"Build the EM Theremin" - altered component placement - comments?

UPDATE: It (mostly) works!!! :-D

See my post with that same headline on page 4 for details.

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Hallo out there! First post here :-)

Thus, short intro first.

I'm new to theremins and building them. Well I know of the existence of the instrument for many years and had in the very deep back of my mind the idea of building one some time, and an aquaintance recently pulled this idea back to the front again ;-)

Do you know this video where scientists filmed a caffeinated spider building a web? That web looks like my knowledge of electronics.
Something a curious kid in adult's body put together, seen things here, tried things there - no formal education with a well rounded curriculum and evenly distributed level of detail.

I have looked at a few different theremin circuits & read about the basic principle of operation.

Read a bit in this forum. I already had ideas for a minimal design, one oscillator per "antenna" and rest digital - from what I've gathered the "dewster"(?) guy will be pleased, others not so much :-D

But this thread is about building the Moog theremin depicted in the PDF floating around the net, "Build the EM Theremin" (and the hotrod pdf).

Let me mention that I'm going to build this twice, for myself and also that mentioned acquaintance who is a total beginner in EE, but has more right to call himself a musician than I have ;-) He is young and I'm not rich either, still don't want to waste much time doing this whole thing on perfboard (& debugging that, hah) so I'd like to have a PCB made at a place doing this resonably cheap for 2-layer boards - if I reduce the size compared to the main circuit board in Moog's article.

So I just made a PCB design out of the recommendations in the document, then shoved the functional blocks around a bit and some parts of some blocks closer together.
I tried to keep *seemingly* reasonable distance between the different oscillator coils, the AC part, and the antennas to the sides of the boards, and the front panel connector such that flat cable will not lie over any coil.

Since not visible in the editor, just in case the inserted images don't work, here links:
layout
rendering

Does that make sense? *Are* the distances acually reasonable?
Would this work, from your experience, or are there problems with this layout?

(red crosses mark antenna board connectors)

I intend to put the "antenna" coils on extra perfboards like shown in the document, keeps cost down.

Note that the LC osc inductors on my board currently are placeholders, when I found the adjustable ones (or created the pattern...) I'll replace them all.

I plan to use a wooden case similar to the standard, but put some harder cardboard stuff as back panel where I'll put the AC jack, since I don't like to drag the AC lines over to the front panel near the audio stuff.

From what I read here, to use the exact "antenna" inductors (those bourns ones) instead of some very small very cheap ones is important? Can someone explain in detail why? Things like capacitance of the coil?

What about the "antennas" - could I use plates instead rods? What's the difference in response? In my imagination, using plates could be more efficient than rods, since such a small hand vs. the usual rod length is using less of the rod to form extra capacitance than a more concentrated plate?

How does e.g. the pitch antenna influence the LC oscillator, when additional capacitance is formed by a "grounded human" approaching, when no part of the LC is grounded itself?

As for grounding - I see no earth jack or something in the circuit, is that not necessary? *confused*

The commercial version has only 3 of those large coils at the antennas, not 4 (only seen the PCB foto), why?

Further comments?

BTW: When using rod antennas: Does this pitch linearization extension module from Thierry also work with this here theremin or only with the commercial one?

Tinker think long and hard about what you are doing. You can buy a new working board  here stuffed with the parts for $96 usd. You will learn more by what you are doing but theremin research is more about discovering what does not work and this is why hands on practical is most important.

Christopher

Tinker think long and hard about what you are doing. You can buy a new working board  here stuffed with the parts for $96 usd. You will learn more by what you are doing but theremin research is more about discovering what does not work and this is why hands on practical is so important.

