Here's a photo of the jig I made for bending the volume antenna, and the result (after cutting the antenna to length). The base of the jig is a scrap pine board; the shaped pieces are made from some scrap particle board, roughly shaped with the jigsaw, shapes finished with the rasp, and the edges grooved with the round file. The dimensions are taken from the RCA antenna measurements posted on thereminworld.com. I had hoped to be able to bend the pitch antennae (I made two lengths, 18 and 24 inches) with the 2 1/4 inch round jig but no dice--the tubing flattened no matter how careful I was. I had to get a tubing bender (a small one--required the assistance of my inner gorilla). Better to replace the small round piece with a grooved rectangular piece for finish bending of the volume antenna. I used 3/8 inch aluminum tubing, 0.035 inch wall thickness [K&S Engineering, Chicago, Stock No. 9315, available in hobby shops and some hardware stores] for both antennae, which is plenty tough. They don't look half bad after polishing with Wright's Silver Cream. I will attach to my Theremax with bulkhead unions from Swagelok Inc. I am familiar with this product from work, and had the boss order me a package (four for $24). Note that on each end there is a nut. Inside each nut is a pair of ferrules, front (cone shaped) and back (a 2-level ring). Slip the nut onto the end of the tubing, followed by the back and then the front ferrule. Put the tubing into the fitting as far as it will go, then tighten the nut as far as it will go, not just to where you feel resistance. Back the nut off, and the ferrules are connected so firmly to the tubing (a little back from the end) that the only way to remove them is with a hacksaw. The only washers I found at the hardware store that would fit these unions are so big I think I will skip them on the outside, since the case will be red oak and pretty hard. On the inside I will put a washer and lock washer, and maybe a spade lug underneath the lock washer will make a good electrical connection. Swagelok also sells straight copper tubing that I think might very well work; it's the same size as the aluminum tubing I used except that it comes in ten-foot lengths. Here's a list of Swagelok dealers.

I am going to put copper-clad printed circuit board on the bottom of the case, underneath the electronics. Besides being a good shielding material, it is a good structural material--very stiff. It is easily cut with either tin-snips or (if sandwiched between pieces of wood) a Skil saw. If the hole you drill in it is a bit small, it can easily be reamed out with the tang of a file.

I succeeded in winding a pair of air coils for my Theremax pitch antenna. Two--because that's what fits easily into the case I am building--air coils in series with taps at 0, 100, 200, 300, 450 and 600 turns, selectable with a rotary switch.

The instructions I used for inspiration are here and here. I used two 2-inch PVC couplings from Home Depot (the long ones aren't in stock) spliced together with 3/4 inch PCV pipe for each coil. (Beware! one end is smooth, the other has lettering that holds the two couplings apart.) 300 turns of 30-gauge enameled wire fit on with just enough room left for a clamp on one end and a bit to spare on the other. Note that the clamps have ribs on the inside; I filed them off. The 30-gauge wire from Radio Shack comes on a 200-foot roll with a couple of other sizes in the same package. I figure each coil uses about 120 feet, so I needed two rolls. Not surprisingly, the second coil (the back one in the picture below) looked better than the first. I learned to put some Scotch tape around the joint in the middle, for one thing. I improved on the winding device linked above by using PVC sprinkler parts I had on hand to make a crank and secured the form to the crank with duct tape. When I started I had to tamp the coils down every two or three turns; by the end I could go ten turns before tamping them down. I finished by pinning things down with a bit of hot glue. I had no need to solder the twists on the taps. I didn't put metal cores into the coils, but can do so if I should so desire. I had to actually wind the coils and get them in place so I could drill various bolt holes in the bottom of the case before I proceed to the sides of the case, otherwise it would be the Devil's own work to assemble.

Coil winder:

Coils, with associated parts in approximately correct positions but not connected; I started winding 1/4 inch from the end of each coil, and wound them so all the coils would be in the same direction (I have no idea if that is going to be helpful or not):

Here's the case I am making; stained but not yet finished. The top will be connected to the bottom with Velcro, but in such a way that the top will be flush with the rest. The little arched opening on the near (right) side is for the 6-position rotary switch that selects the number of additional pitch antenna coils. In the back (left) is a mounting hole for a DC power supply plug; I prefer being able to detach the wall wart from the theremin. The hole in the bottom is for the photo tripod connector, which will connect with a nut salvaged from an old camera flash (85 cents the thrift store) glued on top of the PC circuit board used for shieding in the bottom of the case. The case is a bit larger than necessary, but it is made from nicely finished quarter-inch red oak pieces I found at Home Depot (they also had ash). The thickness and length are exact, but the widths are nominal. Using ready-made pieces (selected for minimal warping) meant I didn't have to worry about staight lengthwise cuts, which would have been very difficult with my limited woodworking skills and equipment. I also used some 3/4 inch square red oak pieces and some thin one-inch pieces of wood I had on hand.

There were various modifications I considered and made. They fall into categories:

My own ideas:

1) Pitch antenna coils. A noble experiment, in progress.
2) Push button mute rather than jack and foot switch. Done.
3) Power supply plug so wall wart can be detached from instrument. Done.
4) Longer case to get the antennae further apart, possibly giving addition flexibility in tuning. Done.

