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Winding the  RS Theremin Coils

"An organic theremin voice is the magical relationship between unshielded coils and the antenna"

"The Coils are the Theremin!"
The secret to my pitch stability against temperature drift was getting rid of variable capacitors.
For perfect pitch linearity on my Ultimate 2 or the PAiA Theremax visit Lev's Antenna
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Below are easy to find coil making components found at Home Depot

Make two identical coils out of PVC  

PVC- Coil This 270 uh coil is made using 100 turns of #30 gauge wire on a 1.375" ( 3.5 cm) dia. form. Operates at 860 khz
I drilled the 3/8" hole in the center plug, press in the chrome coupling nut for the brass screw pitch adjuster.


I discovered later that turn 50 the A3 center tap not seen in the photo is an excellent point for Lev's Antenna to connect. This has a better impedance match which reduces background noise from sneaking into your theremin sound. 		Use enameled wire only! #30 gauge  (A.W.G. Metric 0,2540)
How to Wind the Coils. Lightly press a 2-1/8” ( 5 cm) strip of the 1/2” foam mounting tape lengthwise centered on the coil form. Use a piece of pvc pipe plugged into both sides as handles for easier turning. Place your first wire under the foam or a piece of scotch tape. Put scotch tape at the wire taps, twist them and fold them over so they don't untwist as you pull around the next wire wrap.
Both coils are 100 turns of wire. Start turning wire 1" (2.5 cm) from the tuning screw edge A1. Pull a loop out and twist it at turn 25 "A2", add another 25 turns which is turn 50 the "A3" center tap. You can see these wire loops in the photo at the left. The center tap turn 50 loop is not shown in this photo. The extra loop at turn 75 is not needed but may be wise to add if for some reason you mounted the coil backward.  "Thank you Hank"

The center tap on L2 is not used, only A1, A2 & A4
After final tuning and the theremin sings, use scotch tape to hold all the wires in place on the coil form.

Make two identical coils out of Salt & Pepper shakers.

Salt-Coil  This 358 uh coil is made up of  100 turns of #30 gauge wire on a 1.63" ( 4.1 cm) dia. form. Operates at 750 khz
 Remove the foil paper outer wrap, a dab of water helps it break loose. You don't need to cut the tube shorter as I did.
Both coils should be as identical as possible,  not perfect. If you miss-count a wire turn or two, you can fix that later while doing the "Tuning the Oscillators" test.  Avoid overlapping wires or spaces between wire turns. Do not put the brass screws in until after you have done Tuning the Coil Oscillator 

 Note: Using PVC or Cardboard for the coil form tube will add slightly different accents to the final sound.
Adding and Cutting the Coil Wire Leads

 On the Left side L1 coil, leave 6" (15 cm) of end wire for coil connection to the board and add 6" (15 cm) wire extensions to the wire loop A2 (Turn 25) & A3 (Turn 50).

On the Right side L2 coil, leave 3" ( 7 cm) of end wire for coil connection and add a 3" ( 7 cm) wire to the extension loop A2 (Turn 25). L2 does not use the A3 center tap and does not need a soldered on extension wire!

Note: The longer left side L1 coil connecting wire is needed for coil rotation to create the best wave shape. Also if you added the turn 75 loop it is not used. I use it to experiment with.

Mounting the Coils

Secure the coil tube in the mounting bracket with about 1" ( 2.5 cm) of foam tape, then screw mount the brackets to the circuit board in the proper direction. You can use the mounting brackets on both coils. I like to use a 3" ( 7 cm) screw pivot on L1 the left side coil which allows me to rotate the coil with a knob outside of the enclosure. You can also mount L1 in a single gray bracket and swivel that.

L1 should swivel to either side relative to L2. Coil distance and angle to one another creates the ideal wave shape. The mounting holes place the coils at the ideal distance relative to one another.

Thank you Clint "Grasshopper"

Ultimate 2b Photo
I have tried many possibilities for the L1 coil angle and this "old photo" shows the ideal angle.
 I did not find a significant difference to the coils being horizontal or vertical relative to the mounted Antenna 

After Mounting the coils feed the wires through their proper holes as indicated below.

In the Ultimate 2b Photo above A1 is the side of the pivot mounting screw of L1.
A1 is on the right side of coil L2 near the brass tuning screw. 
It is critical that A1 is facing same direction or side on both coils

 The first coil wire closest to the brass screw is A1 and fed through board hole A1 
A2 is the 25 turn loop after A1 which is fed through board hole A2.

A4 is the opposite end wire of both coils which is fed through board hole A4.

The extension wire on to the L1 coil A3 center loop is fed through the board hole L1-A3.

On L1 take each wire and pull out the slop but leave enough extra so you can rotate the L1 coil from relative to coil L2. Cut the wire about 1" from the bottom of the copper side of the board. Take a butane cigarette lighter and heat up the tip of the wire and the enamel burns off nicely. Clean the tip with one of those green kitchen scrub pads. Also Tin "solder" the tip to make certain its clean. Now back the wire out slightly through its hole and solder the tip to the board.

On L2 take each wire and pull out the slop and cut about 1" ( 2.5 cm) from the board. Take a butane cigarette lighter and heat up the tip of the wire and the enamel burns off nicely. Clean the tip with one of those green kitchen scrub pads. Also Tin "solder" the tip to make certain its clean. Now back the wire out slightly through its hole and solder the tip to the board.

The coil wire connections will be your weakest link, use care when cleaning and soldering.
Note: Do not bunch the coil lead wires together, leave them loosely spaced from the coil to the board holes.


It is now time to do Tuning the Coil Oscillators  (This is the most exciting moment!)


Note: The Ultimate 2b air coil design does not like being boxed in tight enclosures. The best enclosure is to mount your circuit board on the bottom of a thin piece of wood and drape a covering like cloth over the sides to conceal the coils & components. This keeps the board mounted pots accessible from the bottom. The Ult2b can become unstable running to many wires off the board. (Like test leads) 

Coil Making Components

Radio Shack Wire 278-1345 B - You will only need to use 50' of the Red #30 gauge wire (A.W.G. Metric 0,2540)

Everything below is at Home Depot

C482-007 Schedule 40  3/4" deep socket coupling (3"long) @ .59 each. No longer available

Use two of the standard 2" short PVC 3/4" sockets. The handles in the above photo give you a method for holding the wire coil form. The total of 100 turns of wire 270 uh are about 1.15" (2.8 cm) in width. Start winding about 1/8" from one side which will leave room for a mounting bracket on the opposite side which will be the tuning screw side of the form. You should add all three center taps to both coils so you don't run into a mounting direction problem later on.

C449-007 Schedule 40  3/4" End Plugs

¼-20 coupling nut on the wall near the threaded rod in orange pack of 3
Not used on the Salt Coil - Instead use a ¼-20 tee-nut as seen in the bottom salt photo.

¼-20 x 3" Brass Screw - not used on the Salt Coil - Instead use ¼-20 x 2" 

1" Conduit Clamps These are found in bags of 5 in the electrical conduit section.
Home Depot brand is Powerline 5133738B

Foam Tape - 1/2" wide double side adhesive use to secure the wire wraps and hold the coil in the clamp.
Remove the enamel from the wire before soldering 

 Last Update 09.27.07