Tony Bassett No 1 Electronics Theremin

Getting a Tony Bassett No 1 Electronics Theremin to hold tune

The original No 1 Electronics theremin circuit is one of those things that looks trivial on paper and refuses to behave when you build it. I spent an afternoon last winter chasing a 60 Hz hum that wouldn't go away, only to realize the ground loop was coming through the antenna shield because I had tied the shield directly to chassis earth instead of letting it float. Moving that connection to a single star point at the op-amp ground net stopped the buzz immediately. The schematic from Tony Bassett No 1 Electronics Theremin works exactly as drawn when the layout stays close to the prototype; stray capacitance and poor grounding are what turn it into a nightmare. Start with a decent supply. The circuit runs off a dual rail, typically ±9 V to ±12 V, and the oscillator stage is where noise sneaks in if your regulator is cheap. I use a linear regulator on the breadboard version rather than a switching supply, even though that means a bit more heat. The pitch antenna and amplitude loop antenna both sit on the same ground plane, so keep them physically separated by at least a few centimeters and route their leads away from each other. When they couple magnetically you get unwanted feedback that sounds like a wobble instead of a clean vibrato. The tuning process is straightforward but unforgiving. Power up and measure the oscillator frequency with the pitch antenna unplugged. It should sit somewhere in the 400 kHz to 800 kHz range depending on your inductor and capacitor choices. If it drifts more than a few hundred hertz over ten minutes, check your capacitors for ceramic microphonics. X7R types tend to shift with temperature. C0G/NP0 are worth the extra cost here. I switched to C0 on the oscillator tank once and the drift dropped to nearly nothing. That was the single biggest improvement I made across dozens of builds.

For the amplitude stage, adjust the pot so the speaker output is quiet when your hand is far from the loop antenna and swells smoothly as you approach. The tricky part is the phase compensation. If you push the potentiometer too far, the loop oscillates on its own and you get a howling feedback tone that won't stop even when you walk away. Back it off until the threshold is just below self-oscillation. You want the gain margin to be small but positive.

Common failure modes and what actually fixes them

The most frequent problem I see is the pitch antenna picking up RF from nearby electronics. A phone charger within two meters can shift your note by a semitone or more. The fix is a short length of braided copper around the antenna lead and a ground connection that returns to the star point I mentioned earlier. This shields the high impedance node without killing the capacitive coupling you need for playing. Another issue is the hand capacitance being too small to move the frequency enough for a useful range. If you're building this for someone with smaller hands or a child, add a parallel capacitor between the antenna and the oscillator input. Even 10 pF makes a noticeable difference. I have seen builders add a metal plate instead of a wire antenna to increase the effective area. It works, but it changes the touch characteristics so the instrument feels less like a traditional theremin and more like a proximity pad. That is fine if that is what you want, but don't pretend it behaves the same way. One edge case that cost me two days: the speaker output was distorted only when playing certain notes. The oscilloscope showed clipping on the peaks. I traced it back to the power supply droop under audio signal load. The oscillator stage was stealing current during high-pitch notes when the hand capacitance reduced the tank Q. A larger bulk capacitor on the supply rail and a decoupling capacitor right at the IC solved it. This isn't mentioned in the basic schematic, but it is the kind of detail that separates a working build from one that sounds okay at rest and falls apart under use.

When this circuit won't work for you

The No 1 Electronics design assumes you want an analog pitch control with a direct capacitive sensor. If you need polyphonic output or MIDI connectivity, this is the wrong starting point. You would be better off building a digital version with a microcontroller reading capacitance through an RC timing circuit. The analog approach is honest about its limitations though. It gives you a pure, continuous pitch curve that some players prefer, but it also means any drift in component value or temperature becomes a performance problem. I also recommend against using this circuit in an environment with significant electromagnetic interference unless you can control the space. A studio with fluorescent lights, dimmers, and WiFi routers nearby is asking for trouble. The circuit is sensitive by design. That sensitivity is what makes it playable, but it also makes it fragile in real-world conditions. For a beginner project, this build is manageable if you take your time with the layout and test each stage before connecting the next. Measure the oscillator first, then the amplitude loop, then combine them. Don't solder everything at once and hope for the best. The schematic from the Tony Bassett No 1 Electronics Theremin publication is reliable, but the reliability comes from careful assembly, not from the circuit forgiving bad breadboard work.