The 203 Theremin

A Battery-Operated Version of the Wien-Bridge Theremin

September 22, 2001
Updated February 6, 2020

Contents

Legal Notice
Safety Notices
Introduction
Assembly Procedure
Calibration Procedure
Schematic
Circuit Description
Parts Table
Drawing Index
Photos

Legal Notice
(back to contents)

The information contained in this document is ©2001-2020 by Arthur Harrison. Any reproduction of the information contained in this document, electronic or mechanical, shall only be used with Arthur Harrison's permission, and shall acknowledge him as the copyright holder and author.

Use of the information contained in this document for personal or commercial financial gain, such as the manufacture and sale of electronic musical instruments or parts thereof, is prohibited. Unless specifically stated in a written contract, Arthur Harrison grants no licence for the commercial exploitation of the concepts and designs embodied in this document. Refer licensing inquiries to: diy@harrisoninstruments.com.

The information contained in this document may only be reproduced in small quantities when the purpose for its use is the dissemination of information to students or hobbyists, and may not be distributed in any form, electronic or mechanical, for the purposes of any party engaged, directly or indirectly, in commercial enterprises.

Arthur Harrison assumes no liability for any damages, direct, or consequential, which may arise from the dissemination, application, or misapplication of the content contained in this site. The User of the information provided in this site assumes all responsibility for any damages, direct or consequential, which may arise from its use. Arthur Harrison retains the right to alter the content within this site at any time without notice.











Safety Notices
(back to contents)

DANGER: Do not play this instrument at a high volume, especially when using headphones. Use headphones that have a built-in volume control, and adjust the volume control for a comfortable level. Hearing experts advise against the continuous, extended use of headphones.

DANGER: Some of the components used in this construction are "polarized," which means that they will only function properly when inserted into the circuit in the right direction. Always observe component polarities carefully, and double-check the orientation of transistors, diodes, integrated circuits and polarized capacitors before applying power to any circuit. Do not reuse a part which has been subjected to improper insertion.

DANGER: Wear safety glasses and use all appropriate safety equipment when working with tools and materials. Always follow safe shop practices and obey safety rules.











Introduction
(back to contents)

The 203 Theremin utilizes Wien-bridge oscillators in a heterodyne configuration to produce an audible tone that corresponds to hand position. The instrument provides the convenience and safety of 9-volt battery operation, in a compact enclosure. It is a pitch-only theremin, primarily intended for use with an external amplifier and speaker, although it has sufficient output power to drive headphones directly.

A jack on the top of the unit accepts the antenna assembly that consists of a support rod and a 6-inch-square metal plate. An on-off switch and a pitch zero control are located on the top panel, and a 1/8th-inch monophonic phone jack for the output is located on the side.

A note regarding external interference:  The 203 Theremin is prone to interference from sources such as power lines, fluorescent lighting fixtures, incandescent lamp dimmers, and computers. Such sources will cause modulation of the instrument's output tone that may render it unsuitable for certain applications where high fidelity is required. In some cases, effects from interfering sources may be remedied by moving the theremin to a different area.

Where absolute assurance of fidelity is not an issue, the 203 Theremin is highly suitable as a practice and demonstration instrument, due to its portability, excellent stability, and pleasing tone quality. The MP3 sound sample below was recorded under typical conditions. Your results may vary.

The following graph illustrates the approximate relationship of pitch and hand distance for a 203 theremin, when constructed and calibrated as specified in this article:

As in previous articles, I have supplied detailed lay-out drawings for the circuit using Vector perforated board and Wire-Wrap® terminals and sockets. To obtain a compact form-factor, this circuit lay-out is dense, requiring an advanced level of construction skill. This article also includes details to fabricate the theremin's enclosure and antenna assembly. Alternatively, builders may devise their own. For those who prefer additional construction guidance, I have included an "Assembly Procedure" section with step-by-step instructions for fabricating the 203 Theremin.

