Liquid filter

3 filter modes are available: 2-pole band-pass, 2-pole low-pass, and 4-pole low-pass.

Self-oscillation is available for all modes and produces a clean sine-wave on the LP4 output. Unlike many classic 4-pole designs, this filter does not suffer from “loudness drop” when resonance is increased. The resonance loudness compensation circuit brings a slight tone coloration reminiscent of the Roland SH/Jupiter filters, with a very round and “liquid” resonance.

The 4-pole low-pass output is additionally sent to a VCA. This makes Ripples a good “final stage” module for an inexpensive/space-constrained subtractive synthesis synth voice.

Controls

• Cutoff frequency, 18Hz to 18kHz
• Resonance (self oscillation at 80%)
• Attenuverter on Frequency CV

Inputs and outputs

• Signal in
• Frequency CV 1 (1V/Oct)
• Resonance CV. Self oscillation starts at 4V. Control voltages above 5V are non-linearly compressed, for fine control of resonance overdrive
• Frequency CV 2 (with bipolar attenuator, from -1V/Oct to 1V/Oct)
• VCA level. Control voltages above 5V are non-linearly compressed.
• 2-pole BP output
• 2-pole LP output
• 4-pole LP output
• 4-pole LP output attenuated by VCA

Technical characteristics

• Fully analog, based on SSM2164 low noise/distortion quad VCA chip, and LM13700 OTA for that extra bit of non-linearity.
• Frequency CV inputs impedance: 100k.
• Audio input impedance: 75k.
• Resonance and VCA CV input impedance: 30k.
• Open-source hardware.

http://mutable-instruments.net/modules/ripples1