Jule T.'s Analog Synthesizer Build Breaks Things Down Into Easy-to-Understand Sub-Circuits

Maker Jule T. has published a guide on building an analog synthesizer, based around an oscillator with selectable — and carefully-calculated — frequency, input keys, and an amplifier feeding an upcycled speaker.

"Analog synthesizers are very cool, but also quite difficult to make," writes Jule. "So I wanted to make one as simple as it can get, so its functioning can be easily understandable. For it to work, you need a few basic sub-circuits: A simple oscillator with resistor selectable oscillating frequency, some keys, and a basic amplifier circuit."

Jule's guide covers the theory behind the synth's operation — an astable multivibrator circuit with an op-amp, which creates a square-wave output that can be turned into audible sound via the attached speaker - as well as the physical build process. There's also some mathematics involved, in the calculation of exactly what resistor values are required to tune the oscillator to a particular note.

"With the individual keys, you select the desired resistance and the desired tone is produced," Jule explains. "By pressing multiple keys at once, you connect more branches of the resistors in parallel and effectively connecting them in parallel, reducing the total resistance. Lower resistance produces higher pitched tone."

The synth circuit is then connected to an AB-class amplifier — "very basic," Jule admits, "but it works well" — with a logarithmic-scale potentiometer for volume adjustment. "Because the potentiometer on its own in the circuit would de-tune the oscillator (added resistance), I slapped an op-amp buffer in front of it which introduces high input resistance for the circuit in front of it and low impedance for the circuits after it. Basically a buffer is an amplifier with a gain of 1."

The full build guide is available on Instructables, along with a PDF for etching a PCB based on the schematics.