Croglin Dual Low Pass Filter

Voltage Controlled Dual Low Pass Filter

The Oakley Croglin Voltage Controlled Filter is based around two unusual and quirky low pass filter circuits that operate either together or against each other.

The filter design is inspired by a peculiar low pass filter topology first seen in a portable synthesizer from the late 1970s. For the Croglin VCF I have completely redesigned the circuit to produce a stable, lower noise design that unlike the original covers a much wider frequency range and gives the user far greater control over the parameters of the filter.
The cut off frequency of each filter can be controlled by a common control voltage and the front panel's Frequency pot. The second filter's cut off frequency can also be offset from the first. This can be done with either the Offset pot or an offset CV. The offset can be both positive, making the cut off frequency of the second filter higher than the first with a positive input CV, or negative, making the frequency lower than the first.
The two filters work in parallel on the audio input signal. The output of the first filter is then passed directly to the output but a proportion of the second filter may be added or subtracted from this. The level of the second filter is controlled by the Duality pot. In the middle position the Duality pot allows no audio to come from the second filter. Fully clockwise the second filter is added to the first filter's output. Fully anti-clockwise the second filter's output will subtract and cancel out those parts of it that are common with the first's output. A wide variety of tonal responses can be made; from the traditional low pass filter and band pass sounds to complex vocal like formants.
The 1U 'Filter Core' format is our way of handling filter modules. Although the 1U module can be used as a filter module on its own, it is expected that users will make use of external mixers to control CV and audio levels going into the filter. In this way, you will be able to have a collection of space saving 1U filter cores that can be used with any generic mixer module. The popular Oakley Multimix is the perfect choice for such a mixer module.
For the 2U design three audio inputs are provided, two of which have their own attenuator. Two CVs can control the cut-off of the filter. One is fixed at 1V/octave and the other one has an input attenuator. Unlike the 1U design the Offset can be controlled simultaneously with a front panel pot and an external CV whose sensitivity is controlled with a reversible attenuator.
The module accommodates either our standard Oakley/MOTM power header or a power header.


  • 25 mA +15V
  • 25 mA -15V
  • 0 mA 5V
  • 78 mm deep
  • Ø 4.00 (1 Votes) Average Rating

This Module is currently available.

$237 Price in € submitted Oct 25th 2012, 21:18 by Squiggletronics | last Change Oct 11th 2017, 04:29 by CoreyBairre