Building a Modular Synthesizer from Scratch

Started building Eurorack modules from scratch last year. Here's what I've learned about the analog electronics involved.

A voltage-controlled oscillator (VCO) core uses an expo converter (matched transistor pair like the LM3046) to convert 1V/octave control voltage to an exponential current. This current charges a capacitor that resets at a threshold, creating a sawtooth. The expo converter's temperature coefficient must be compensated — a thermistor in the expo converter biasing network corrects for the transistor's -0.33%/°C tempco.

The Moog-style transistor ladder filter (VCF) is four low-pass RC stages with JFET voltage control. The resonance feedback path creates the characteristic self-oscillation. Getting the tuning right requires matching the JFETs — measure Idss for each and select matching quads. It's tedious but the sound difference between a matched and unmatched ladder is audible.