Assembly Procedure
(back to contents)

If you have a populated printed circuit board, then use the Alternative Parts Table and Assembly Procedure. Otherwise, use the following procedure to fabricate both the Wire-Wrap® board and the enclosure.

1. Cut the Vector 64P44WE perforated board to obtain a piece with 48 by 21 holes. The board may be cut by deeply scoring along a column of holes with an awl, and then breaking it along the score. If you use this technique, securely clamp the board and a straight-edge guide to a sturdy work surface. You will need 48 columns of holes, so score and break the material along the 49th column. Then, use the same procedure to reduce the vertical dimension of the board from the stock size of 44 holes to the required size of 21 holes. Sand or file the edges of the board smooth. Be aware that the glass-epoxy material used for the perforated board is very abrasive, and will quickly dull tools. CAUTION:  Wear a respiratory mask when cutting or sanding the glass-epoxy perforated board, because breathing the dust may be hazardous to your health. Once the board is cut to the proper size, refer to the Symbol Key and Component Locations drawings to locate and drill the four 0.113"-diameter mounting holes at each corner of the board. Also locate and drill the nine 0.086"-diameter wire strain-relief holes.
   
   
2. It is convenient to use the board, before it is populated with terminals and components, as a drilling template to locate the four 0.113"-diameter mounting holes in the enclosure cover. Using a dial caliper or other suitable measuring device, mark the location for one of the four circuit board mounting holes on the top surface of the enclosure, as shown in the Enclosure drawing. Align the circuit board with the marked location. Position the board so that it is parallel with the edges of the enclosure. Secure the circuit board to the enclosure with tape, and drill the four holes. Remove any metal burrs from the holes using a center reamer.
   
   
3. Refer to the Enclosure drawing. Locate and drill the seven remaining holes. Remove metal burrs from the holes using a center reamer. Using steel wool, remove sharp edges from both pieces of the enclosure. CAUTION:  Keep the steel wool away from electronic components, and thoroughly remove any remnants from the workpiece promptly.
   
   
4. Refer to the Component Locations drawing and the Symbol Key to locate and insert the Vector T68 solder fork/Wire-Wrap® terminals in the appropriate holes. The terminals are inserted with long-nosed pliers. If they are hard to insert, use a slight alternating clockwise-counterclockwise twisting motion while you push. Make certain that each terminal is securely and squarely seated. The components are soldered to the fork-side of the terminals on the "component side" of the board. Wire-Wrap® connections are made to the pin-side of the terminals on the "wiring side" of the board.
   
   
5. Once the terminals are inserted, you will find it convenient to temporarily install four 4-40 threaded standoffs in the circuit board's mounting holes, which will serve as "feet" to keep the board level on the work surface during soldering. Attach the standoffs to the wiring side of the board, so that the board stands on the work surface with the component side upward.
   
   
6. Before they are inserted in the board, the integrated circuit sockets for U1 and U2 require the addition of capacitors C5 and C6, connected from pins 4 to pins 7. Refer to the Bypassed IC Socket drawing, and prepare the two sockets as indicated. The leads of the capacitors should be wrapped partially around the socket pins and soldered. Make sure that all the metal surfaces are completely free of oxidation, so that the soldering can be performed quickly, or the body of the socket will melt. Also be very careful to avoid solder "bridges" to adjacent pins. The capacitor's body and leads should be pressed against the base of the socket, so that they do not prevent the socket from being fully seated against the board. Note that "open frame" sockets are specified for this construction, meaning that there is a space in the body of the socket for the added capacitor.
   
   
7. Repeat the above step for U3's socket, but connect the capacitor (C13) from pins 4 to 8, instead of pins 4 to 7. Set the three prepared IC sockets aside.
   
   
8. Solder the components to the terminals as indicated in the Component Locations drawing. Be certain to observe the polarities of the five 100uF electrolytic capacitors, C8, C11, C20, C24, and C25. Also be careful to orient rectifier CR1 and transistors Q1, Q2, and Q3 correctly. Note that the transistors' flats face upward. Although the orientation for the resistors and non-polarized ("electrostatic") capacitors makes no electrical difference, it is customary to insert them so that they can be read "left-to-right." The value markings on the capacitors should be oriented so that they can be identified, and the resistors should be oriented so that the color bands representing the most significant digit of the values are on the left, and the tolerance bands (gold for ±5%) are on the right.
   
