First build, some basic questions about concepts, metal case, earthing.

Hi. First post here and first theremin build. I have some fundamental questions before starting on it. I've appreciated the theremin and its concept for some time but rather passively. Recently I was at this concert which revived my interest and made me want to build my own
http://www.theatredelarchipel.org/fr-show-730.html I'm going to build something based on the etherwave, perhaps with a bit of improvisation as I go along.

I have a background in physics and I'm not too bad with electronics so I'm not too daunted by the build but I do have some very basic questions. 

First, the case:  What happens if you use a metal case? I appreciate that this seems a rather basic question for someone who purports to understand the principle of operation but, assuming the casing is earthed then we'd have a fixed field between antenna and base. The player's hand would interrupt this field but I'm guessing that the physical size and shape of this field would make playing difficult. Is that right? Exactly the same applies if I use a metal base only (and the rest of the case in wood). How far from the case would I need to place the antennae in order to have a reasonable playing space?  Would placing the antennae on booms say 2ft in front of a metal case be enough? And, if it weren't for the issue of a field between antenna and earthed box, surely encasing the electronics in a metal box is a good thing to do. Any thoughts? 

I've referred a fair bit to https://www.element14.com/community/docs/DOC-16794/l/thereminamjphysexcellentpdf . If you look at fig. 8, you can see that in this build, each module is contained in a metal box for screening (and connected by screened cable). That screening appeals to me as good practice, as does the idea of each circuit on a separate board.  In this case, with all those earthed metal boxes in there, surely it's the same as a metal casing from the pitch antenna field point of view.

I also want to make my theremin dual voltage so that it can run on a 220v ac supply or 12v dc. If the latter then it will need earthing so a connection from metal case to ground seems like a good idea.


Can you guide me and let me know if I'm getting things really wrong here. Will the antenna be ok mounted on an insulator 10cm above a metal casing or do I need more distance? And is a metal casing simply a bad idea?

Thanks in advance for your help.

Matthew.

Hi Matthew,

IMO, the reason they tell you to not have any metal in the vicinity of the antenna is due to the design of the Etherwave (EW).  In the simplest Theremins you have two oscillators: one that is fixed, and one that is variable depending on hand capacitance.  If the pitch control has enough range you can accommodate just about any environmental C, thought the absolute pitch will probably be affected.

In the EW, the variable oscillator has two resonances that fight each other: the LC variable oscillator stimulates a second LC resonance - the series inductor and the antenna / hand capacitance - and these two resonances have to have a certain relationship in order to somewhat linearize the nearfield pitch response.  Environmental C can be rather variable, and the pitch knob on the EW varies the fixed oscillator, so the user is kind of stuck in terms of accommodating this variable environmental C because there is no easy way to adjust the resonance relationship (there probably should be two pitch knobs on the EW, one for the fixed and one for the variable oscillator).

I imagine the EW design could probably function fine in a metal chassis, though you would have to make allowances for the increased C.

Theremins necessarily expose fairly sensitive points to the environment via the antennas, so I'm not convinced that a metal chassis will help that much in terms of reducing interferers.  The earliest Theremins used tubes, and so metal chassis, and one of the reasons for the long EQ coil was likely to distance the antenna from the chassis.  On the other hand, a wooden cabinet isn't entirely transparent to the C field.

The paper you point to is rather academic and rightly tends to get criticized around here.  Their antenna static C formula is probably pretty close, but their variable hand C formula is way the hell off of any real / simulated data I've generated and analyzed.

You really need a digital oscilloscope to do Theremin design / construction (minimum 2 channels & 50MHz bandwidth - I've got a Tektronix TDS210 that is entirely sufficient).  Plus an OK DMM.  A function generator that goes to 1 or 2 MHz is really handy for researching resonance.  An LC meter that goes sub 1pF/1uH is nice to have now and then as well (particularly if you wind your own coils).

Hello Dewster,

Thanks for your helpful reply and happy new year to you (my excuse for not getting back to you sooner!). Happy New Year to everyone else on here too.

So you've reassured me that I'm not doing something too unreasonable by attempting to use a metal case (and yes, will need take into account the increased C). At least I'm encouraged to the point where I'm going to push ahead with it on the assumption that it probably won't be a disaster.

You also said "there probably should be two pitch knobs on the EW, one for the fixed and one for the variable oscillator"...... and I think that's a good idea. I'll probably build that into mine, it seems like a good addition for not much trouble. That does bring me onto questions about the choice of oscillator. I'll start another thread about that. Hope that the right etiquette, otherwise this just turns into a "my build" thread whereas a thread on each subject is far more useful to others, right?

Your list of what I need for theremin design / construction is mostly ok from my end....... but I have an analogue scope, not a digital one. What's the big advantage of a digital scope in this case? Is it that you can perform manipulations to simulate the mixer or is there something else? I'm a cheapskate on the function generator and just use a software one on my pc.

Matthew.

Hello Matthew,

You can post just about however you like here, whatever seems to be the most appropriate for your particular project.  I keep a couple of project threads going and they can get fairly wide-ranging depending on what is happening and who is participating (alas, I fear most are stuck over in the book of mugs these days).

A decent digital scope will have good trigger delay, which can help you understand sensitivity, environmental noise, etc. in resonant LC circuits.  A function generator that can go to 1 or 2MHz can help you play with LC resonance independent of all the Theremin stuff. I've found both of these invaluable in understanding what the heck is going on (they didn't teach us much about inductance in EE school).