I found this series of videos, created by Moritz Klein, in which he explains how to build a synthesizer in 8 -quite simple- steps. Most of the parts are created with simple components, such as TL074 opamps and 40106 inverting buffers.
The really interesting part of this DIY project is that Moritz Klein explains a lot about the workings of each essential electronic component and its application. It certainly is a clear introduction in electronics so even if you are not planning to build a synth this series is very valuable.
The series consists of the following parts:
- DIY Vco Part 1: the core oscillator
- DIY Vco Part 2: exponential converter
- DIY Vco Part 3: tuning
- DIY Vco Part 4: more waveforms
- DIY Vcf Part 1: Filtering basics
- DIY Vcf Part 2: active filters and resonance
- DIY Vcf Part 3: resonant high-pass & vactrol based voltage control
- DIY Vca: designing a classic transistor VCA from scratch
- DIY ADSR: designing a simple ADSR envelope generator from scratch
- DIY S&H: designing a sample & hold circuit from scratch
- DIY Mixer: designing a 3 channel mixer
This list is not complete, but this is enough to build a complete working synthesizer.
Moritz Klein offers more construction projects (still including all explanation) to expand your DIY synth, such as voltage controlled PWM, looping envelopes, PSU (power supply), saw to triangle converter, 3VCO module, diode ladder filters and more. Klein also found collaboration with Eric Synths: some of his modules are for sale as DIY kits