

If anything here looks the same as anything I posted before, it probably is!  I am losing track of the bits I take from my tome, and paste here..

Boring technical hot air follows...

In my obsessive state prior to heart failure, I was examining (and trying to overcome) everything which could affect theremin linearity, and my conclusion was and is that, unless the player is connected to the theremins ground by a constant and unchanging capacitive/inductive/resistive "network", consistant linearity aint going to happen ! ..

Player to ground coupling is, I believe, of paramount importance WRT linearity.. In the following I am discussing the pitch antenna, but the arguments also apply to the volume antenna..

If one has a theremin with no (external) ground connection, it effectively becomes a 2 plate capacitive sensor - One plate is the theremins internal ground to which the oscillator / equalizer / resonant circuit is referenced, the other 'plate' is the antenna. To change the pitch, a changing capacitance is required between the pitch antenna and this "ground".

As the player (by this I mean their hand, arm, body) approaches the antenna, they are also aproaching the theremin.. They are capacitively coupling to both the theremins ground and its antenna, the coupling to BOTH is increasing with an inverse square function, so these functions are additive - No doubt others will give the precise maths.. But what one gets is a much greater increase in capacitance as one gets closer to the antenna - a much more 'emphasised' curve.

If the player is electrically connected to the theremins ground, there will be no capacitive coupling between them and the theremins ground, because both are at the same potential.. Coupling to the antenna will be a simple inverse square function.

If the theremin is designed to convert an inverse-square function into the required exponential function for musical linearity, then this theremin will be giving a linear musical response when the player is connected to its ground..

But remove this connection, and this theremin will be highly non linear.

In reality, having an ECG pad wired to your theremin and stuck to your body is a bit awkward ;-) .. So "remote capacitive coupling" is employed.. By connecting the theremins ground to "actual" ground means that any equipment or metal or conductive matter connected to ground will act as an extention of the theremins "ground plate" - Usually, this gives a background coupling to the player of about 100pF.. Direct coupling of the player to an ungrounded theremin' s ground will probably be less than 10pF, even when really close to it..

The larger the "background" capacitive coupling of the player to ground, the less significant any coupling of the player directly to the theremins ground will become, and the closer a theremin designed to achieve linearity will actually get to linearity.

Oh, just to add a little more detail - Even if the theremin is not connected to "external" ground, it will still couple capacitively to this ground - So one never gets the worst-case situation where the [player-theremins"ground"- Antenna] is the only coupling mechanism - The theremin couples to ground, and this coupling capacitance will be in series with the players coupling to ground - then there is the players coupling to the antenna, and the players direct capacitive coupling to the theremins ground.. resulting in loads of silly little variable capacitances in series and parrallel combinations between the pitch antenna and the theremins ground...

I strongly suspect that it is impossible (due to the nature of groung to player coupling, and its variability) to design / build a truly linear theremin unless one (a) wires the player to it or (b) has a control on the theremin to allow the player to adjust the linearity to suit the environment..

And even then, perfect linearity (or linearity which has no noticable deviation) is, IMO, only obtainable if everything about the environment is unchanging.

Ah Fred.

A non EU approved cucumber; it's not straight!

I have come to know that the more I think I know about theremin behavior............... the less I understand it!

So, I figure you just gotta go with whatever's what................ and play! That won't suit you Fred as like you I used to have to know exactly what did what in all things, but even cars and their ECU's baffle me these days; but I still drive them. Actually, come to think of it even 'Smart' phones do too. It's an age thing I reckon!!!!!

"That won't suit you Fred as like you I used to have to know exactly what did what in all things" - Chris

Oh, it would suit me - I know absolutely nothing about cars (except to avoid buying one which is full of electronics, LOL ;-) but am ok with that - have no interest in what makes them tick..

But - If one is crafting something, hopefully to as near to perfection as can be achieved, then you need to know everything you possibly can about it.. And this is where im at.. If I was only interested in playing the theremin, I would have bought the best I could afford and concentrated on playing - and discovered that I couldnt... ;-)

One reason the subject of this thread (or at least the effect of vertical movement on pitch) is so important to me, is that I have seen this as one of the flaws in a lot of theremin designs, and gone to great lengths to reduce it..

If you look at my loop thread, you will see that I have a deliberate "dead zone" on the pitch antenna - the effect of this (particularly as I intended to use metal case on the theremin) is to compensate for the capacitive contribution of the case, so that pitch didnt rise due to vertical movement when the hand was close to the antenna and theremin.

Another (and IMO worse) effect is from the volume hand on theremins where the loop is close to a ground influence at the theremin.. One gets a pitch increase as the volume hand gets closer to the theremin, due to player ground coupling increasing..

Ok, the above is easily solved by moving the volume antennas operating point further from it.. And the pitch shift occurs usually at the lowest volume setting, but it still bothered me - My volume rod completely overcomes this "problem".

Fred.

"Another (and IMO worse) effect is from the volume hand on theremins where the loop is close to a ground influence at the theremin.. One gets a pitch increase as the volume hand gets closer to the theremin, due to player ground coupling increasing..

Ok, the above is easily solved by moving the volume antennas operating point further from it.."

Fred- so you you're saying that this interaction is caused by the loop (or wire running to it) being  physically too close to a ground point , like a wire or trace, in   the theremin circuit?   Or did you mean externally, but nearby ( like a huge radiator or house plumbing)?