Oakley Modular Euro COTA – VC ‘Cascaded OTA’ Filter

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.