EM theremin build

Posted: 12/8/2012 1:32:36 AM

Joined: 5/9/2012

Hi again all,

Still chuggin away when I get the chance, and had a question about the fixed coils.  I have some 10 mh coils (although measured seems to be closer to 15 for each winding) that are similar to http://www.digikey.com/product-detail/en/EH35-3.0-02-10M/817-1027-ND/1928606 

The layout is quite a bit different than the ones used in the schematic and bill of materials, and the actual inductance seems to be off by a good amount from what it calls for aswell. 




Posted: 12/8/2012 3:13:39 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

Those inductors are useless for the EM.

You need Hammond or Bourns/Miller inductors as discussed in multiple threads here on TW - and the reasons why you cannot just use any inductor are also discussed.

Some things to note are the low SRF (means the capacitance is too high) - Also, tolerance +/- 30% (which will easily give you the readings you are seeing, as inductance measured depends on the meter you are using and its frequency).. And there is no specification on its thermal charactaristics - could vary hugely over temperature.

See http://www.thereminworld.com/Forums/T/28479/problems-with-ew-build?Page=1

Or search for miller here.


Posted: 12/12/2012 7:33:39 AM

Joined: 5/9/2012

I have the correct coils for the antennas ordered and on the way! One other question about the variable 68uh coil in the schematic, the variable ones I ordered state 66uh variable inductors, and after playing with several, they only seem to go up to 62 ish uh.  They will go down quite a ways but won't go any higher than that.  Would it be possible to use a couple of those inductors, turned down about halfway ( to give about 30 uh each) in series a couple inches apart and fine tune it that way when it comes down to the final tuning?  I ask because I have been searching the websites I buy from and have had a heck of a time finding the 68 uh inductors. I thought I had it close with the 66 but I guess not. 

A thread I found http://www.thereminworld.com/Forums/T/26293/toko-inductors-needed-for-em-theremin?Page=0

Seems to say ( on the first page) that the revised circuit someone found for this em theremin actually uses two 47uh instead of a 100 uh , so I was hoping the same would work for the 68uh. 


Thanks for your patience in answering my noobish questions. Still in the learning phase for all this, and while there's still a lot I don't fully understand about the more intermediate to advanced electronics, I feel like I'm learning quite a bit from this project.  I try not to ask questions until I've searched quite a bit for an answer, and either I'm bad at researching or there's just some things that don't seem to have a lot of info out there.  I'm excited to get this built and start learning how to play it.  Every now and then I'll take a break, lay out the pieces I have on the table in front of me, and start playing air-theremin and humming.. My wife probably thinks I'm losing it. 

Posted: 12/12/2012 8:38:44 AM

From: Colmar, France

Joined: 12/31/2007

"Normally" these variable inductors have a tuning range of about +/- 20%. 

In order to simplify things (100uH and 68uH variable inductors are more and more hard to get in the 10mm housings), Moog replaced them by a 47uH variable inductor in series with a 47uH fixed inductor on the pitch side and a 22uH fixed inductor on the volume side.

The needed variable inductors (TOKO EZ1017) can be found here 


Posted: 12/14/2012 5:01:25 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

There is another way to get the required inductances - a way I have been using quite a lot because of the difficulty in obtaining and stocking various values of adjustable inductors (and also extremely useful in tuning the Lev oscillator).

One can connect inductors in series, where the total inductance = L1+L2.. Or one can connect them in parallel, where the inductance = 1/((1/L1)+(1/L2)..) - As fixed inductors of differing values are more easily obtainable than variable ones, I use a 42IF106 IF transformer from Mouser, and take the required tappings to obtain the adjustable inductance I want - either in parallel or in series with a fixed inductor.

It should be noted that only IFTs like the 42IF106 which do not have a fitted capacitor, can be used in this way.

I have measured the following inductances across the 42IF106:

Pins 1 - 2  (103 turns)    Up = 221.8uH  Down = 517.7uH

Pins 2 - 3 (50 turns) Up = 43.6uH   Down = 114.6uH 

Pins 1 - 3  (153 turns)   Up = 1.16mH   Mid = 680uH  Down = 489uH

Pins 4 - 6  (27 turns ?)  Mid = 20uH Down = 29uH

Pins 1 - 6 with pins 3+4 connected, L across pins 1 and 6: Up = 703uH  Down = 1.6mH

Up = Slug as high (near top of can) as it will go comfortably, Down = Slug to the bottom.

