Oakley Sound Fourmix User manual

Oakley Sound Systems
5U Oakley Modular Series
Fourmix
CV and Audio Mixer
Fourmix PCB Issue 1
User Manual
V1.1
Tony Allgood
Oakley Sound Systems
CARLISLE
United Kingdom

The suggested panel design for the MOTM format version of the Oakley Fourmix.
2

Introduction
This is the User Manual or the issue 1 Fourmix 5U module rom Oakley Sound. This
document contains only an overview o the operation o the unit.
For the Builder's Guide, which contains a basic introduction to the board, a ull parts list or
the components needed to populate the boards, and a list o the various interconnections,
please visit the main project webpage at:
http://www.oakleysound.com/ ourmix.htm
For general in ormation regarding where to get parts and suggested part numbers please see
our use ul Parts Guide at the project webpage or http://www.oakleysound.com/parts.pd .
For general in ormation on how to build our modules, including circuit board population,
mounting ront panel components and making up board interconnects please see our generic
Construction Guide at the project webpage or http://www.oakleysound.com/construct.pd .
3

The Oakley Fourmix Module
The prototype issue1 Fourmix module behind a natural finish 1U wide S haeffer panel.
Our simple but use ul our channel mixer or use with control voltages and audio. Each
channel has an independent level control that goes rom ully o to unity gain (0dB).
Two outputs are provided. OUT (MAIN) is a low impedance driver that simply presents the
sum o the input signals as controlled by the level pots. The other, OUT (SOFT), is also the
sum o the inputs but this time the signal is passed through a special 'so t clipping' circuit. This
so t clipper is designed to sound like an overdriven valve ampli ier and adds a orm o
harmonic distortion that is musically interesting.
I no jack plug is inserted into the IN 3 socket then by adjusting the LEVEL 3 pot you will
add a positive o set voltage to the output. Likewise, i no jack plug is inserted into IN 4 then
the LEVEL 4 pot will control the amount o negative o set that is added to the output. Each
pot there ore can either act as a constant voltage source which can be added to other CVs or
controlling other modules. Furthermore, it can also be used to generate asymmetrical signal
clipping in either any ollowing modules or by using the so t output o the Fourmix itsel .
The module also eatures two output level LEDs. These monitor the signal level at the main
out socket. The 5V LED lights when the signal exceeds +/-5V and indicates that so t clipping
is about to take place rom the so t output. The peak LED lights when the output signal is
getting near hard clipping, ie. around +/-11V. Special circuitry is employed to ensure the
LEDs do not cause power supply clicks when they turn on or o .
To allow all the unctions required by this module we have had to reduce the spacing between
the pots. Instead o our usual 1.625" (41.3mm) we have opted or the smaller 1.375"
(34.9mm). Used in conjunction with the smaller 20mm (13/16") diameter knobs this still
allows or an attractive module design and inger riendly tweaking.
4

The OUT (MAIN) output
This produces a signal that is equivalent to the sum o the our inputs as controlled by the our
level control pots. As with all mixer circuits it is possible to exceed the maximum allowed
output voltage i the input signals and input levels are set too high. For example using three
input signals o 6V peak with their pots set to ull should produce a theoretical 18V peak
signal. This is beyond the capabilities o the Fourmix since the power supply to the module is
only +/-15V and the circuitry can produce at best a +/-13V signal rom the output. This
discrepancy causes what is called 'clipping'. It is called this because the output wave orm is
essentially clipped o at the top and bottom where it wants to go but cannot. This hard
clipping tends to sound harsh and overly bright. It can be musically interesting though and
doesn't actually cause any harm to the module.
The two LEDs on the ront panel help you avoid hard clipping. I the red peak LED is coming
on o ten then it is more than likely that you are hard clipping the MAIN (OUT) signal. I you
want to avoid such clipping you should be looking at never allowing the peak LED to light up
at all. The green LED indicates a signal above +/-5V and this is the normal signal level
expected within a modular synthesiser. Typically, the input pots should be adjusted so that the
green LED should be coming on occasionally.
The OUT (MAIN) output signal is bu ered. That is it will not droop signi icantly with
increased load. In other words, unlike the normal outputs o audio modules, the output level
will not change no matter how many modules (within reason) you connect up to it. Thus it is
use ul or distributing sensitive control voltages to multiples o modules. It should be noted
that any Oakley modules that process KeyCV (the main pitch controlling control voltage in the
modular) will also eature the same bu ered output circuitry as the Fourmix.
The main output o the Fourmix can be used as a precision voltage source. Channel 3 and
Channel 4 produce positive and negative re erence voltages respectively when no jack plug is
inserted into their input socket. With their level pots set to their maximum the voltage is just
under +4.5V or channel 3 only and just under -4.5V or channel 4 only. Since each channel is
o opposing polarity using both channels simultaneously will produce a cancellation e ect.
That is, i both 3 and 4 are set to their maximum settings the inal output voltage will
completely cancel and produce zero volts rom the output.
5

The OUT (SOFT) output
With signal levels under 3.5V or so the so t output produces pretty much the same signal as
the main output. However, as the signal level rises on the main output, the signal seen at the
so t output becomes more and more distorted. It is e ectively being gently compressed at the
top and bottom o the wave orm. This is clipping o a kind since it does restrict the maximum
output voltage but because it comes in grace ully it is called so t clipping.
One can think o so t clipping as being much like ordinary audio compression but acting so
ast it it operates on each cycle o the audio wave orm. The maximum signal level likely to be
seen at the so t output is not much above 6V peak.
The so t clipping output is use ul to limit a CV or audio signal in a musically interesting way.
For audio it tends to bring out the undamental tone o the signal in a subtle way. Do not
expect screaming Marshall lead tones – this is a mild e ect and one that is akin to overdriving
a discrete VCA or ully open Moog ladder ilter.
You can use the so t output as reasonable triangle wave to sine wave convertor. I you do this
you can also create asymmetrical distortion by using the voltage sources built into channels 3
and 4 on the Fourmix.
The output is not bu ered and has an output impedance o 1K.
6

Final Comments
I hope you enjoy using the Oakley Fourmix module.
I you have any problems with the module, an excellent source o support is the Oakley Sound
Forum at Mu wiggler.com. Paul Darlow and I are on this group, as well as many other users
and builders o Oakley modules.
I you have a comment about this user manual, or have a ound a mistake in it, then please do
let me know.
Last but not least, can I say a big thank you to all o you who helped and inspired me. Thanks
especially to all those nice people on the Synth-diy and Analogue Heaven mailing lists and
those at Mu wiggler.com.
Tony Allgood at Oakley Sound
Cumbria, UK
© April 2011 – updated March 2017
No part o this document may be copied by whatever means without my permission.
7