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.
Project Downloads
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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