Croglin - Voltage Controlled Dual Low Pass Filter

PCBs for this project have been discontinued

The panel design for the 1U wide 'filter-core' Croglin.

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. Current consumption is approximately 25mA per rail.

The PCB is 7.8cm (deep) x 14.4cm (height). As with all Oakley projects the PCB is double sided with through plated holes, has tough solder mask both sides, and has bold component legending for ease of construction.

The suggested front panel design for the 2U version of the Croglin VCF.

Sound Samples

The above track was done with four overlaid tracks using a small Oakley modular. A simple drum track was added for extra spice. All the filtering on each analogue part was done with a Croglin.

The above track is a solo of the drone used in the first track. Again you can hear the dual filters of the Croglin module at work.

The issue 2.1 Croglin 'Filter Core' VCF as a single width MOTM format module in a natural finish Schaeffer panel. Note also the use of the optional Sock4 socket board to help keep the wiring to the sockets neat and tidy. The empty resistor and header locations are used in the 2U wide full version.


Croglin VCF User Manual

Croglin VCF issue 2 Builder's Guide

Construction Guide Our handy guide to building Oakley DIY projects

Parts Guide Our handy guide to buying parts for Oakley DIY projects.

Schematics are only available to purchasers of the PCB and will be sent via e-mail when the board(s) are shipped.

Use 'save as...' button to download and view the files. IE users need to right hand mouse click and then choose 'save as'.

Front Panel database

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

5U format 1U wide (Filter Core) module in traditional black

5U format 2U wide (Full Version) module in traditional black

To read these files you will need a copy of 'Frontplatten designer' from Schaeffer. The program also features on-line ordering, so its now even easier to buy your panels from Scheaffer.

The company are based in Berlin in Germany and will send out 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.

Schematics and Projects: Projects

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Copyright: Tony Allgood.
Last revised: February 12, 2024.