The Oakley Fourmix Module
The prototype issue1 Fourmix module behind a natural finish 1U wide Schaeffer panel.
Our simple but useful four channel mixer for use with control voltages and audio. Each
channel has an independent level control that goes from fully off to unity gain (0dB).
Two outputs are provided. OUT (MAIN) is a low impedance driver that simply presents the
sum of the input signals as controlled by the level pots. The other, OUT (SOFT), is also the
sum of the inputs but this time the signal is passed through a special 'soft clipping' circuit. This
soft clipper is designed to sound like an overdriven valve amplifier and adds a form of
harmonic distortion that is musically interesting.
If no jack plug is inserted into the IN 3 socket then by adjusting the LEVEL 3 pot you will
add a positive offset voltage to the output. Likewise, if no jack plug is inserted into IN 4 then
the LEVEL 4 pot will control the amount of negative offset that is added to the output. Each
pot therefore can either act as a constant voltage source which can be added to other CVs for
controlling other modules. Furthermore, it can also be used to generate asymmetrical signal
clipping in either any following modules or by using the soft output of the Fourmix itself.
The module also features two output level LEDs. These monitor the signal level at the main
out socket. The 5V LED lights when the signal e/-5V and indicates that soft clipping
is about to take place from the soft output. The peak LED lights when the output signal is
getting near hard clipping, ie. /-11V. Special circuitry is employed to ensure the
LEDs do not cause power supply clicks when they turn on or off.
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 the smaller 20mm (13/16") diameter knobs this still
allows for an attractive module design and finger friendly tweaking.
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