   
9. Once the components have been soldered in place, carefully inspect each connection using a magnifier and ample light. Ensure that each soldered connection appears bright and shiny, and that the contour of the soldered surfaces are clearly discernible. Make sure that "flashing" from the component bodies do not interfere with the solder joints. Also ensure that there are no solder "bridges" among adjacent terminals, which is a very common cause of problems.
   
   
10. Refer to the Wire Locations drawing. Note the two locations (columns 23 and 32) where bus wire is used to bridge adjacent terminals. Insert the bus wires, and solder them in place. The bus wires may be made from a scrap component lead or a piece of bare Wire-Wrap® wire.
    
    
11. Insert the six IC sockets as indicated in the Component Locations drawing. Be sure that the sockets with bypass capacitors are properly located. The notches on the sockets are oriented toward the top of the board, as shown in the IC Socket and IC Orientation drawing.
    
    
12. Insert variable capacitor C4 into the board as indicated in the Component Locations drawing. The capacitor's pins will be soldered to Wire-Wrap® wires in a later step.
    
    
13. Remove the four 4-40 threaded standoffs and remount them on the component side of the board.
    
    
14. Refer to the Wire Locations drawing, or if you prefer, the Mirrored-Image drawing. Connect the terminals as indicated using Wire-Wrap® wire. Do not omit the connection, denoted with the letter "A," that connects terminal 5 to pin 8 of U3's socket.
    
    
15. Connect C4 as indicted in the Wire Locations drawing. Two of C4's terminals are internally connected to the "rotor" of the capacitor (the part that turns). The right rotor terminal is soldered to two Wire-Wrap® wires, and the redundant, left rotor terminal is left unconnected. C4's center terminal, which is its "stator," is soldered to one Wire-Wrap® wire. These are the only soldered connections on the wiring side of the board, and are required because C4's terminal pins are not suited for wrapped connections. Be sure that the capacitor is completely seated against the board before soldering it in place.
    
    
16. Using an ohm meter or continuity tester, verify that there are no improper or missing connections.
    
    
17. Refer to the Component Locations drawing. Insert the six integrated circuits in their respective sockets. Each integrated circuit is inserted with pin 1 oriented toward the top, left corner of the board, as shown in the IC Socket and IC Orientation drawing. The circuit board is now complete, and may be set aside until a later step, when it will be installed in the enclosure.
    
    
18. Cut the RV1 potentiometer shaft to a length of 3/8 inches.
    
    
19. Solder two 4" lengths of hook-up wire to the potentiometer, RV1, as shown in the Wire Locations drawing. Note that one of the wires connects to both the slider terminal and the counterclockwise terminal of the potentiometer. Mark the free ends of the wires with tape: Mark the wire connected to the slider/counterclockwise terminals "1," and the wire connecting to the clockwise terminal "2." Twist the two wires together for a length of 1.25 inches, being careful not to break the soldered connections.
    
    
20. Solder two 4" lengths of wire to the output jack, J2, as shown in the Wire Locations drawing. Identify each wire with tape: Mark the wire connected to the tip terminal of the jack "3" and the wire connecting to the sleeve terminal "4." Verify that the tip and sleeve terminals are correctly identified with a continuity tester or ohm meter:  The sleeve terminal should have continuity (0 ohms) to the jack's threaded bushing. Twist the two wires together for a length of 2.5 inches, being careful not to break the soldered connections.
    
    
21. Solder two 4" lengths of wire to the switch, S1, as shown in the Wire Locations drawing. The switch has three terminals, but only two will be connected. Use the center terminal for one wire, and the terminal opposed to the bushing's slotted "keyway" for the other wire. Identify each wire with tape: Mark the wire connected to the center terminal of the switch "5" and the wire connecting to the end terminal "6." Twist the two wires together for a length of 1 inch, being careful not to break the soldered connections.
    
