The Oakley SVCO-A and SVCO-B
The Two compact well specified audio VCOs for MOTM and MU based modular
synthesisers.
The 'A' variant has a single tune control and three way octave switch. While the otherwise
identical 'B' variant has an increment-decrement switching system (with three way momentary
switch and LED ladder display) to allow for easy control over five octaves and a wide range
LFO setting.
SVCO-A modules are built with just the SVCO main board. The B variant uses the same
SVCO main board with an additional SVCO-B daughter board that attaches to the main board
via a tidy 10-way ribbon cable.
Pitch is controlled by either a precision 1V/octave input socket or by a CV (control voltage)
input whose sensitivity is controlled by a front panel pot. The SVCO can also be connected to
the Oakley Buss for hassle free keyboard or sequencer control.
Both variants have a variable width pulse wave as well as a sawtooth waveshaper to generate
a fully varying shape from ramp through triangle to sawtooth. An external CV input may be
used to control the waveshape. The Shape CV input features a reversible attenuator control so
that the incoming CV may be inverted as well as simply level controlled.
The SVCO uses a sawtooth core but with a different topology than our existing double width
VCO. The core of the VCO circuit is based on designs used in a series of classic Japanese
synthesisers from the mid to late 1970s. However, it should be said that the SVCO does not
copy any particular synth but instead takes certain attributes from several of them. The pitch
control circuitry is a development of my usual design, while the sawtooth waveform shaping
is, to my knowledge, entirely unique.
Hard synchronisation is possible from an external VCO that has a falling edge sawtooth
output.
To allow all the functions required by this module we have had to reduce the spacing between
the pots. Instead of our usual 1.625" (41.3mm) we have opted for the smaller 1.375"
(34.9mm). Used in conjunction with smaller 20mm (13/16") diameter knobs this still allows
for an attractive module design and finger friendly tweaking.
The module accommodates either our standard Oakley/MOTM power header or a
Synthesizers.com power header. Current consumption is approxi40mA and -35mA
for the SVCO-A, and +47mA and -38mA for the SVCO-B.
In LO mode the SVCO-B will act as a voltage controlled low frequency oscillator. In this
mode, and without any additional CVs patched in, the SVCO-B's frequency pot will operate
over a range of 0.01Hz (one cycle every 1.5 minutes) to around 500Hz. The SVCO-B will
always initialise in the LO mode on power up.
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