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Oakley Sound Systems One of Three Voltage Controlled Oscillator Module Oakley Sound Systems
One of Three
Voltage Controlled Oscillator
Module
PCB Issue 4
Users Guide
V4.4
Tony Allgood
B.Eng PGCE
Oakley Sound Systems
PENRITH
CA10 1HR
United Kingdom Introduction
This User Guide is for the Issue 4 board set only. Check that you have an Issue 4 board. The
issue number is written at the bottom left hand corner of the main board
The Oakley One of Three is a voltage controlled oscillator module that was inspired in part by the
VCO design of the later MiniMoogs. In this module I decided to create a VCO that sounded good
and performed well enough to keep up with the digital oscillators of modern day synthesisers.
This is the fourth issue of this popular project and module. The new issue mainly contains
mechanical updates and still features the same powerful analogue sound that won the last three
issues much acclaim.
The VCO features sawtooth, pulse, triangle and a low distortion sine output. The duty cycle of the
pulse output may be controlled directly by a PCB mounted pot, and also an external voltage, the
sensitivity being controlled by another PCB mounted pot. The frequency of the VCO can be
controlled by two PCB mounted pots, one for fine, the other for coarse adjustment. These four pots
are designed to be fitted to the main board, and when special pot brackets are used, the PCB can be
firmly supported to the front panel.
The output levels for the sawtooth, sine and triangle are standardised at +/-5V, ie. 10V peak to peak.
The pulse wave output is also 10V peak to peak, but the peak levels vary with pulse width. The
Oakley VCO uses an interesting technique to maintain the average voltage over one cycle to zero
volts. This new circuit essentially adds an offset to the pulse output to compensate for the non zero
average voltage for any pulse wave that isn't a square wave. Ordinarily, one would see average DC
values varying from +5V to -5V as the PW is swept from one end to the other. In the new VCO, this
average level is kept at zero. This means that for narrow pulses, you now have a wave that goes
from just below 0V [down] to just below +10V [up]. For square waves, you have the usual +5V up
and -5V down. For wide pulses, you have a wave that is just below 0V [up] and just below -10V
[down]. The reason for this is that fast pulse width modulation no longer adds thumping to the
audio output. ie. fast EG sweeps of PW will sound great. Although, no switch is provided to turn
this feature off, you can simply omit one resistor and it will function like other VCOs.
All outputs have an output impedance of roughly 1K.
The pulse output is also switchable between centre modulated and edge modulated forms of
pulse width modulation. Most VCOs will only offer edge modulated, in which only one edge of the
pulse wave is affected by the pulse width pot or modulating CV. The Oakley VCO allows you to
modulate either one edge (edge) or both (centre). In fixed pulse width applications this generally
makes no audible difference. However, when used with fast moving modulating CVs the difference
in timbre is apparent.
A high impedance synchronisation input is provided to prematurely reset the VCO waveform. With
this you can force the VCOs operating frequency to that of an external sawtooth signal, say from
another VCO module. This input is level sensitive, so hard sync is possible with inputs of about
+3V or above. Inputs below this will cause only occasional synchronisation leading to interesting
harmonic structures.
Please note; that to cause hard sync effects the master sync signal must be a sawtooth (falling
ramp shape) waveform of 10V peak to peak. The slave VCO will not sync to a ramp waveform
which typically has a rising ramp and does not feature the fast rising edge the VCO needs to lock on to. These ramp waveforms are erroneously called sawtooth by some manufacturers. However, a
simple inverting circuit will be sufficient to turn the waveform around the right way so it can be
used as a sync master.
The VCO supports the standard 1V/octave exponential voltage to frequency relationship. However,
a linear control input is also provided for constant depth frequency modulation.
Temperature compensation is performed by using a matched transistor pair in the exponential
convertor, and a high quality temperature sensitive resistor.