    
22. Twist the battery connector wires together for a length of 1.5 inches. The black wire corresponds to the negative terminal, and the red wire corresponds to the positive terminal.
    
    
23. Solder a 3" length of hook-up wire to the antenna jack, J1.
    
    
24. Install the output jack, potentiometer, switch, antenna jack, and battery holder in the enclosure as shown in the Assembly drawing, details "A" through "E." The switch is furnished with an anti-rotation washer that has a two tabs; the inner tab engages the switch bushing's keyway, and the outer tab is inserted into the small hole that is above the hole for the switch bushing. The switch is oriented so that the instrument is "on" when its toggle is toward the antenna jack.
    
    
25. Refer to the Assembly drawing, detail "F." Remove the four 4-40 threaded standoffs from the component side circuit board and remount them on the wiring side of the board. Install the circuit board in the enclosure. Orient the board so that terminals 1 through 8 are adjacent to the potentiometer and switch.
    
    
26. Insert the wires from the potentiometer, output jack, switch, battery connector, and antenna jack through their respective strain-relief holes in the circuit board. Insert the wires through the strain-relief holes from the wiring side of the board.
    
    
27. Trim the wires so they they extend about 1/2" past the holes. Strip 1/16" of insulation from each wire, and solder them to their respective terminals, marked 1 through 9, in the Wire Locations drawing. Be sure that the battery connector's red wire goes to terminal 7, and that its black wire goes to terminal 8. The wire connecting the antenna jack, J1, to terminal 9 should be kept short, with only a slight amount of slack. The wires connected to the output jack, J2, should be routed away from the circuit board.
    
    
28. Attach the knob to the potentiometer so that the knob's index line corresponds to the potentiometer's shaft position; the line should point to the 12 o'clock position when the potentiometer is centered.
    
    
29. Fabricate the antenna plate as shown in the Antenna Plate drawing. The drawing specifies 0.063" thick 6061 alloy aluminum, however, neither the thickness nor the type of metal used is critical, provided the size is about 6 inches square. Assemble the antenna as shown in the Antenna Assembly drawing. The banana plug, P1, is furnished with a screw and solder lug, which may be discarded. The threaded rod is furnished as a 2-foot length. Cut a 6" length of the rod for the assembly.
    
    
30. Install a 9-volt battery, B1, in the holder and attach the battery connector. Refer to the Calibration Procedure to adjust the theremin.

Calibration Procedure
(back to contents)

1. Connect the theremin's output to headphones* or an external amplifier via its output jack, J2. If the theremin is used with external amplification, set the amplifier volume control to a very low level. If headphones are used, set the headphone volume control to a very low level.
   
   
2. Set the PITCH ZERO potentiometer (RV1) to the middle position. Set the POWER switch (S1) to the "on" position. Insert the theremin's antenna into the antenna connector (J1).
   
   
3. Hold the theremin in the air with one hand, at least for feet from any objects. Grasp the theremin by its case, not its circuit board. Keep your other hand away from the antenna.
   
   
4. Using your free hand and a small straight-blade screwdriver, engage the tuning stud on the PITCH ZERO CALIBRATION capacitor (C4). Rotate the capacitor's stud while observing the pitch of the tone. Stop rotating the capacitor when the pitch is lowest, or completely absent.
   
   
5. Install the cover, being careful not to pinch any of the wires. Place the theremin on a table, at least 2-feet distant from objects. (To facilitate battery replacement, it is recommended that the four self-taping screws supplied with the enclosure are omitted.)
   
   
6. Move your hand directly over the antenna, keeping it centered and parallel. The theremin should not produce an output until the hand is approximately 15 inches away. The output pitch should increase as the hand approaches the antenna. Touching the antenna should squelch the output.
   