Christopher

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Thanks for the link, 96,- USD is not such a bad price. (EDIT: + shipping 42,-..50,- $US )  "My" PCB manufacturer will get me *two* of depicted layout boards for 33 EUR (~42 USD) plus shipping within Europe, if I ditch soldermask & labels which I don't need for those humongous THT components, which I chose b/c that aquaintance wouldn't mind also soldering his own board as an excercise. Considering that I have almost all of the parts lying around anyway besides those (physically) big inductors, my DIY price could be considerably better. As for excercise, this would be the first board that I'd have made, so I'm axious to gain some initial experience with that & won't put it off for other projects as well & waste more time with perfboarding everything - so there's a source for extra motivation to do a PCB ;-)

EDIT #2: I will, however, offer this alternative to the aquaintance anyway, maybe he'd actually prefer to opt out and get his hands on a working theremin more quickly, so the link is def. appreciated. (he does want to learn about electronics & building things, but I don't know his priorities for this particular project).

I, for one, would not want to buy this.
I'll DIY or die, just like I'm doing with my synth projects :-D Hopefully soon with PCBs instead copper eyed boards with yards of "coil wire" and QFP adapter PCBs ontop ^^

hi tinkerdude,

some parts are quite difficult to find. like variable inductors or coils. so make first sure you can gather all the needed parts first. many posted about their moog-clones, that it doesn't work, no sound, hissing noises hums etc. 

so thats the cool about the ready made board. you could still diy the "hotrodding" for  v/oct-cv and gate and expand your  diy synth with it. 

some people built succesfully a 145theremin (and others of course) from here:  http://www.theremin.us  

but since it's about theremin, everything else to say is relatively subjectiv, regarding sound, playability etc.

so have a cooltime and keep the iron hot.

dani

some parts are quite difficult to find. like variable inductors or coils.

Oh yeah, I found that out meanwhile, those darned inductors... I've found the huge fixed ones for the antenna linked in some thread here. But the newer version's 47µH variable ones mentioned I could not find anywhere.

What if I tried to "shoot" some cheap old variable coils with can on eb*y with "any" value but suitably big can & unwind the wire & put my own on it, just more perhaps? I guess I might spoil the parameters like Q doing that.
Then again, I built just the variable pitch oscillator on perfboard using some cheap 1/4 W resistor style case inductor (plus extra transistor to buffer) and it works. Don't have anything yet that displays such slight frequency variations from hand distance changes yet, though, but if I *touch* the antenna I do see it on the scope ;-)

Ah, can someone explain to me what exactly the "antenna coils" do? I would have thought that they were to keep peace with neighbors listening to AM radio back then, but I read someone mention here something about altering the response curve - I wouldn't know how to calculate that, though...

many posted about their moog-clones, that it doesn't work, no sound, hissing noises hums etc. 

I must admit, that sounds a bit discouraging, when rebuilding something pre-designed (more or less) fails to work. Though as a kid I also built a lot of things that didn't work :-)
I do have an oscilloscope, even if it's just a chinese one. (to have a 100x probe would be nice here on the oscs, but oh well...)

you could still diy the "hotrodding" for  v/oct-cv and gate and expand your  diy synth with it. 

I read that PDF, it's not interesting for me personally. Though I had in mind to perhaps build a pure MIDI controller "theremin" - but actually I doubt that I'd even try to become a proficient player of this son of a gun of an instrument control non-mechanism, I read about "it all", "now you may breathe here, but watch your chest", well, basically... :-D
My DIY synth is of own design (save for copying a certain well liked VCF, I hope uncle Bob forgives me ;-) I'm not advanced enough to design my own & it's not my goal on that level), and won't have v/oct inputs. It's digital every inch before the mixer / VCF, probably lame but I have a chance of designing greater parts of the whole thing with roughly my current knowledge plus what I learn along the way, without everything exploding over my head ;-)

Hello Tinkeringdude,

I really dont advise messing with the inductors unless you really know what you are doing! - Quite honestly, unless you are doing this with a mind to learning, and have 'scope and frequency counter and time and patience, buy a ready built EW board! - It will end up a LOT cheaper!!

