Oakley Modular Euro Croglin – VC Dual Filter

The 10HP wide Oakley Croglin Voltage Controlled Filter (VCF) is based around two low noise ‘Sallen-Key’ filter circuits that operate either together or against each other.

The cut off frequency of both filters, named A and B, can be controlled by a 1V/octave control voltage (CV) input, a variable (0% to 200%) CV input, and the module’s own Frequency pot. Filter B’s cut off frequency can be made to be different to Filter A with either the Offset pot and/or an external offset CV. This frequency offset can be both positive, making the cut off frequency of Filter B higher than Filter A, or negative, making the frequency lower than Filter A.

The two filters work in parallel on the same audio input signal. Filter A is a -12dB/octave low pass filter, while Filter B can be switched to be either a -12dB/octave low pass filter, or a +/-6dB/octave band pass filter. Each filter has its own output on the front panel.

A third output, called simply ‘Output’, combines the signals from each filter. The output of Filter A is summed together with a proportion of Filter B, the actual proportion, from -100%, through 0%, to +100%, being set by the Duality control. In its middle position the Duality pot allows no audio to come from Filter B, so you just hear the signal coming out of Filter A. Fully clockwise, the signal from Filter B is added to Filter A, so the two filters augment each other. Fully anti-clockwise, Filter B’s output will subtract and cancel out those parts of the signal that are common with Filter A’s output. A wide variety of tonal responses can be made; from a traditional low pass filter and band pass sounds to more complex vocal like formants.

At higher levels of resonance both filters will self-oscillate, with a sine like waveform, over the full audio band and beyond.

The module requires +/-12V and has a current consumption of +60mA and -60mA.

The PCB Set

The Croglin module comprises of two printed circuit boards (PCBs) connected together with six 0.1 (2.54mm) single in line (SIL) headers and
sockets. The main board on the rear of the module houses the power input and conditioning, the filter cores, half of the audio output circuitry, and CV control circuitry. The pot board has the pots, switch, and sockets that are attached to the front panel, as well as the resonance, CV input and the other half of the output circuitry. The main board is a four layer design, while the pot board is a two layer design. Each board is the same size; 49 mm x 107 mm.

The majority of the components are surface mount parts. The ICs are SOIC, and the resistors and surface mount capacitors are 0805.
All components are standard parts.