Compact 12/24dB LP/HP/BP Analog VCF filter

Compact analog 12/24dB LP/HP/BP Analog VCF filter

This compact voltage-controlled filter Eurorack module is based on the SSI2140 integrated circuit, famous for its use in the Prophet 5 and other high-end synthesizers.

Filter IC offers  four quality transconductance operational amplifiers (OTAs) controlled by a temperature-compensated exponential converter. Each OTA, along with a precision capacitor, creates a filter pole with a frequency slope of 6 dB per octave. SSI2140 IC in this module is soldered on back of the PCB shielded from both sites between ground planes.

In this Eurorack analog filter module, these OTAs are wired into two 2-pole (12 dB) filter blocks based on a multimode filter topology. These two blocks are connected in series with dedicated outputs for a filtering response of 12dB and 24dB.

All filter poles are controlled by a precision exponential cutoff, which can be adjusted via a dedicated cutoff potentiometer, FM CV, and 1 volt/octave pitch CV.

In subtractive synthesis the same pitch CV is sent to both the oscillator and the filter, so the filtering frequency is automatically adjusted according to the pitch of the note being played. This ensures that all notes sound similar. Without the pitch CV input, the filter would remain static and over-filter high notes, making them less audible.

Filter module offers two analog switches to select the filtering mode for each of the two multimode filter blocks.
When both VCF switches are in the LP position, the module functions as a 12 dB and 24 dB (2 or 4 pole) voltage-controlled low-pass filter. Such filter mode is the basis of subtractive synthesis, where high frequencies are filtered out from the selected cutoff point.

When both switches are in the HP position, the module provides 12 dB and 24 dB high-pass outputs. High-pass is the opposite of low-pass, as it filters out low frequencies above the cutoff point. In addition to filtering low frequencies, a high-pass filter can be used to boost low frequencies at the cutoff point by increasing their gain using resonance.

Switching one section to low pass and the other to high pass provides bandpass filtering where the signal is filtered from both sides around the cutoff frequency.

While the SS2140 offers an integrated resonant VCA, we didn't consider it suitable for our filter topology because it's hardwired to one of the outputs. In our Eurorack VCF module, resonance is implemented using a discrete VCA based on matched monolithic transistor pair. The VCA allows control of the resonant amount with CV. Our Eurorack filter module features a resonance potentiometer as well as resonance CV input that responds to voltages from 0 to 5V.

The filter's resonance VCA is also configured to decrease the input signal gain when resonance is increased.
This feature is implemented to maximize the voltage across the filter poles just below the clipping level. This increases the filter module's dynamic range and reduces its background noise.

Our filter offers resonance with subtle diode distortion. The distortion adds a pleasant and distinctive sound to the filter's output when resonance is engaged. The diodes also limit the maximum resonance gain.

Audio demos are available on module page.

Power consumption: +12V – 85mA, -12V – 75mA
Module width: 4HP
Module depth: 3.8 cm
Inputs: Audio IN (±5V), Pitch CV (1 volt/octave), Resonance CV (0 to 5V), FM CV (±5V)
Outputs: 12dB (2 pole), 24dB (4 pole)
Frequency range: 1Hz - 30kHz
Input impedance: 50kΩ
Output impedance: 1kΩ 0.1%
Noise level: -80dB
Indicators: Led indicators for audio output and FM CV inputs
Warranty: 3 years from purchase date, warranty is voided if disassembled and pins bent, damaged by liquids or soldering works done on the module.
Testing: Fully tested before dispatch

https://www.lfusionmodular.com/eurorack-modules/compact-lp-bp-hp-vcf-voltage-controlled-filter/