The 6HP wide Oakley Sound MOSPhaser is a six stage
phaser (five variable all pass filters and one fixed all pass
filter) that generates a classic sound. Partly based on a well
loved 1970s guitar effects pedal the MOSPhaser offers improved
frequency range, a more comprehensive modulation oscillator and
external voltage control.
Each
of the five all pass filter stages uses one of the MOSFET
transistors inside a CMOS invertor chip, the 4009UBM, to control
the centre frequency of the stage. By using a single integrated
circuit for all transistors means that no matching of devices is
required as would be the case if we were using discrete
components.
The
module is easy to build and requires only a simple voltmeter to
set up.
The unit includes a useful
internal low frequency oscillator (LFO) with accompanying
bicolour LED. The centre frequency of the phase shifter can also
be controlled by an external control voltage. This allows
envelope generators to be used to sweep the phaser to create
filter type effects.
Four front panel pots are
provided:
Frequency:
this controls the frequency of the two notches created by the
phase shift network.
Emphasis:
this accentuates the feedback signal within the phaser to create
a deeper effect.
Modulation: A
simple attenuator to adjust the level of the internal LFO or
incoming CV. A switch provides the option of inverting the CV’s
polarity.
LFO Rate: adjusts
the speed of the LFO.
The internal LFO produces a
triangle wave output, +/-5V, which is also available from a front
panel socket, LFO OUT. This allows you to use the MOSPhaser’s
internal LFO for other modulation purposes. The frequency range
of the LFO is designed for slow sweeps covering 5Hz to 0.01Hz (one
cycle every 90 seconds) although this can be simply changed to
something faster by using a smaller value timing capacitor in
your build. The LFO signal is automatically routed to the
modulation depth pot when no jack is inserted into the CV IN
socket.
The unit is designed to work
with standard modular signal levels, although it could be easily
converted to be driven from other sources including an electric
guitar.
The module’s current draw,
with no signal passing through, is approximately +/-38mA at +/-12V.
The PCB set
The MOSPhaser comprises of
two printed circuit boards (PCBs) connected together with four 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,
and the all pass filter cascade. The pot board has the pots,
switch, and sockets that are attached to the front panel, as well
as the input and output circuitry and LFO. The main board is a
four layer design, while the pot board is a two layer design.
Each board is the same size; 29 mm x 107 mm.
The
majority of the components are surface mount parts. The ICs are
SOIC, while the resistors and capacitors are 0805. All components
are standard parts.
Sound Samples
Pink Noise through the phaser
using the internal LFO. First half is with no emphasis and then
the emphasis increased during the second half returning to no
emphasis at the end.
Two Oakley VCOs set to
sawtooth and using the phaser’s internal LFO. The emphasis and
rate controls are manually altered during the sample. Some light
reverb is used for ambience.
Single sawtooth VCO used to
play a simple riff. The phaser’s frequency control is manually
adjusted.
Two VCOs, one sawtooth and the
other a triangle wave an octave below. The phaser is being swept
with a fairly fast envelope triggered on each note. The phaser
frequency is manually adjusted. No filter modules are used but a
simple delay is used to add interest.
Project Downloads
MOSPhaser (Euro) issue 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
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:
6HP Eurorack
format in natural silver
To read, edit and print this file
you will need a copy of ‘Frontplatten Designer’ from Schaeffer. Panels can be ordered via Schaeffer’s website.
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
order your Schaeffer panels from Front
Panel Express.
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