However - You can use 42IF106 IFT transformers to create the variable inductance values you need, without unwinding them or modifying them in any way.. There are enough taps on these to get whats needed, and between pins 2 and 3 you can adjust from about 58uH to 105uH (the parts do vary a bit in their adjustment range - table below is pushing extremes of adjustment - I prefer to be nearer the mid point ;-)  but better is to put one in parallel with another to allow one to be a course adjustment and the other a fine adjustment. (inductances in parallel  L= 1/((1/L1)+(1/L2)) so putting the (L1) "1-3 + 4-6" || to (L2) "2 - 3" allows for real fine adjustment with L1 (a couple of uH to about 6uH) and course adjustment on L2, and a span of 54uH to 98uH.

But as I say, unless you are prepared to battle the theremin beast, buy a Moog board !

Fred.

 Oh, not shown.. 1 to 3 is 680uH at centre. 2 is a tap on the winding 1 - 3, 4 to 6 is a separate winding that can be connected to increase the inductance of any other winding or tap, 2 or 3 must then be connected to 4 to keep the phase right.

"Ah, can someone explain to me what exactly the "antenna coils" do? I would have thought that they were to keep peace with neighbors listening to AM radio back then, but I read someone mention here something about altering the response curve - I wouldn't know how to calculate that, though..."

I do wish the article by Thierry on this subject was in the technical repository here somewhere! - I will try to find the link (or someone else may) because its essential reading.. Ok, there is some controversy these days, but the undisputed function is that it gives a massive increase in sensitivity IF tuned correctly, and I believe that IF correctly tuned it can greatly increase linearity.. The key though is that tuning this inductor (as in, tuning the oscillator to cause it to be close to resonance in the far field) is critical.. There is absolutely no point in slapping a big inductor onto a theremin not designed for one...... Bob Moog's theremin's are designed for them and they are critical to the performance of these.

Full table based on measurements of one IFT at extremes of adjustment

tinkeringdude, there is a huge value in knowing what you are up against, and in having as much experience as you can with established products.  I personally bought an EWS a while back, and lately the Theremini - mainly as research subjects, but also for doodling around with, so I believe I am acutely aware of their associated strengths and weaknesses.  Had I known about the EW raw board I most likely would have opted for that.  Anyway, you can buy these things and resell them later for not too much in the way of financial loss.  Certainly less than the cost of making a prototype or two, that chances are won't work all that well.  I'm not saying the EWS or Theremini are the best examples, but they are standards against which you can judge your own work.

You can breadboard your design on a standard plastic breadboard and have it work pretty good, even at >2MHz, so I encourage you to experiment for a while before committing to a PWB.  Theremins are all about LC oscillators and inductance/transformers in particular, so the more and the sooner you understand coils the better off you'll be.  Oscillator feedback is critical, as there are designs that won't recover from a stall.  Download LTSpice and use it to gain insight into everything you do.  Oscillators that simulate well (don't need a kick in the pants to start them up) tend to behave well in real life.

Thank you both for the tips so far.
Neat cheap replacement that transformer ;-) I might try that.

Breadboard, I have none, could be quicker I guess, but never liked it.
I use perfboard with copper "eyes" (not the strips, it confuses my brain that one dimension is preferred there conenction wise ^^). It "wastes" some money to throw away most experimental perfboards at some point, but not too bad. At least if I sneeze the connections still remain intact :-D

I understand coils very basically, but it's the one basic electronic part that I have the least experience using, much less calculating (self winding) and taking into account anything beyond just the inductance.
So, yeah - there is learning potential - it's also why I built just the one oscillator and experiment with it.

The idea of cloning the EW is actually what you mention - having one widely known standard device and get a feeling for it. There is great reluctance paying 400 EUR for such basic thing because uncle Bob's name is on it. (or now 300,- with the Theremini you mentioned, not sure that'd be the first kind of "Theremin" I'd like to get to know, though).

