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The OUT (MAIN) output
This produces a signal that is equivalent to the sum of the four inputs as controlled by the four
level control pots. As with all mixer circuits it is possible to exceed the maximum allowed
output voltage if the input signals and input levels are set too high. For example using three
input signals of 6V peak with their pots set to full should produce a theoretical 18V peak
signal. This is beyond the capabilities of the Fourmix since the power supply to the module is
only +/-15V and the circuitry can produce at best a +/-13V signal from the output. This
discrepancy causes what is called 'clipping'. It is called this because the output waveform is
essentially clipped off 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 front panel help you avoid hard clipping. If the red peak LED is coming
on often then it is more than likely that you are hard clipping the MAIN (OUT) signal. If 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 buffered. That is it will not droop significantly with
increased load. In other words, unlike the normal outputs of audio modules, the output level
will not change no matter how many modules (within reason) you connect up to it. Thus it is
useful for distributing sensitive control voltages to multiples of modules. It should be noted
that any Oakley modules that process KeyCV (the main pitch controlling control voltage in the
modular) will also feature the same buffered output circuitry as the Fourmix.
The main output of the Fourmix can be used as a precision voltage source. Channel 3 and
Channel 4 produce positive and negative reference 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 for channel 3 only and just under -4.5V for channel 4 only. Since each channel is
of opposing polarity using both channels simultaneously will produce a cancellation effect.
That is, if both 3 and 4 are set to their maximum settings the final output voltage will
completely cancel and produce zero volts from the output.
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