Widely recommended:

5) Antennae more like RCA ones. Done.

Suggestions from PAiA web site:

6) 10K ohm R from R4 to R9 (on side nearest to oscillator coils) to improve tone.
7) Delete C44 to improve timbre in square wave

Suggestions from Fred Nachbaur:

8) Extra shielding around oscillators--very difficult; not done.
9) Not explicitly stated, but photos indicate fly wires soldered to underside of board for better appearance. I tried it, and it calls for soldering skill greater than mine. Not done.
10) To decrease birdies, C22 and C34 220 pF-->1000pF
11) To improve timbre fader response, pot. R81 100K --> 100K log taper (audio taper) and R40 470K --> 150K.

HOWEVER, there are new mods suggested by PAiA. [Yes, they DO have outstanding customer service!] They currently suggest 100 pF capacitors for C39, C44, and C37, plus a 6.8K ohm resistor between R81-3 and R81-2 to deal with all the items addressed in mods nos. 6, 7, 10 and 11 above. (These suggestions are not in the manual or on the PAiA web site.) I followed the PAiA suggestions (they sent me the parts for free). They say these new mods also improve some of the stuff (I'm kind of vague here) related to timbre effects of rapid attack and (I think) CV outputs related to same.

Here are the innards. Printed circuit board [PCB] to the left, air coil assembly to the right. Top attaches with Velcro (not quite as well as I had hoped, but OK and can be adjusted if I really want to do it). The black thingy in the clips at the back is a trim tool for the oscillator coils--so I am less likely to lose it. I put unetched PCB underneath all the electronics, all connected together and all connected to ground. Seemed like a reasonable thing to do. PAiA says to do it for the PCB. In the center you can see the nut to attach the mounting gizmo for the camera tripod I will use in place of a mic stand; that piece of PCB is held in place by two blocks of wood that also serve to flatten the bottom, which was slightly warped. Power supply plug at left rear. The gray clamps for the air coils ultimately turned out to be superfluous; I found it very helpful to tack the coils in place with hot glue, and that would have been enough. But I had the bolt holes all drilled, so installed the clamps anyway. The clearances were REALLY tight; if I were doing it again, I would mount the air coils vertically (which would require longer taps). All the various parts are connected to the bottom of the case with 3 mm and 4 mm nuts and bolts with washers and lock washers as appropriate; a bit more expensive in the U.S. than U.S. standard, but looks good. The PCB standoffs are 15/16 inches (24 mm) from Radio Shack (which have 3 mm threads).

And here is the final result, assembled and mounted on the camera tripod; 18 inch/45 cm ruler for scale. Testing and tuning later this week.

MY RESEARCH PROJECT, after I get the T-Max built, will be to test the linearity at various coil settings and two different pitch antenna sizes. A friend with an oscilloscope will help, and I will post the results here once I have them. Of course no two types of theremins will have just the same quantitative response to such tinkering, but the qualitative responses should be similar, and therefore it should be of general interest, I think. The electronics experts probably figured it all out long ago, but I don't have their knowledge, so I must resort to empirical tinkering.

Well, the air coils experiment seems to have been a mistake, at least combined with the big RCA-style antennae, or at least the way I did it. I couldn't get the TMax tuned. I got buried in other stuff the past several weeks, and only today I got back to work and bypassed the air coils. (Thanks for the coax, Philip!) After a little tinkering, I seem to have it tuned about right, both for volume and pitch response. There is a VERY faint pitch response over a short part of the volume control range, but that's close enough for tonight. To compensate for the RCA-style volume loop I had to adjust L4 a bit more than for the standard antenna, but it could still be done without difficulty. Now the question is, what to do with that space where I put the air coils, and the hole in the right side of the case. Pitch preview seems like an obvious substitution.

I seem to have the pitch tuned OK, and I have discovered that for volume adjustment I must tweak L3, then put the cover on and try the response. But one problem remains. Once I have the volume responding about right, as I move my hand toward the volume loop the volume drops off as it should, goes through zero, then goes through a zone of reversed response (louder near the antenna) and finally drops to zero. As I adjust L3 inwards, this reversed zone becomes smaller and closer to the antenna, but never goes away completely. Does this sound familiar to anyone? Any suggestions?

From kkissinger:
Alan,
Depending on how you tune L4, the Theremax will get softer as you approach the loop, louder as you approach the loop, or softer then louder as you approach the loop. Just tweak L4 to get the response that you want. -- Kevin

Thanks, Kevin--I don't think that's in the manual, but it ought to be! I was beginning to catastrophize about some odd electronic mismatch. I will set to work at once!

[later] That did it, all right! With L4 cranked in two turns, the volume response seems to be just as it should. I spent some time fiddling with the FX presets on my amp (list downloadable at http://behringer.com/K900FX/index.cfm?lang=eng link in right column) and could get quite a variety of sounds. Many thanks, Kevin!

Site last updated 10th December 2007