1V/octave, the same as the unattenuated input. At
lower settings, the CV2 input is less sensitive.
IN inputs Audio inputs to the filter. The upper in-
put, marked with a diode symbol and the notation fw
rect, is subjected to full-wave rectification (positive
and negative voltages translated into their absolute
values) before being applied to the filter. The lower
input is a direct connection. Both inputs may be used
at once; their effects are summed.
The rectified input includes a phase inverter (both
positive and negative voltages are translated into neg-
ative) to cancel out the naturally-occuring phase in-
version between the input and LP output in this filter
topology. As a result, if you feed an audio signal into
the rectified input with the filter cutoff significantly
below the frequency of the audio, it will be rectified
and filtered into a positive voltage tracking the overall
amplitude of the input signal. This way the module
can be used as an envelope follower.
The inputs can accept any voltages between the
module’s power rails (−12V to +12V) without dam-
age. The module my be overdriven, creating signifi-
cant distortion, with inputs beyond about ±5V.
CV inputs Exponential control voltages for filter
cutoff frequency. The upper socket (CV1) has a
nominal sensitivity of 1V/octave; but the tracking
of this filter is not meant to be very accurate, and
it cannot be made highly accurate because of the
somewhat unpredictable properties of the inductors.
Tracking will differ in different parts of the audio
spectrum. The CV-processing circuit is partially
temperature-compensated, with zeroth-order “offset”
compensation but not first-order “tracking” compen-
sation. The lower socket’s (CV2) sensitivity is ad-
justable with the att knob, to a maximum of the same
sensitivity as CV1. The CV1 input, attenuated CV2
input, and TUNE knob setting are all summed to
produce the control value for the filter core.
Both CV inputs can accept voltages anywhere be-
tween the module’s power rails (−12V to +12V) with-
out damage. Which voltages are useful depends on
the patch and the setting of the TUNE knob, but a
typical user might aim for 0V to 5V.
HP, BP, and LP outputs These are the three out-
puts of the filter core: high-pass, band-pass, and low-
pass. Because this is a two-pole filter, the asymptotic
slopes of the response curves are 12dB/octave for the
high-pass and low-pass, and 6dB/octave on each of
two slopes for the band-pass.
All three outputs are active simultaneously,
driven by the combined input from the two IN jack
sockets. The phase relationships among the three
outputs will change with frequency as the filter shifts
between using its capacitors and its inductors; that
means mixing outputs to produce other filter func-
tions (such as notch filtering) may produce results
that sound good, but they are unlikely to be strong
on measures like stopband attenuation.
Voltage levels on the audio outputs will normally
be similar to the voltage levels on the inputs, with the
maximum possible voltage limited by possible clip-
ping in the op amp chips at around ±10V. Output
level at maximum oscillation will be about ±5V. At
the lowest resonance setting, the BP output will be
a little quieter than the other two, an effect which
tends to disappear at higher resonance.
Specifications
The nominal input impedance is 100kΩfor all inputs
except the rectifier input, which varies between 50kΩ
and 100kΩ. Nominal output impedance is 1kΩfor all
outputs.
Any voltage between the power supply rails (nom-
inally ±12V) is safe for the module, on any input;
output voltages are limited by the capabilities of the
op amps to about ±10V and will clip if the inputs
are driven sufficiently hard. Distortion resulting from
limiting in internal feedback paths may show up be-
fore the outputs actually clip.
The circuit is DC-coupled throughout; as a result,
it can operate at very low frequencies, but small DC
offsets may appear on the outputs. Trimmers are
provided for minimizing offset effects.
Briefly shorting any input or output to any fixed
voltage at or between the power rails, or shorting
two to each other, should be harmless to the module.
Patching the MSK 009’s output into some other mod-
ule’s output should be harmless to the MSK 009, but
doing that is not recommended because it is possible
the non-MSK 009 module may be harmed.
This module (assuming a correct build using the
recommended components) is protected against re-
verse power connection. It will not function with the
power reversed, but will not cause or suffer any dam-
age. Some other kinds of power misconnection may
possibly be dangerous to the module or the power
supply.
In normal operation the maximum current de-
mand of this module is 25mA from the +12V supply
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