Under my fiancee’s and my brand RE:Flex Sound, I’m working on a versatile Eurorack format filter, with intent to bring it to market.

While working on my polyphonic synthesiser, I refined the features for its state variable filter. This filter is based on the Thomas Henry VCF-1 but has some noteworthy additions:

• Voltage controlled resonance
• Drive (pre-amplification), also voltage controlled
• “Attenuverters” instead of attenuators for modulation sources, allowing to invert modulation within the same knob rotation
• “Notch” (band reject) output

This will be my first module I intend to bring to market. Since it’s not made to work only in my setup, I’m developing it to a much higher standard than my older ones.

The module has a defacto standard 10-pin power connector, and comes with reverse polarity protection. Calibration for the 1V/Octave response is available with a trimpot. There is space on the board to install a multi-turn trimpot for those who desire even more precision.

Work Involved

• Circuit design for all the additions, including simulations (LTSpice)
• Schematic capture (KiCad)
• Circuit board design (KiCadD, 4-layer design)
• Front panel design (Inkscape, KiCad)
• Branding/logo design (Inkscape)
• Some assembly for through-hole parts and a few SMD parts
• Extensive testing with function generator, oscilloscope, and in context of actual modular synthesiser

Work left to be done for a new revision:

• Fix some pinout errors on the PCB
• Circuit revision for “drive” control (ranging, compensation at output)

Additional Images

Here’s a look at the back of the module, showing the component side of the board, including some bodges to address issues found.

The top level schematic: