The Air Between My Fingers Wouldn't Sing

So here’s the thing about building theremins: you’re not really building an instrument. You’re building two radios that fight each other, and the sound of that fight is what you play.

Let me back up. Yesterday I was winding guitar pickups—eight thousand turns of copper around alnico magnets, trying to turn string vibration into voltage. Straightforward transduction, Faraday 101. But somewhere around turn six thousand, while I was thinking about how the coil’s inductance interacts with cable capacitance to shape the tone, I remembered the beat-frequency oscillator in my receiver. The BFO. The thing that makes CW signals audible by mixing them with a local oscillator to produce an audio-frequency difference tone.

And I thought: wait. That’s a theremin. That’s exactly a theremin.

You’re probably thinking I’m overcomplicating this. A theremin is the spooky sci-fi instrument, right? You wave your hands and it makes wobbly sounds. Clara Rockmore played classical repertoire on one. It’s in every B-movie from 1951. What’s there to build?

Everything, as it turns out. The RCA Theremin from 1929 ran two oscillators around 260 kHz—well below the AM broadcast band, deliberately chosen to avoid interference. One oscillator has a fixed frequency. The other has an antenna, and your hand becomes one plate of a variable capacitor. Move closer, the capacitance increases, the frequency drops. The two oscillators heterodyne together—that Canadian word, courtesy of Reginald Fessenden, Greek roots meaning “different power”—and the difference frequency falls into the audio range. That’s your note.

It’s the same physics I use for receiver tuning. It’s the same physics that makes the aurora hiss when you sonify it. Two frequencies collide and you hear the ghost of their difference.

But here’s where the circuit voicing comes in, and why I ordered parts instead of a kit. A cheap theremin—and I’ve heard a few—has terrible pitch response. The notes are cramped near the antenna and stretched far away, which makes playing anything melodic nearly impossible. Professional theremins fix this with a linearization coil: you wire an inductor in series with the antenna to create a resonant circuit that flattens the response curve. Without it, you get maybe eight inches of playable range. With it, you get three feet.

That coil is where the voicing lives. Its inductance, the antenna’s capacitance, the operating frequency of your oscillators—these interact to set the instrument’s personality before you’ve played a single note. You know how I wound those guitar pickups tighter or looser to change the resonant peak? Same principle. Different instrument. The oscilloscope shows you what you’re doing: the heterodyne frequency dances as you move your hand, and the waveform shape tells you whether your output stage is distorting or your coupling is too tight.

What I don’t have yet is the ear for it. Clara Rockmore—who was a violin prodigy before tendinitis ended that career—once said you cannot sway on your feet while playing. You cannot nod your head. Any body movement shifts your capacitance and changes the pitch. Her technique involved keeping her thumb and forefinger touching to stabilize her hand and control vibrato deliberately rather than accidentally.