It should be noted that although it is possible to obtain the entire EW tuning range using pins 2 - 3, adjustment is quite course doing this - My preferred method is to put a 100u in parallel with pins 1 - 2, as this combination gives a comfortable adjustment range of about 69uH to 83uH, or using pins 1 - 3 gives 83uH to 92uH, so one could have "handbags" or links to select which winding one uses, and cover 69uH to 92uH.

One can mix and match series and parallel inductors - Rule of thumb is that if the tunable inductor is in parallel with a fixed inductor, the bigger the tunables inductance is, the less the adjustment range will be, and therefore if one wants fine adjustment, select the largest available combination from the 42IF106.

One can also use the 42IF106 configured as a 704uH - 1.6mH in series with the antenna coils, to give one some fine tuning on the antenna equalization.

One other nice thing about the 42IF106 is that its not a tiny component.. It fits on a 0.1" board if you angle it at 45 degrees.. It is also quite easy to dismantle and rewire - (I have calculated the core AL as about 29 when in mid position.. and about 21 with slug at top and about 48 with slug at bottom - but this is not absolutely consistant and I suspect measurement errors due to meter frequency shifting)  one can quite easily remove turns to reduce the inductance.. This calculator helps!



Posted: 12/15/2012 1:56:49 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

ok - I HAVE NOT TRIED THIS.. but it looks to me like possibly an ideal solution to tuning tank inductors..

If one uses two 42IF106 IFT's in parallel, with no fixed inductors :

IFT1 pin 1 to IFT2 pin 1   (top of the combined inductors - to +Ve)

IFT1 pin 2 to IFT2 pin 3   (bottom of the combined inductors - to transistor / antenna CCT)

IFT1 is adjustable from about 44uH to 115uH, and would be the COURSE adjustment

IFT2  is adjustable from about 490uH to 1.164mH, and would be the FINE adjustment

In parallel the combined adjustment range would be about 40.4uH to 103uH

The FINE adjustment would be about 3uH of the total inductance when IFT1 was tuned to minimum, and about 11uH when IFT1 was at its maximum inductance.

Cost of a 42IF106 is only about 50c more than a fixed inductor. By setting IFT2 to mid position, then adjusting IFT1 as close as you can get to desired frequency, one should be able to finely tune the frequency with IFT2.



Posted: 1/2/2013 6:03:04 AM

From: erf

Joined: 9/15/2012

Hi luc234,

My senior design group just made a theremin. We used the EM Theremin as our primary reference, as well as Fred's helpful comments! Fred, we cited you :)

See the "Final Paper (II)" for our discussion and my github for the eagle pcb and multisim files.



WARNING: At the time of writing, the inductors need to be isolated from the ground plane in the eagle file! Will update that soonish. Also, need to change orientation of a couple of the silkscreen images.

I hope these help!

Posted: 1/2/2013 7:28:35 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

Hi AntiVapor,

Impressively presented paper! Thanks for the citation, but I really dont merit it - nothing you used originated from me, I was merely quoting Dr Bob Moog - The way it comes over in the paper one would think that I was the originator and Bob had got the ideas from me! - This does make me feel a little uncomfortable.. I dont deserve the citation, particularly in that context - Also, I dont actually do my volume controls in that way...... LOL ;-)

Anyway, Glad I was of some help .. And I do appreciate being acknowledged!

Its a bit of a shame that you used a comparator to produce the difference square wave - its a really messy way to do things! - All you needed was one D FF (1/2 4013), with reference and variable oscillators driving the clock and data inputs - this would have given you a far cleaner, independant square wave output at the difference frequency.. And you could have legitimately have cited me as the inventor of that! ..  oh well.... ;-)


I have no doubt that the layouts will be a useful resource for theremin builders - Seems (have not had time to really study them) that the EM Theremin front-end analogue stuff is all there to be used/abused by analogue theremin builders..

Posted: 1/5/2013 5:14:30 AM

From: erf

Joined: 9/15/2012

Hi Fred,

Thanks for the feedback. There is definitely going to be a version 2 and Ill try that out!



Posted: 4/30/2015 12:58:48 AM

From: Moncton, New Brunswick, Canada

Joined: 4/24/2015

Luc234... how did your EM theremin turn out?  you last posted in 2012.... i just read through this thread and i'm dying to know!

lots of good info here.


especially about the amps... i was wondering about just working with a guitar amp.  i see that moog sells 'theremin amps' but i thought that might just be a marketting thing... but i see here there is an issue with overdrive.


i too was thinking of building LM386 amps for each of the 13 theremins in requiem for radio... i've built LM386s in the past and they're lovely and sweet... but maybe don't have the necessary oomf.  Good info here!  I'll do a search and see if there are some threads dedicated to amps now.


and Luc234.... how did it turn out????


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