   
7. Use the PITCH ZERO potentiometer (RV1) to "fine tune" the instrument as necessary for the desired sensing distance and sensitivity. When properly calibrated, the theremin should have an optimal response when RV1 is centered.
   
   
8. The above steps might have to be repeated to achieve the correct calibration. Once the procedure is accomplished, the instrument should remain calibrated.
   
   
   
   *if you have stereo headphones with a 1/8" plug, use a 1/8" mono plug to 1/8" stereo jack adapter, such as Radio Shack catalog number 2740368 (one of  four adpaters included in catalog number 2740069).
   
   
   
   

Schematic for the 203 Theremin
(back to contents)

Circuit Description
(back to contents)

The 203 Theremin uses two Wien-bridge oscillators, one fixed in frequency, and the other that has a frequency influenced by hand capacitance in the proximity of its antenna. It is the difference frequency between the two, obtained with heterodyning, that provides the theremin's audible tone. Wien-bridge oscillators were chosen for their good temperature stability and moderate immunity from electromagnetic and radio frequency interference. In addition, they do not require obscure air-core inductors.

The fixed-frequency oscillator is comprised of operational amplifier U2 and the frequency-determining elements in its Wien network; C2, R5, C3, and R6. Variable components C4 and RV1, also part of the Wien network, provide a means of adjusting the frequency so that a zero beat condition can be obtained when the hand is away from the antenna. The frequency of oscillation is described by the equation 1/2piRC, where R=R5=R6 and C=C2=C3. This equation yields a frequency of about 84kHz, although in practice, the actual frequency will be somewhat lower due to the addition of the two variable components and stray circuit capacitance.

Resistors R9 and R10, and capacitor C8 provide a one-half-supply reference for the oscillator. Resistor R8 converts the oscillator's output voltage into a current. Transistor Q1 provides half-wave rectification of this current, and permits it to be negative-rail referenced, as required by U1, an operational transconductance amplifier (OTA). Resistor R7 and capacitor C7 provide an averaged value of this current to the OTA's amplifier bias control input (pin 5). Note that the OTA is the generic type CA3080, once made by several IC manufacturers, but now discontinued. The part indicated is a direct replacement sold by NTE Electronics, NTE996.

The OTA's positive input is fed from the oscillator's output, substantially attenuated by R1 and R2 to prevent non-linearities in its transfer function. The OTA's output feeds the negative input of U2. The criteria for oscillation is that the inverting gain of the operational amplifier be exactly 2. This gain is partly determined by the ratio of R4 and R3, which, in itself, is greater than 2. However, a path parallel to R4, via the OTA, reduces this ratio according to the current at its amplifier bias control input (pin 5).

As Q1's collector voltage rises, the OTA's transconductance increases, effectively increasing the oscillator's negative feedback, and thus reducing the amplitude of oscillation, until the base-emitter junction of Q1 falls below the threshold of conduction. This, in turn, lowers Q1's collector voltage, sustaining oscillation. The output amplitude is primarily a function of Q1's base-emitter voltage and the value of R8.

Capacitor C1 suppresses out-of-band high-frequency components at the output of U2. Capacitors C5 and C6 provide decoupling for U1 and U2, respectively. The sine-wave amplitude at U2's output (pin 6) is between 2 and 3.5 volts peak-to-peak, centered about +4.5 volts. This amplitude is affected by the two variable components, C4 and RV1. Note that strict regulation of the oscillator amplitude with variations in the Wien network is not a criteria in this circuit; the amplitude control loop only satisfies the requirement that oscillation is sustained and remains sinusoidal for all values of C4 and RV1.

