Croglin

Voltage Controlled Dual Low Pass Filter

The Oakley Croglin Filter is based around two unusual and quirky low pass filter circuits that operate either together or against each other. This peculiar low pass filter topology is first seen in a portable synthesizer from the late 1970s. For the Croglin VCF the circuit is completely redesigned 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. In addition, the second filter's cut off frequency can be offset from the first. This can also 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, or negative, making the frequency of the second filter 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 is subtracted from the first filter's output. It will then cancel out those parts of it that are in 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.

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