EM theremin build

Hi there,

I'm looking to build a Theremin, and so far the best info I've found on an entire build is a pdf from cs.nmsu.edu . ( http://www.cs.nmsu.edu/~rth/EMTheremin.pdf ) . It's written by Robert moog, has detailed instructions and a parts list. I was just wondering if anyone's had experience with this particular article and if so, what's their opinion.  Also, does anyone have any other reference material for making one?  

I have a good amount of the items listed from the parts list, and have a number of others coming in on an order.  I plan on fabricating most of the items I can, which is why I like this article since it gives good info.  I also plan on making my own coils.  I'm hoping to be able to start building here within this next month or so, after I finish my current project, which is a 5w vacuum tube amplifier I'll be making mostly for my guitar.  I was considering, probably far down the road, if all goes well with these two builds, of making a vacuum tube theremin, so if anyone has any reference or opinion, I'd appreciate anything you guys want to offer.

I'm an electronics student and so this is something I've been wanting to do for quite a while, not only because I like playing and creating music, but also as a learning experience.  

Thanks

Tubes!

I've found a kindred spirit. I also play guitar through a home built tube amp.

Note that a theremin usually has a line level output and will overdrive a guitar amp.

The EM theremin is a Moog Etherwave. I don't think you could pick a better project for your first theremin build. You're going to have a decent, playable instrument when you get done. If I remember right, the VCA IC is obsolete, but there is a pin compatible replacement available. I forgot the number. I'll post it when I find it.

Winding your own coils however, will probably yield less than optimal results. They are made in a way that's practically imposable to duplicate with hand winding. They are available from Digikey, and are only a few bucks. I don't want to discourage you from trying. The attempt could also be a learning experience.

Digikey

Tube theremins. Two words. Mark Keppinger.

I'm currently in the beginning stages of building his Model K tube design. Winding coils at home is practical for this design, but one of our members here can make them for you.

Chobbs

Awesome, thanks for the reply!

I haven't had extensive experience winding my own coils yet, just made some of my own pickups for an instrument I made (a kind of electric diddly bow/cigarbox guitar), but those don't exactly require the precision a coil for a theremin requires. At school, though, we do have the equipment to measure the inductance of a coil, so that, along with a few calculations, I hope to get close.  Since I'm still a month or so off from actually being able to start this, and while I wait for my parts for my tube amp to arrive (mostly the transformers) I'll play around with coils and see how it goes.  I've been collecting the ferrite beads out of power supply cords, and I hope to figure out a good way to make my own variable inductor so I can really get it precise.

I have this really bad diy-habit.  I like to make things from scratch as much as possible, which is great for learning, but does have drawbacks as far as completing projects because my own stuff doesn't always function like it should.  If the coils prove to be too much of an issue i'll definitely  buy some since they are so cheap, and I am determined to get this theremin built.

I'll say it again. Kindred spirit.

My Keppenger theremin is being built from dead ham radio gear.

The brick wall with inductors is their capacitance. Ask your instructor about self resonant frequency.

randkg (at) att.net

Drop me a note and we'll talk tube :)

FInally got one of my orders in a couple days ago that have some parts for the theremin, so while waiting for my tube amp parts, I putting the theremin circuit together! Putting it together on a breadboard at the moment, but its mostly just a practice run and to see what parts I'm missing and need to get still (seems like no matter how many times I go through a check list for parts, come time to build, there's something missing) 

From what my instructor tells me, a breadboard may not be the best place to try and build, since there will be quite a bit of extra capacitance from the metal strips on the board itself, which, for most circuits won't matter much, but may result in a problem with this build.  For the most part, it seems fairly easy to construct, so I'm thinking after this test run I'll go ahead and start building the circuit on some protoboards. 

One idea I was hoping to get some feedback on involves building the main circuit on one protoboard, or breaking each section into it's own small board?  Such as putting the VPO on one, then the Fixed pitch osc/pitch tuning on one, the volume oscillator and volume tuning on another, then the detector and vca amp/processor on one.  I've always had better luck breaking circuits down like that for me, but is there any extra issues I may run into doing this?

