The 2U version of the COTA VCF
This is a four pole multimode filter
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. It’s temperature compensated
too. The module also features a feedback input. This is similar
to the one we introduced in the groundbreaking Multiladder module.
In conjunction with the 24dB/octave output this input allows the
resonance signal path to be split. This means that additional
modules can process the resonance loop independentally. This
allows a large variety of new timbres to be created. And it also
allows for voltage controlled resonance with the use of an extra
single VCA module.
The design is intended to fit into a 1U
wide ‘filter-core’ module or a more fully featured 2U wide panel
with seven control pots.
The compact 1U ‘filter core’ module
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 Oakley Multimix is an ideal choice for a handy mixer
module.
For the 2U design three audio inputs are
provided each with its own attenuator. Three CVs can control the
cut-off of the filter. One is fixed at 1V/octave, the other two
have input attenuators. CV1 features a reversible type attenuator
with inverting/non-inverting properties.
Power (+/-15V) is provided to
the board either by our standard Oakley 4-way header or
Synthesizers.com header.
The board size is 104 mm (high)
x 104 mm (deep). Current draw is around 20mA per rail.
The issue 3 filter core version of the COTA
VCF in a natural finish Scheaffer panel. Note the optional Sock8
socket board used for easier construction.
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.
The following two
samples use the same equipment as above but add a Oakley
Overdrive module at the output of the COTA.
A lightly overdriven solo sound
using the COTA to shape the harmonics heard
More overdrive with the COTA set
to highish resonance and swept with an ADSR, firstly on slow
attack, then quicker.
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 PCB(s) and will only be sent via e-mail when
the board(s) are shipped.
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 filter core
module in traditional black
5U format filter
core module in natural silver
5U format full width
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.
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