Used for contouring sounds, and various modulation depth control in my DIY modular synthesiser.

This dual VCA module is essentially Ray Wilson from Music From Outer Space’s Dual Log/Linear VCA circuit design, with small tweaks for my power supply differences.

It comes in handy not only for standard contour control, but also for what you could call “meta-modulation”, controlling the depth of one modulation connection with another control voltage. Further, it can also be used for ring modulation which lets you produce bell-like effects. VCAs are essential in any synthesiser—it’s often said you can never have too many of them.

This design is based around the dual operational transconductance amplifier, LM13700, which has two units in one IC. As such it’s only natural to design the module to have two VCAs.

Work involved

• Schematic capture (EAGLE)
• PCB design (EAGLE)
• Panel design (Inkscape)
• Laser cutting at local makerspace
• PCB assmbly (through-hole soldering)
• Panel assembly

Here’s a view of the pretty purple board fabbed by OSHPark:

You can find all the design files on GitHub: VCA module