W
e designed the Optodist using SR3 optocouplers; the SR3's give a tighter,
snappier CV response and have less bleed through of the signal when gain is
fully off and signal in is set to low settings for a somewhat clean signal or
lighter distortion effect.
P
lease note that due to the nature of the vactrol-style optocouplers, response
behaviour may vary between different factory batches.]
Al
so on board is an LED limiter circuit which soft-clips the signal through a
set of 4 LEDs (visible on the front panel), for adding a different flavour
to your distortion. the choice of LEDs is up to you, experimenting with
different types will affect the sound.
Two colours of LEDs were used and tested in development of the Optodist,
green being standard but also some investigations into yellow were made that
revealed that it has a subtly brighter tone and is much brighter visually as
well. Any 3mm LEDs from infra red up to piercing bright white or violet will
work, although there may need to be an adjustment to a resistor value to get the
correct brightness.
T
he Optodist is a novice-friendly project, it is a low part count, single-PCB
build that only requires the most basic experience in PCB soldering and
module assembly:
T
he amplifier has an exponential response to control voltages;
and accepts
between 0V and 8V cv by default.
T
he amplifier accepts both DC and AC signals,
though it should be noted that due to the slew rate of the optocouplers, audio
rate modulation of the gain CV is limited in frequency to a
degree dictated by
the choice of either SR2 or SR3 optocouplers.
T
he module has one audio input and one audio output;
the signal in knob
d
ictating the audio input level. The VCA has a control voltage input with
attenuator (gain CV), and manual gain control which sets either the VCA initial
open level and/or the amount of overdrive gain.
T
he module can be used as a basic attenuator;
by disconnecting cv/keeping gain
cv fully CCW, and adjusting the signal in and gain control as desired.
W
hen the cv input is presented with a control voltage between 0V and 8V, in
this setup, the module will behave as a traditional relatively clean
sounding VCA;
albeit with some character and limitations as dictated by the
optocouplers.
D
epending on your calibration settings and choice of optocouplers, opening
either the 'signal in' knob and/or opening the 'gain' knob past approximately
50% will introduce distortion;
this distortion will vary in character
depending on the settings, but can be tamed by engaging the LED clipping
circuit, although this will further alter the character of the distortion.
(when gain is between fully off [CCW] and 50% the module can be used as an
attenuator or as a relatively clean VCA where gain dictates the VCA initial
level; opening the 'gain' knob past approximately 50% will introduce
distortion -
from 50% up to max gain (full cw) the Optodist will boost the
signal with increasing distortion).
Summary of Contents for Optodist
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