Eurorack systems are the most popular format for modular synthesis, because they are readily available in variety of modules, and they’re more portable than traditional large-format 5U systems.
Eurorack synth guru DivKid shares his thought on what you’ll need to get started. And he touches on case considerations, power, patching, modules.
According to DivKid, a good starter system will include: an oscillator (VCO) or other sound generator; a filter (VCF); an amplifier (VCA); and some modules that can be used for modulating controlling or automating the settings on your synth (keyboard, sequencer, LFOs, envelopes, etc).
To understand how to work with Eurorack Modular Synthesis, watch this video.