I started a new Quad VCA project. This has been on hold for too long time. I started my modular synthesiser DIY-projects in the beginning of year 2000 with some Oakley Sound System modules (VCO, VCF). I have the schematics for Oakley's Basic Series VCA that uses the BA6110 OTA-chip. This chip is a high quality replacement for Rolands BA662-chip that has been used in several Roland synths (TB303, System 100M etc.). I have several BA6110 chips so it's time to put them in use (I think this chip is these days also an obsolete hard-to-find component).
Usually VCA (voltage controlled amplifier) input comes from audio output of filter or oscillator and the CV (control voltage) comes from an envelope generator (ADSR). A good VCA amplifies also other CV's. So you can use VCA to adjust the output level of LFO's and ADSR's.
VCA's are always needed in modular setups but I have a specific application in my mind. I will use this Quad VCA to control my Nord Drum 2 module together with Intellijels Quadra AD envelope generator. Here the AD-generator simulates hitting of an drum-pad connected to Nords trigger-input and VCA simulates how hard the pad is hitted.
Here is a short example how this sounds. With Quad AD and VCA I can use this method to control dynamically four Nord Drum trigger-inputs. As you can hear, Nord Drum sounds respond nicely to the hitting velocity.
This is a MOTM-size project. All the components are mounted on the PCB. Here is a picture of the 50% ready PCB-layout.