The Oakley CV/gate bussing
You really should think about it if you have a medium sized Oakley system. Using the Dizzy board
(also available from Oakley Modular!), it allows Keyboard CV and Gate signals to be piped around
the back of the modulars case along with the power supply rails. Any VCO and VCF can be
connected to the KEY-CV line, and this will save you having to patch KEY-CV to every module
that needs it. Inserting any patch lead into the 1V/OCT socket will override the CV bus line
connection. The gate signals are treated similarly to the CV line but for use with the ADSRs and
other EG modules.
The Oakley CV/gate buss uses a common three way 0.1 header to carry the two signal lines. A
third, as yet unused connection is also present for future expansion.
The new VCO issue supports the Oakley CV/gate buss natively. Previous issues had the CV buss
being connected to the module via a wire tail attached to the 1V/octave sockets normally closed
(NC) lug. The new socket board features an optional three way header that can be fitted to allow
direct connection to the CV/gate buss on an installed Oakley Dizzy system. If not required the
header can simply be omitted altogether and a solder link put in its place. Or, the header can be
fitted and a simple two way jumper, like those used on computer motherboards, can be fitted to
allow to the buss to implemented at a later date.
The new issue PCB set
The main PCB is 108 (height) x 143 (depth) mm in size. All three boards use double sided copper
traces and have through plated holes. The solder pads are large and are easy to solder and de-solder
if necessary. They have a high quality solder mask on both sides for easier soldering, and have clear
legending on the component side for easier building.
If you are building the standard design there are no components mounted off the boards. All
components including sockets and pots are soldered directly to the boards.
Previously, many Oakley modules have had the sockets, switches and extra pots wired to the board
by individual wires. This module allows all the socket wiring to be done via the socket PCB and
two MTA solderless or Molex connections. If you are building this module in the standard Oakley
format this new system will reduce assembly time and possible wiring errors.
Some people will wish to use this Oakley design in a non standard format, such as fitting it to
another manufacturers rack or one of their own invention. This is perfectly easy to do. Simply do
not use the socket board and wire the main board to the sockets as per usual. I have provided space for four of the control pots on the main PCB, whilst the other three pots are
fitted to their own board. If you use the specified pots and brackets, the PCBs can be held firmly to
the panel without any additional mounting procedures. The pot spacing is 1.625 and is the same as
vertical spacing of the MOTM modular synthesiser.
There are detailed instructions later in the document about how to build the boards. The whole
project takes around 3 hours to build and test.
Oakley VCO Issue Changes
For those interested the Oakley VCO has evolved slowly over the last six years:
Issue 1: The start; the VCO is based primarily on the third series Minimoog VCO with plenty of
additions. eg. in built 10V reference, sync, different exponential convertor and summing stage,
sinewave output, pulse width selection, output buffering, Omeg pots.
Temperature compensation was generally provided by a +3000ppm/K 900mW film resistor. This
was later upgraded to the KRL wirewound types for last ten or so boards that were sold.
Issue 2: Boards had the following features added:
Added -10V reference to go with the +10V internal reference voltage already on issue 1. This
stabilises absolute pitch drift so that supply rail potential doesn't affect the pitch of the VCO. All
pitch pots use the +/-10V references.
The range of the TUNE trimmer was reduced and this was coupled with a reduction in the quiescent
operating frequency.
Added SYNC input buffer circuit. This has the advantage of providing a high impedance input. The
old issue had a 10K input resistance which caused a slight drop in amplitude to the other (master)
VCO connected to the SYNC input. An additional benefit is that unwanted cross coupling between
two or more slave VCOs was eradicated. With the older VCOs, any two slave VCOs tended to sync
to each other as well as the master. Interesting effect though.
I swapped the positions of the power supply regulator with the CV summer circuitry. This enabled
me to move the two tuning multiturn trimmers to the edge of the board. The old issue 1 board had
these parts in the centre of the PCB, and they were impossible to trim when the board was fitted
i