This filter is part of my Eurorack series. The design is based on the Steiner-Parker diode filter, and I stripped it down to a minimal set of components while keeping its characteristic sound. It has controls for frequency cut-off, resonance and CV level. There are distinct inputs for low-pass, band-pass and high-pass, which allow all sorts of filter responses when combining them.

Key features

• voltage controlled diode filter
• inputs for low-pass, band-pass and high-pass response
• controls for frequency, resonance and CV input level
• two CV inputs for controlling the cut-off frequency
• easy to build, all through hole components
• 10 HP

How it works

It is basically an RC-filter circuit where the resistors are replaced by diodes to have voltage control over the filter cut-off frequency. The diodes change resistance when their forward bias voltage is changed by the differential amplifier formed by Q1 and Q2. Inputs are introduced in different places of the circuit to have low-pass, band-pass and high-pass responses. The inputs are buffered and can be combined to achieve different filter responses. There are controls for filter cut-off frequency, resonance and level for one of the CV inputs.

The kit includes

• all needed quality components
• optionally matched transistors

More technical details and assembly guide can be found at https://polykit.rocks/vcf