The 10HP wide Oakley Croglin Voltage
Controlled Filter (VCF) is based around two low noise ‘Sallen-Key’
filter circuits that operate either together or against each
other.
The cut off frequency of both filters,
named A and B, can be controlled by a 1V/octave control voltage (CV)
input, a variable (0% to 200%) CV input, and the module’s own
Frequency pot. Filter B’s cut off frequency can be made to be
different to Filter A with either the Offset pot and/or an
external offset CV. This frequency offset can be both positive,
making the cut off frequency of Filter B higher than Filter A, or
negative, making the frequency lower than Filter A.
The two filters work in parallel on the
same audio input signal. Filter A is a -12dB/octave low pass
filter, while Filter B can be switched to be either a -12dB/octave
low pass filter, or a +/-6dB/octave band pass filter. Each filter
has its own output on the front panel.
A third output, called simply ‘Output’,
combines the signals from each filter. The output of Filter A is
summed together with a proportion of Filter B, the actual
proportion, from -100%, through 0%, to +100%, being set by the
Duality control. In its middle position the Duality pot allows no
audio to come from Filter B, so you just hear the signal coming
out of Filter A. Fully clockwise, the signal from Filter B is
added to Filter A, so the two filters augment each other. Fully
anti-clockwise, Filter B’s output will subtract and cancel out
those parts of the signal that are common with Filter A’s output.
A wide variety of tonal responses can be made; from a traditional
low pass filter and band pass sounds to more complex vocal like
formants.
At higher levels of resonance both filters
will self-oscillate, with a sine like waveform, over the full
audio band and beyond.
The module requires +/-12V and has a
current consumption of +60mA and -60mA.
The PCB Set
The Croglin 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 cores, half of the audio
output circuitry, and CV control circuitry. The pot board has the
pots, switch, and sockets that are attached to the front panel,
as well as the resonance, CV input and the other half of the
output circuitry. 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.
All components are standard parts.
Sound Samples
All the following samples use an Oakley D-VCO
which connects to an Oakley Multimix and then to the Croglin.
A bass drone using Filter B in low pass
mode and manually tweaking frequency and resonance. A little
light reverb has been added for ambience.
A dry sample of the same bass note but this
time using Filter B in its band pass filter mode.
Using the Croglin’s main output with
Duality set to its maximum positive position and Filter B sightly
offset from Filter A. The Croglin’s cut-off is modulated by a
looping envelope. The resonance is increased slightly about half
way through this sample.
This time the Croglin’s slightly offset
dual filters are swept from a triangle wave LFO. The Duality
control is set fully negative and the Resonance about half way. A
little reverb has been added.
A simple sequence is being played while the
Croglin’s controls are being manually tweaked. The Croglin’s cut-off
is being swept by an Oakley Envelope. A delay effect has been
added for extra bounce.
Project Downloads
Croglin Builder’s Guide
Construction guide and parts list.
Construction
Guide My handy guide to building
Oakley DIY projects
Parts
Guide My handy guide to buying
parts for Oakley DIY projects
Schematics are only available
to purchasers of the PCBs and will only 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
web based 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