On a somewhat related note, the tube amp I'm constructing is based on an ax84.com high octane single-ended 5w amp.  I would like to use the theremin with the tube amp if possible, so is there any tweaks to the amp I might make that would make it more compatible with the theremin?

After wanting to make a theremin for quite some time now, with the planning, gathering componants etc, it's such a relief to actually start putting it together, even if it's only a little test run.  I was planning on putting this off for another month or so while I work on my tube amp, but after starting it, I'm not sure I can put it off that much longer!

"One idea I was hoping to get some feedback on involves building the main circuit on one protoboard, or breaking each section into it's own small board?  Such as putting the VPO on one, then the Fixed pitch osc/pitch tuning on one, the volume oscillator and volume tuning on another, then the detector and vca amp/processor on one.  I've always had better luck breaking circuits down like that for me, but is there any extra issues I may run into doing this?" - Luc

All my experiments with the EM / EW circuits have been using small modules, and there has never been a problem.. The only real issue is the antenna.. You want the antenna lead as short as possible and see the other EM thread up at the moment regarding this.

With modular assembly, the link from the oscillators to the mixer is quite inportant as this is High Z - Unfortunately it is not buffered in standard form - if you were to buffer this signal with a BJT before piping it to the mixer, you should be fine. Once the signal has gone through the mixer and into the LM13700 you dont need to worry about capacitance.

Fred.

Regarding your amplifier.....

I've had a look at what you're building. It uses the standard, and effective Fender/Marshall tone sack. 

It's going to sound great with guitar but not so whoopee with theremin. It will be grossly over driven, and have a rather notable mid range dip. You can usually get a fairly flat response with the treble and bass at zero, and the mid at 10. 

The best solution would be a second low gain, full range preamp with standard, hi fi style tone controls and mix that and the guitar preamp at the power amp.

The quick and dirty way is a resistive voltage divider or pot at the input.

A word on speakers.

If you go with an 8 inch, I can almost promise you won't be happy with it.

I found THIS ONE. I have a pair of these in a 50 watt tube amp, and a single one in a 30 watt tube amp. My Fender Deluxe Reverb has a pair of the 10 inch version in it. They sound a lot more expensive than they are.

randkg (at) att (dot) net

Drop me a note and I'll send diagrams of what I use.

Rob.

Wow... you all are up early to buildthis project! I am inspired! :)

"With modular assembly, the link from the oscillators to the mixer is quite inportant as this is High Z - Unfortunately it is not buffered in standard form - if you were to buffer this signal with a BJT before piping it to the mixer, you should be fine. Once the signal has gone through the mixer and into the LM13700 you dont need to worry about capacitance."

Fred.

Thanks for the reply.  I'm currently looking into this before going on with the build.  So far, I've only worked with buffers on digital logic, and haven't done with them on the transistor level. Lots of stuff to learn, but that's half the fun!

"The best solution would be a second low gain, full range preamp with standard, hi fi style tone controls and mix that and the guitar preamp at the power amp.

The quick and dirty way is a resistive voltage divider or pot at the input.

A word on speakers.

If you go with an 8 inch, I can almost promise you won't be happy with it.

I found THIS ONE. I have a pair of these in a 50 watt tube amp, and a single one in a 30 watt tube amp. My Fender Deluxe Reverb has a pair of the 10 inch version in it. They sound a lot more expensive than they are.

randkg (at) att (dot) net

Drop me a note and I'll send diagrams of what I use."

Rob.

Thanks for the heads up.  Since this, along with the theremin build, are both my first attempts in these areas, I think I'll just try and keep it simple and just try and get these functioning first with the tried and true designs. 

Afterwards, I'll hopefully attempt other builds.  While looking the tube amps, I see alot of people using tube amp builds for HIFI setups, is this more along the lines you were referring to?

 Since the theremin will mostly be played in private, small settings, would something like an lm386 amp work? I've made plenty of those and they can do fairly well for such a small, simple build. 

Thanks