A lot of synth modules schematics by Ken Stone http://www.cgs.synth.net/modules/#cgs48

Navs modular lab http://navsmodularlab.blogspot.de

Arduino audio input http://www.instructables.com/id/Arduino-Audio-Input/step8/Clipping-Indicator/

Interface Arduino to I2C SRAM http://www.instructables.com/id/Interface-an-SRAM-bus-to-your-arduino/step3/Two-RAM-Chips-Madness/

Nootropic Designs audio hacker shield

http://nootropicdesign.com/projectlab/2013/07/05/audio-sampler/

http://nootropicdesign.com/audiohacker/

LM13700 – OTA – an opamp that outputs current instead of voltage, good for VCA http://www.ti.com/lit/ds/symlink/lm13700.pdf