In light of the Theremin’s strong association with mid-twentieth-century science fiction, a primary goal set for the GT Theremin was to pay credence to its history by making it resemble a classic sci-fi movie prop.  This requirement was reflected in nearly every detail, from the circuit design to the physical construction.

Principles of operation

The GT Theremin fundamentally functions similarly to Lev Termen’s original invention.  Pictured above is a functional block diagram showing the interconnection of core components of the GT Theremin’s signal circuitry (the power supply is not shown).  Tones are generated by mixing the outputs of two closely-tuned RF oscillators together in a nonlinear mixer (also called a “converter” in radio contexts to help distinguish from linear mixers).  The mixer operates on the signals similarly to mathematical multiplication, resulting in the generation of frequencies that are the sum and difference of the input signal frequencies.  This action, known as heterodyning, is used in many radio architectures to convert the information-carrying signal to different frequency bands (it might be pointed out that a Theremin design like this one rather resembles a mixed-up AM/FM radio receiver).  Heterodyning may be understood by considering the following trigonometric identity:

In a Theremin, the RF pitch oscillators are tuned very close together, such that the difference frequencies produced by heterodyne action are in the audible band (less than about 22 kHz).  The original (inaudible) RF signal and sum frequencies are discarded by a low-pass filter.  One pitch oscillator runs at a constant reference frequency, while the other variable pitch oscillator is de-tuned slightly by the capacitance between the pitch antenna and the player’s hand.  Because the variable pitch oscillator runs at a fairly high frequency, the relatively small capacitance change associated with the player’s hand movement is sufficient to effect a frequency difference of a few kilohertz, giving the Theremin its pitch range.  In the GT Theremin, the nominal frequency chosen for the pitch oscillators was originally 700 kHz, but this was later reduced to 500 kHz to lower the pitch range and sensitivity somewhat.

The volume modulation circuitry begins with another RF oscillator, tuned to a frequency significantly different than the pitch oscillators to avoid interference.  The frequency of oscillation is modified slightly by the presence and position of the performer’s hand relative to the volume antenna.  To help detect this frequency change, a band-pass filter with a steep fall-off or high Q-factor (these aren’t the same thing, but I won’t make a distinction at this point) greatly reduces the volume signal amplitude when it deviates from the filter’s center frequency.  This yields a signal whose amplitude corresponds to the distance of the performer’s hand to the volume antenna.  The frequency and amplitude-varying signal is converted into a DC voltage by a peak detector circuit.  This DC control voltage corresponds to the player’s volume hand distance and is used to set the gain of an amplifier which acts upon the pitch signal.

Circuitry overview

In line with our goal of making a “sci-fi” instrument, and giving credence to Termen and RCA’s designs, I decided early on that the GT Theremin would use only vacuum tubes as active devices (as opposed to transistors).  This presented a number of unique challenges, since this was my first experience with vacuum tube circuits. Before I proceed any further, I feel a warning is in order for anyone thinking about building a similar design:

ACHTUNG! The electrical potential present in powered vacuum tube circuits is hazardous and potentially lethal.  Do not even think about working with vacuum tube circuits unless you are very confident of your electronics abilities and there is someone else nearby to notify your next of kin.

Building a vacuum tube Theremin is not a project for beginners, and high voltages are nothing to be trifled with.  While the voltages specified in the GT Theremin design are lower than those used in many vacuum tube circuits, they can still kill you under the right (or wrong) circumstances.  Please don’t mess with tubes if you’re at all unsure about your electronics abilities, and please exercise caution otherwise. That said, if you are comfortable working with typical AC mains voltage, you should be fine. I lived to tell this tale by exercising due caution and always being careful to power down the high voltage circuitry before working on it.

Vacuum tubes

If you were born after the year 1960 or so (like myself), there’s a fair chance you don’t know what a tube is.  Vacuum tubes (also called thermionic valves) are electronic devices which were prevalent during the early twentieth century before being mostly supplanted by transistors in the 1950s and 1960s.  They work on the principle of thermionic emission; in general, where charged particles gain enough thermal energy to overcome an energy barrier.  In this case, the charge carriers are electrons, and the energy barrier is the interface between a metal and a vacuum.  An electrode called the cathode is heated until it emits electrons, forming a negative space charge around it.  When a potential difference is applied between the cathode and another electrode called the anode (informally, the plate), electrons drift from the cathode to the anode due to the electric field between them.  If the bias is reversed (anode at lower voltage than cathode), very little current flows because the anode is not hot enough to emit significant quantities of charge carriers. This is a basic vacuum diode, and by adding additional electrodes called grids, the flow of current between the anode and cathode can be strongly controlled.

The common three-terminal vacuum tube is called a triode (pictured right), which can be used as an amplifier or switch, its small-signal model being more or less identical to that of a FET.  There is a lot more to be said about different types of tubes, how they work, and how they were used.  If you’re interested in learning more, James Calvert at the University of Denver has written an excellent and extensive page on the subject, which I highly recommend as a starting point.  For the purposes of the GT Theremin, knowing that a vacuum tube diode is roughly analogous to its semiconductor counterpart and that a triode is somewhat reminiscent of a FET is sufficient.  I also used a third, more exotic type of tube, but I’ll explain its function later.

Other components

While I opted to use vacuum tubes exclusively, I did not attempt to recreate “authentic” early-twentieth-century Theremin circuitry. Many of the components and construction techniques I have used were not available then. I did this largely as a matter of convenience and expense; given the constraints on my time and funds, it would have been infeasible to add the additional burden of creating a historically-accurate design. Therefore, you’ll notice modern electrolytic capacitors, molded chokes, LEDs, protoboards, heat shrink tubing, pin headers, etc. Additionally, many of the parts we used were scrounged from ECE Senior Design Lab stock, so some aspects of the design were influenced simply by what we had on-hand. As much as possible, I’ll describe easy component substitutions as I detail the circuit design.

You may be wondering why I bothered to use vacuum tubes at all if our aim wasn’t to build a true 1930s-style Theremin. The official answer is “in order to appeal to the well-known musician preference for vacuum tubes and add to the instrument’s aesthetic appeal as a performance piece.” The real reason is simply curiosity and an appeal to panache. I wanted to do a project with vacuum tubes for the challenge and the fun of discovery. What’s more, hopefully you’ll agree with me that a vacuum tube looks infinitely cooler than a packaged transistor. There’s no reason I couldn’t have designed a very similar (and cheaper!) Theremin circuit using discrete transistors or op-amps, but I wouldn’t have had nearly as much fun building or playing with it. We wanted to design something in the spirit of the original, but with a healthy dose of our own style mixed in.

So without further ado, here is the full schematic for the GT Theremin (PDF).  Read on for circuit details, or skip to the end if you’d prefer to just see the results.

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