I would have made the PCB exactly as suggested by the moog article, if that wasn't so huge that it'd drive the cost up immensely.
Cloning such a small-ish (thanks to the OTA I guess ^^) circuit seemed more or less foolproof to me - except for the slight insecurity in that antenna coil & component layout / proximity business.
Hearing of "many people reporting it doesnt work" certainly makes me think again.
But it's not like I have never built anything that worked, if it sounded that way, describing of what I did 15 years ago :-)

I do use LTspice, but from the moderate experience I have in this program, stuff in there is more oscillation happy than in the real world - or vice versa, depending on what you do absolutely *not* need at the moment :-D
Take the variable pitch oscillator of the EW - I changed it to work with single supply but forgot to change the resistors for it to work with 5V instead 24V r-r. It still oscillated in spice with 5V, but heck not on my board :D
(I did use the spice supplied model for bc547 which I also used for real)

Generally those are not bad points you raise, I'd guess you're right about those instruments not losing much value when reselling. Unless I manage to damage them, that is ;-)

I guess now I'll wait for the other guy (who also wants one) to reply whether he'd prefer to buy that ready-to-go EW board, and then decide based on that how to continue.
If I'm on my own, I'm ready to fail. If I can't debug it within 1..2 weekends, it might be on hold for months until I understand this or that detial better due to doing other stuff. No probs ^^
And btw, as long as the problem is not too close components - my PCB will have extra pads 'n stuff for bodge work :-)

But... it's not like there wasn't that plywood box with a few dozen knobs & LEDs on it in that corner, jealously staring at those pile of inductors that just arrived for the attention I suddenly give them...
Initially "theremin" was, for the time being, thought as a side project (ya know "just clone that tried & proven thing") & getting into this stuff more deeply later.
Well I'll certailny further play with & examine that one oscillator & see what I get it to do "antenna" wise.
Perhaps a decision to the whole project will make more sense after I gained some experience in this greater distance proximity sensing.

"Take the variable pitch oscillator of the EW - I changed it to work with single supply but forgot to change the resistors for it to work with 5V instead 24V r-r. It still oscillated in spice with 5V, but heck not on my board :D
(I did use the spice supplied model for bc547 which I also used for real)"

Yeah - I have no idea how you managed to get it to run even in simulation! ;-) .... How far in time did you run the sim, and how did you set the start conditions? ... Wouldn't surprise me if what you were seeing was a transient anomaly.. you could probably get the board to produce a few hundred cycles before it petered out if you whacked the supply just right ;-) .. did you check the amplitude of the oscillations? sure they weren't 50uV rather than "real" ?

There are actually loads of better oscillators to use with 5V !

Here is my advice - If you want the EM/EW, build it as close to the original as you can manage - inductors, supplies, the lot... Or buy a board.

If you want to tinker, use Dewsters simple series LC fet oscillator, if you want a low voltage oscillator that replicates the EW, use one of the many parallel LC oscillators and use the value inductor and capacitor for the tank, and the same value antenna inductor..

But dont go trying to make Bob's oscillator run at 5V! - not unless you are a real analogue oscillator genius.. I dont think theres any chance that even Bob would have done it - he would have had the good sense to use a more suitable circuit!

If you want to tinker, I suggest something based on the Silicon Chip theremin - You could build an Enkelaar clone - I have schematics and photos at element 14 - just search "Enkelaar" there, heres the schematic

Oh, you cannot add an EQ coil to this circuit, but it is playable, and < 1/2 the price of an EW board for the SC kit ... But I would buy the EW board! ;-)

"I use perfboard with copper "eyes" "

Good choice! - I think thats what most of us call breadboard these days - strip board is almost never used by anyone anymore. A small solderless plug-in breadboard is extremely useful to have for quick tests.. Often I work with both at the same time, particularly if I dont know what im doing ;-) ... Build a small section on the plug board, F about with component values, measurements etc, move to breadboard when its working, build next doubtful section on plug-board, interface with wires to previously transferred section if building a complex circuit, and so on.

Fred.

"Here is my advice - If you want the EM/EW, build it as close to the original as you can manage - inductors, supplies, the lot... Or buy a board."  - FredM

I second that.  The parallel LC tank oscillator with series EQ is a strange beast that requires careful design and tuning.  You could clone it perfectly and not know you got it right if you aren't fairly familiar with how it ought to behave.  I personally don't think it's worth the trouble, but then again I don't have a lot of experience with analog Theremin design (and don't intend to, but I guess you never know).