Croglin - Voltage Controlled Dual Filter

Constructional Difficulty: Easy to moderate with surface mount parts

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.

Sound Samples

All the following samples use an Oakley D-VCO which connects to an Oakley Multimix and then to the Croglin.

A bass drone using Filter B in low pass mode and manually tweaking frequency and resonance. A little light reverb has been added for ambience.

A dry sample of the same bass note but this time using Filter B in its band pass filter mode.

Using the Croglin's main output with Duality set to its maximum positive position and Filter B sightly offset from Filter A. The Croglin's cut-off is modulated by a looping envelope. The resonance is increased slightly about half way through this sample.

This time the Croglin's slightly offset dual filters are swept from a triangle wave LFO. The Duality control is set fully negative and the Resonance about half way. A little reverb has been added.

A simple sequence is being played while the Croglin's controls are being manually tweaked. The Croglin's cut-off is being swept by an Oakley Envelope. A delay effect has been added for extra bounce.


Click here for ordering details.

Croglin (Euro) main board issue 1 PCB.................................... 12GBP
Croglin (Euro) pot board issue 1 PCB ........................................ 6GBP

Both boards are required to build the Oakley Croglin for Eurorack project.

All other parts can be purchased from your usual electronic component supplier.

All prices include VAT at UK rates. Shipping is additional to these prices. See also the FAQ page.

Project Downloads

Croglin Builder's Guide Construction guide and parts list.

Construction Guide My handy guide to building Oakley DIY projects

Parts Guide My handy guide to buying parts for Oakley DIY projects

Schematics are only available to purchasers of the PCBs and will only be sent via e-mail when the boards are shipped.

Front Panel database

A Schaeffer front panel can be made for this module. The database can be found by downloading the following link:

10HP Eurorack format in natural silver

To read, edit and print these files you will need a copy of 'Frontplatten Designer' from Schaeffer. Panels can be ordered via the program using its web based ordering system.

Schaeffer are based in Berlin, Germany and can send panels to anywhere in the world. Delivery to the UK normally takes around ten days. For North American users you can also order your Schaeffer panels from Front Panel Express.

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Copyright: Tony Allgood.
Last revised: June 30, 2020.