The variable oscillator, comprised of operational amplifier U5, OTA U4, and transistor Q3, is essentially identical to the fixed-frequency oscillator. Antenna A1 provides a means of frequency modulation for the oscillator with hand capacitance. Resistor R24 protects U5's input from static discharges. Charge-pump voltage inverter U6 and associated components develops a negative supply bias that approximately doubles the battery voltage to the oscillator. The increased supply voltage permits a large sine-wave voltage amplitude at the antenna, required for adequate signal-to-noise performance of the theremin. Although the charge pump is capable of self-oscillation, this is avoided to prevent unwanted audible frequency products. Instead, the charge-pump waveform is synchronously controlled by the variable oscillator via C23 and R28. The sine-wave amplitude at U5's output (pin 6) is about 13 volts peak-to-peak, centered about ground, with the antenna disconnected. This value drops to about 8 volts peak-to-peak, with the antenna inserted. The variable oscillator output voltage is determined by the value of R27, and was selected to ensure a sine-wave output with the lowest practical battery voltage.

The fixed-frequency oscillator's output is applied to R14. The output from the local oscillator is applied to the gate of Q3, a depletion-mode, N-channel, junction field-effect transistor, via C12. The levels at Q2’s gate are reestablished symmetrically about the one-half-supply reference (AC ground) via R13. With Q3's gate positive in respect to its source, it turns on, shunting the signal at the junction of R14 and R15 to AC ground. With the gate negative, Q3 turns off, allowing the signal to be present at R15. Heterodyning, which results from the gating of the fixed-frequency oscillator output by the variable oscillator output, takes place at the junction of R14 and R15.

U3A, in conjunction with R14, R15, R16, C9, and C10, form a Sallen-Key low-pass filter that removes the sum frequency from the heterodyne process, leaving the audible difference frequency. The output of the filter is AC coupled via C14 to an inverting gain stage consisting of U3B and associated components. R17 and R18 establish the DC gain for this stage. C15, in conjunction with R18, determine the U3B stage's high-frequency roll-off, and C14, in conjunction with R17, establish its low-frequency roll-off. The combined low-pass functions of the Sallen-Key and U3B sections result in an audio output that has constant loudness over the instrument's 20 to 2500Hz range. Capacitor C11 blocks DC from the output, resistor R12 discharges C11 to prevent residual charges from causing audio transients, and resistor R11 isolates U3B’s output from load capacitance and serves as a current limiter. Capacitor C13 provides decoupling for U3.

The output wave shape is essentially sinusoidal from 100Hz to the terminal frequency, with a maximum amplitude of about 2.8 volts, peak-to-peak occurring at about 180Hz. Below 100Hz, the output wave-form exhibits substantial, although pleasing, harmonics that result from oscillator coupling. The circuit produces about 15 millivolts peak-to-peak of noise, measured at zero-beat, yielding a signal-to-noise ratio of 44dB. This ratio, however, may be degraded due to external interference, which will vary considerably depending on the instrument's proximity to emissive sources such as power lines, fluorescent lighting fixtures, incandescent lamp dimmers, and computers.

The oscillators exhibit excellent frequency immunity to supply variations, with only 0.04 percent change from 6 to 9.5 volts. This supply voltage immunity, in addition to first-order cancellation between the two similar oscillators, permits circuit operation without input voltage regulation. Current consumption from a 9-volt battery is approximately 23 milliamperes, and the circuit will operate with no performance compromise down to 6 volts, thereby extending battery life considerably.

Parts Table
(Updated February 6, 2020)
(back to contents)

DISTRIBUTOR LINKS

Notes:

1. These items are included in Harrison Instruments item number 99999-5977-006. The antenna plate, without the other parts, is also available as item number 99999-5977-005.
   
   
2. 116 terminals required
   
   
3. Approximately 4' of hook-up wire required
   
   
4. Approximately 20' of Wire-Wrap® wire required
   
   
   
   
   
   
   
   
   
   
   
   
   
   

Drawing Index
(back to contents)

Photos
(back to contents)

Kynar® is a registered trademark of Elf Atochem North America, Inc.
Teflon® is a registered trademark of E. I. du Pont de Nemours and Company.
Wire-Wrap® is a registered trademark of Cooper Industries, Inc.

Text, drawings, and photographs ©2001, 2012, 2014, 2016, 2020 by Arthur Harrison

Contact Author

Back to Index Page