The 6HP wide Oakley Classic VCA is a new
version of one of my first Eurorack modules. This is a voltage
controlled amplifier module inspired in part by a classic vintage
design, the ARP4019 sub-module which was used on the wooden case
ARP2600P semi-modular synthesiser. The design has a fully
discrete core but uses op-amps for input summing and the output
drivers.
Two signal
inputs are provided, one DC coupled and the other AC coupled. The
former allows audio and low frequency signals to be processed.
While the latter has a inbuilt high pass filter so that it can be
only used to process audio frequencies. Both inputs can be used
simultaneously and each one features an input level control.
Two control
voltage (CV) inputs are provided. CV1 can be operated in either
exponential or linear mode, determined by the position of the
switch on the front panel. CV2 is linear and is provided with a
standard attenuator pot that can control the sensitivity of the
input.
CV1 features a
reversible attenuator. When the pot is turned right of its
central position, the pot acts as a normal attenuator; increasing
the sensitivity of the CV input. Left of centre, the pot will act
in inverting mode and the CV will be tipped upside down so that
increasing positive voltages will decrease the gain of the VCA.
When no input
jack is inserted into either CV input socket, the CV pot of that
input becomes a gain control. This allows that pot to be used as
a simple volume control. With no jack inserted into the CV2 input
and the CV2 pot is set high it allows the CV1 input to turn the
VCA off.
This newer
version of the Classic VCA has two outputs. Out A is the standard
output of the VCA. Out B, gives you the difference signal between
the sum of the two audio inputs and the final output of the VCA.
This is a sort of anti-output; when the Out A output signal goes
up in level this output goes lower. You can use it as a ring
modulator (with either CV pot set to max) or in conjunction with
Out A as a simple voltage controlled pan.
A single +5V CV
input with its associating pot turned up full with produce unity
gain (0dB) through the VCA.
Current draw is approximately +/-30mA at
+/-12V.
The PCB set
The Classic VCA module
comprises of two printed circuit boards (PCBs) connected together
with three 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 VCA core. The pot board has
the pots, switch, and sockets that are attached to the front
panel, as well as the CV and audio input circuitry. 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 and
THAT340 array are SOIC, while the resistors and surface mount
capacitors are 0805. The PNP transistor array is SOT457. All
components are standard parts.
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 PCBs 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 these
files you will need a copy of ‘Frontplatten Designer’ from Schaeffer. Panels can be ordered via the program using its
secure online 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.
