COTA - Voltage Controlled 'Cascaded OTA' Filter

Constructional Difficulty: Easy to moderate with surface mount parts

This is a 10HP wide four pole multimode filter module capable of -24dB/octave low pass, -12dB/octave low pass and +/-6dB/octave bandpass outputs simultaneously. The filter core is based on four identical cascaded current controlled integrators. This is similar to the topology found on the filters in the SH series of monosynths as well as the CEM3320, IR3109 and SSM2040 filters of the classic polyphonics.

At high resonance, I call it 'feedback' here, the COTA VCF will oscillate well over the whole of the audio band with a clean sinewave. The module also features a feedback input which breaks the internal feedback path that controls the resonance. This means that additional modules can process the -24dB or BP output before feeding it back into the COTA's feedback input. This allows a large variety of new timbres to be created. And it also allows for voltage controlled resonance by using a VCA module to go between the COTA's -24dB or BP output and the feedback input.

The module has two audio inputs, each with its own mini level control, which are mixed together before being sent to the filter core. The mixer can be operated in either 'clean' or 'soft clip' modes. Clean mode will simply mix the signals together and feed them cleanly into the filter core. Soft clip mode will gently limit the maximum voltage output of the mixer rounding off the top and the bottom peaks of the audio signal. In soft clip mode the asymmetry control allows you to force the mixer's output harder into the negative soft clipping zone distorting the audio in an interesting way.

The module requires +/-36mA at +/-12V.

The PCB Set

The COTA 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 core, exponential convertor and output circuitry. The pot board has the pots, switch, and sockets that are attached to the front panel, as well as the mixer, soft clipping circuit, and CV summing circuit. 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. The four transistors are SOT23, and the PNP array is a SOT457. All components are standard parts.

Sound Samples

The following samples were recorded with a small set up comprising of one Oakley DVCO, one Oakley COTA filter, two Oakley Envelopes and a Classic VCA.

A simple dual sawtooth drone using the -24dB/octave low pass output. The cut-off frequency is modulated by a triangle wave LFO (from an Oakley MOSPhaser). The sample is first recorded completely dry, and then about half way through some echo and reverb is added. The final addition is a triangle wave from the DVCO added to the VCA's second input bypassing the filter.

An Oakley Sequencer drives the DVCO whose VCOs, one saw and one square an octave lower, feed the COTA and then to the final VCA and out. The sequencer's parallel load function is played manually to create several different sequences from the same eight notes. The COTA's settings and the envelope's decay time are manually tweaked as the set plays. A simple mono delay is added for that Berlin School feel.


Click here for ordering details.

COTA (Euro) main board issue 1 PCB........................................... 12GBP

VCF (Euro) pot board issue 1 PCB .................................................. 6GBP

Both boards are required to build the Oakley COTA 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

COTA (Euro) Builders 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 PCBs and will 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 secure online 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: September 25, 2023.