SetonixSynth Macropod User manual
Macropod Variable Bandwidth VCF User’s Manual - 4U
Copyright 2021 SetonixSynth. All rights reserved.
Hello and thank you for using the Macropod Variable Bandwidth VCF for 4U modular. We hope you will
find its tones most pleasing!
Technical Specifications (4U, Loudest Warning standard)
Width: 2”
Depth: 25mm
Peak Current Draw: 48mA @ +12V, 48mA @ -12V
1. Connecting your Macropod
Connect a power cable to the MTA-156 header at the top of the Macropod pcb. The +12V side of the
connector must be aligned with the silkscreen indicator that says “+12V” for proper use. This module is
reverse diode protected, but it is still not recommended that you plug it in backwards to see what
happens.
2. Theory of Operation
Macropod signal routing, simplified
This module uses the AS3350 Dual VCF chip, a reissue of the CEM3350 designed by Doug Curtis and
used in a few 80s synthesizers. This is our second design using this IC, coming after the Marsupial Dual
VCF for Eurorack. While that design highlights the chip’s rich sound and resonant characteristics, the
Macropod leverages its versatility to create a space-friendly and highly-functional Variable Bandwidth
VCF with several features previously excluded from the two best-known filters of this type (the Serge
VCF2 and Buchla 291): Resonance control, onboard Notch output, and a “squared-off” distorted output.
In this design, a two-channel audio mixer sums Audio In A and Audio In B and routes the mix to half of the
chip, which is configured in High Pass/Band Pass mode. These two outputs are directly accessible. The
High Pass output is then routed to the input of the chip’s other half, configured as a Low Pass/Band Pass
filter. This filter’s Low Pass output creates our Variable Bandwidth A output via the master Bandwidth
control, which introduces an inverted voltage to both filters to control their distance from the center Cutoff
(e.g. a positive voltage input to the Bandwidth CV jack will simultaneously increase the cutoff frequency of
the Low Pass Filter section and decrease the cutoff frequency of the High Pass Filter section in equal
amounts).
This filter takes the concept further, however! In traditional filtering, summing a phase-inverted Band Pass
signal with the filter input creates a Notch filter, and the concept works well with this filter’s Variable
Bandwidth output. In this case, the inverted input is summed with the regular Variable Bandwidth A output
and then the mix is inverted again; this nominally produces a Notch output which is in phase with the
input.
We also added a transistor-based switching circuit through which the Variable Bandwidth A signal is
routed, which creates the Variable Bandwidth B output. This output is lovely for creating electric guitar or
bowed cello sounds, or can approximate the sound of Pulse-Width Modulation in some configurations.
This function was inspired by the lovely aggressive sounds of the bottom section of the Serge Wave
Multipliers module, though with a different circuit.
3. Summary of Functions
Cutoff: Sets the central frequency of the filter relative to the cutoffs of each filter section.
Bandwidth: Manual control of the Bandwidth of Variable Bandwidth A and B where CW = greater
bandwidth up to approx. 12 octaves at Maximum setting.
Freq Atten: Attenuverter for the Frequency CV input, where center = 0 Modulation, CCW = Inverse
modulation and CW = Regular modulation.
BW CV Atten: Attenuverter for the Bandwidth CV input, where center = 0 Modulation, CCW = Inverse
modulation and CW = Regular modulation.
Freq CV: Input for Cutoff Frequency CV. Amount is controlled by the knob labeled “Freq Atten.”
BW CV: Input for Bandwidth CV. Amount is controlled by the knob labeled “BW CV Atten.”
1V/Oct ↑:Un-attenuated CV input which controls the cutoff of the High Pass filter section.
1V/Oct ↓: Un-attenuated CV input which controls the cutoff of the High Pass filter section.
Audio In A: Audio Input with gain controllable via the “In A Gain” knob up to 1.5x gain. Audio from this
input is mixed with Audio In B after gain processing and before the filter stages.
In A Gain: Controls the Gain of Audio In A up to 1.5x the original gain. Ideal for feedback patching.
Audio In B: Unity-gain Audio input. Audio from this input is mixed with Audio In B before filter stages.
HP Out: 12dB/Oct High Pass Filter output. The cutoff of this output is controlled by the master Cutoff
Frequency, Bandwidth, and 1V/Oct ↑ controls.
Resonance: Controls the Resonance/Q setting of both filters simultaneously.
BP Out A: Output of the first Band Pass Filter. The central frequency of this filter output is the same as
HP Out and Bandwidth can not be varied.
BP Out B: Output of the second Band Pass Filter. The central frequency of this filter output is the same
as the Low Pass Filter which forms the final stage of the Variable Bandwidth A output, and can be
controlled directly via the 1V/Oct ↓ control input.
Notch Out: Output of the Notch Pass Filter, produced by summing the Variable Bandwidth A output with
the inverted Audio input post-Mixer section. This output will cancel the phase of frequencies within the
bandwidth of the Variable Bandwidth A output effectively with Resonance turned fully CCW, while
boosting those frequencies more and more as Resonance setting is increased.
VBW Out A: Output for the Low Pass Filter, which is filtering the output of the other filter section’s High
Pass Filter to form a filter topology with Variable Bandwidth. Use this output for “clean” processing of the
input signal.
VBW Out B: Created by routing Variable Bandwidth Out A through a single-transistor clipping stage with
AC Coupled output. The base of the transistor needs approximately 0.7V input to activate, so lower
Bandwidth or Cutoff frequency settings which produce a quiet output may cause this output to “mute” until
the voltage threshold is reached.
4. Calibration
Calibration of the Macropod is simple and should be done by ear. Patch a triangle-wave VCO within the
audible frequency range (probably 1kHz or so is best) into “Audio In B” and turn the Resonance control
fully CW. Monitor VBW Out A and adjust the rear trimpot until maximum Resonance doesn’t cause
self-oscillation (added frequencies besides the input). However, if you like the way the self-oscillation
sounds, turn up the trimmer and go with it!
5. Patch Ideas
"Extra-Grit Variable Bandwidth"
Patch Input Audio to “Audio In B” and patch VBW Out A to “Audio In A”, then monitor a different output of
the filter. Use “In A Gain” to add an adjustable amount of distortion to the input signal pre-filter.
“Extra Resonance”
Patch BP A or BP B outputs into “Audio In A” and input signal to “Audio In B”. Use the “In A Gain” control
to dial in extra resonance of a flavor dependent on the output used for feedback.
“Heavy Metal Guitar”
Turn the Bandwidth control fully CCW and send any waveform to any audio input. Send an envelope or
gate pulse to BW CV and Attenuate to taste. Monitor VBW Out B to create a chunky, palm-muted like
sound.
These are but three patch ideas, and a thorough exploration of the Macropod can surely yield many more.
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