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 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 output and 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, while the resistors and surface mount capacitors are 0805. The two transistor arrays are SOT457. All components are standard parts.

Sound Samples

All the following samples recorded with a small set up comprising of one VCO, one COTA filter, two ADSR/VCA and a midiDAC to control them all. Some reverb was added for a little ambience and because I can't help myself....

Using the -24dB/octave low pass output. Simple 16 note sequence with an input from the sawtooth output of a single Oakley VCO. The filter is manually being swept via the 'freq', 'resonance' pots and altering ADSR sweep depth.

Using the BP output. Another simple 16 note sequence.

Using the -12dB/octave output. Another simple 16 note sequence.

Using the -24dB/octave output. Another simple 16 note sequence making use of lots resonance to show how squelchy this one can get.

Using the -24dB/octave output. Lots of little blips, bleeps and cheesy riffs.


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.

Back to Eurorack projects page: Projects

Back home

Copyright: Tony Allgood.
Last revised: September 09, 2019.