The panel design for the 1U wide ‘filter-core’
Croglin.
The Oakley Croglin Voltage
Controlled Filter is based around two unusual and quirky low pass
filter circuits that operate either together or against each
other.
The filter
design is inspired by a peculiar low pass filter topology first
seen in a portable synthesizer from the late 1970s. For the
Croglin VCF I have completely redesigned the circuit to produce a
stable, lower noise design that unlike the original covers a much
wider frequency range and gives the user far greater control over
the parameters of the filter.
The cut off
frequency of each filter can be controlled by a common control
voltage and the front panel’s Frequency pot. The second filter’s
cut off frequency can also be offset from the first. This can be
done with either the Offset pot or an offset CV. The offset can
be both positive, making the cut off frequency of the second
filter higher than the first with a positive input CV, or
negative, making the frequency lower than the first.
The two filters
work in parallel on the audio input signal. The output of the
first filter is then passed directly to the output but a
proportion of the second filter may be added or subtracted from
this. The level of the second filter is controlled by the Duality
pot. In the middle position the Duality pot allows no audio to
come from the second filter. Fully clockwise the second filter is
added to the first filter’s output. Fully anti-clockwise the
second filter’s output will subtract and cancel out those parts
of it that are common with the first’s output. A wide variety of
tonal responses can be made; from the traditional low pass filter
and band pass sounds to complex vocal like formants.
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 popular
Oakley Multimix is the perfect
choice for such a mixer module.
For the 2U
design three audio inputs are provided, two of which have their
own attenuator. Two CVs can control the cut-off of the filter.
One is fixed at 1V/octave and the other one has an input
attenuator. Unlike the 1U design the Offset can be controlled
simultaneously with a front panel pot and an external CV whose
sensitivity is controlled with a reversible attenuator.
The module accommodates either our standard
Oakley/MOTM power header or a Synthesizers.com power header.
Current consumption is approximately 25mA per rail.
The PCB is 7.8cm (deep) x 14.4cm (height).
As with all Oakley projects the PCB is double sided with through
plated holes, has tough solder mask both sides, and has bold
component legending for ease of construction.
The suggested
front panel design for the 2U version of the Croglin VCF.
Sound Samples
The above track was done with
four overlaid tracks using a small Oakley modular. A simple drum
track was added for extra spice. All the filtering on each
analogue part was done with a Croglin.
The above track is a solo of
the drone used in the first track. Again you can hear the dual
filters of the Croglin module at work.
The issue 2.1 Croglin ‘Filter Core’ VCF as
a single width MOTM format module in a natural finish Schaeffer
panel. Note also the use of the optional Sock4 socket board to
help keep the wiring to the sockets neat and tidy. The empty
resistor and header locations are used in the 2U wide full
version.
Downloads
Croglin
VCF issue 2 Builder’s 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 PCB and will be sent via e-mail when the
board(s) are shipped.
Use ‘save as…’ button to
download and view the files. IE users need to right hand mouse
click and then choose ‘save as’.
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 1U wide (Filter
Core) module in traditional black
5U format 2U wide (Full
Version) 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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