Potential EM build

Posted: 3/5/2012 1:13:28 PM

Joined: 3/5/2012

So I've gotten it into my head that I need a project over spring break and hey, since I've always wanted a theremin, why not that (even though it may well turn into spring break and beyond depending on how things go)?  After much researching designs and poking around, pricing out components, and not finding others, I'm leaning toward the EM theremin.  I am, however, at a bit of an impasse.

I've actually managed to find all of the inductors (or currently available equivalents) except for L11 (68uH such as the Toko T1019z).  That's not even totally true - I can still order one from JAB if necessary.  I have come across the option of running a 68uH variable (Toko, http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=111360052&uq=634665265201543672) in series with a 2uH fixed (http://www.mouser.com/Search/ProductDetail.aspx?R=511R-28Jvirtualkey51820000virtualkey807-511R-28J).  I did just realize they have (vastly) different self-resonant frequencies, so I may be sunk here.  They also have lower Q-values which, from what I've gathered from what I've read here, shouldn't matter -- but the fixed 2uH component has one significantly lower than the adjustable one.  Are these non-issues, or am I back to the drawing board?

Another concern I have is with the power supply and grounding.  I'd like to isolate the transformer and the PS in a separate box (like a laptop charger).  My thought is to carry the grounding wire from the third prong on the mains plug through to the component box and to the ground rail, along with the converted +/- 12VDC rails through a DIN connector and cable, or something similar.  I can elaborate more on this idea later, though.  The component issue is the more pressing one, and the PS is more of a matter of design convenience.

I have read through several threads on the EM/EW articles and theremins, and gleaned a fair bit of information.  Some of it is "obsolete" because certain components are no longer available, and I'm sure I've missed something along the way, so forgive me if my question has already been asked.

As a little bit of background on me, I'm studying mechanical engineering.  I am currently in a circuits class, so changing some components and re-analyzing the circuit and changing cap values, etc is a learning experience for me.  Any information on doing this sort of analysis as it relates to changing L values would be appreciated, as I've only done basic stuff like node voltage and mesh current analysis.  



Posted: 3/5/2012 5:22:00 PM

From: Colmar, France

Joined: 12/31/2007

The EM is a kind of predecessor of the Etherwave. It will not be as good as an Etherwave because Bob Moog optimized the circuit later. Thus I can't understand why people make still the effort to build this rather unfinished prototype.

Posted: 3/6/2012 12:03:21 AM

Joined: 3/5/2012

It's out there and available; that's probably why.  I don't really know why I decided on that one ... it seemed fairly well documented and straightforward.  I am definitely open to other options as well, if you have any suggestions.  

I do recall now seeing some mention here of improvements on the EM/EW design, such as using three coils of higher inductance in series for the antennae circuits instead of four, and using CA3083s in place of some of (all?) of Q1-8.  Is this a step in the right direction, or a wild goose chase?

Regarding that variable coil, at this point I think it'll probably be easier to stick with 66uH coil (not 68 as I said in my original post) and reevaluate the circuit and change the cap values as necessary.  

Going perhaps the other direction, does anyone have an opinion on some of the tube designs, namely the Forbes?

Posted: 3/6/2012 2:42:51 PM

From: Brooklyn,NY

Joined: 12/1/2009

Dont bother with the Forbes, unless you happen to already have most of the parts(TX especially =$$$) and you want a cute/quirky science-fair project to build with the "kids".   A lot of problems with this design... starting with the powersupply,  then plate antenna,  poor linearity and reversed pitch... ugh its a mess. 

If you have some electronics experience and you are interested in a tube circuit...you should check out mark Keppinger's tube theremin(MK1).    A handsome and worthwhile build, in my opinion.

Posted: 3/6/2012 10:04:27 PM

Joined: 3/5/2012

Poor linearity is definitely a putoff ... these things are hard enough to play well (so I hear) when they have good linearity ...

I didn't know about the reversed pitch on the Forbes theremin.  Another one I was thinking about is Art's 126, but that has the whole reversed volume thing.

I had thought about the Keppinger design too, but I haven't spec'd/priced out the components yet.  I did notice that one of the recommended transformers was pretty spendy.  It's certainly a beautiful machine, though. Add those big beautiful coils on and it's probably out of my budget range for now (which is pretty nebulous and undefined as it is). 

I'm starting to think I might push this project back to the summer (hopefully then I'll have a nice internship to help pay for it), plus it'll give me some time to think about exactly what I want.

RS, I'll try and read through your website and shoot you an email later this week.  I've got a dynamics test in a few days that I'm most certainly NOT ready for ...

Thanks for the input so far, everyone.

Posted: 3/6/2012 10:34:10 PM

From: Brooklyn,NY

Joined: 12/1/2009

oops...meant reversed volume.  reversed pitch would be totally nuts.

dont know much about Art's, but they use plate antenna so id wouldnt waste my time and cash.

here's a secret- you could eliminate the power amp portion of the kep design and thus avoid spending $200+ on hammond tx's.  Just build the osc chassis and     make a simple solid state (diode rectified) PS using two cheap 12.6v tx's back to back to get about 150vdc, with 6.3 VAC (from between the transformers) to the tube heaters.   (or use a dual tx, like mark uses for his Model K  (still under developement)

Posted: 3/7/2012 8:40:56 PM

From: Victoria, Canada

Joined: 9/4/2009

Last year i did a build of Art Harrison's "126" theremin, with some modifications:

- used a wooden cabinet;
- used traditional rod & loop antennas;
- added a circuit module to "un-reverse" the volume response
  (i did this with a solid-state opamp, which meant totally redesigning the PSU)

Here's a link to my build log.

This was my third theremin, and my first-ever tube project.  (My first theremin was the PAIA Theremax, 2nd was the EM Theremin, and i've since built a Mk1 Keppinger, which is my ultimate lifetime theremin!)

In my opinion, the 126 is a worthwhile instrument.

It sounds lovely.  I did not expect to be as blown-away as i was by how much nicer this tube theremin sounds than, say, the EM theremin.  (Of course, tone is all about subjective preferences.)  The tone control gives a nice range of timbre.

Volume response is nice and smooth, and works the traditional way with my volume reverser switched in.  The closer=louder volume was going to be a deal-breaker for me, so designing this modification was the first step in my project.

Pitch range is something like 4 octaves+.

Pitch linearity isn't stellar, but is certainly playable.  It possibly could be improved with a series inductor...? 

(I would love to see some discussion on how to select/calculate/design a "proper" inductor for linearising the pitch field.  Though i suspect there's no general-case rule of thumb...)

Posted: 7/31/2016 12:54:47 PM
Art Harrison

Joined: 3/17/2010

A while back, I altered the volume circuit for the 126 so that it can produce either volume response. The 126 theremin article is annotated with a link to the newer version:



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