High tech 4HP analogue filter Eurorack module
Listen to how the filter module sounds
Here are our audio demos produced with this Eurorack filter module:
All the sounds in these demos are entirely analog. Our Eurorack dual VCO module generates wave shapes for the filter using a mix of two oscillators; two envelopes, for the VCA and filter, are generated from a single gate using our Dual Envelope Follower paired with a Gate-to-Pulse Mult.
The drum section in all the demos is produced using our two percussion modules:
Four pole analog filter based on transconductance amplifiers (OTA)
This compact voltage-controlled analog 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 temperature-compensated transconductance operational amplifiers (OTAs). Each OTA, along with a precision capacitor, creates a filter pole with a frequency slope of 6 dB per octave. SSI2140 IC is integrated on bottom side of the PCB shielded from both sites between ground planes protected from RF interference.
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 12 dB and 24 dB.
Precision Voltage-Controlled Filter Cutoff
Filtering frequency is controlled by a precision exponential cutoff, which can be adjusted via a dedicated cutoff potentiometer, FM CV as well as 1 volt/octave pitch CV.
1 Volt/Octave Pitch CV Input
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.
Lowpass, highpass and bandpass audio filtering modes
The filter module offers two analog switches to select the filtering mode for each of the two multimode filter blocks.
Lowpass filter for subtractive synthesis
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.
Additional highpass and bandpass filtering modes
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.
Voltage-Controlled Resonance Based on a Discrete VCA
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 5 V.
Maximized Dynamic Range
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.
Subtle resonance distortion circuit
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 control the maximum resonance gain.
More info, where to order and full module specs
For more info and full specs of this analog filter Eurorack module please visit its dedicated page in products section.
Really enjoyed hearing how the SSI2140-based design handles both subtle movement and more driven modulation—those demos highlight a nice balance between character and precision. Pairing the dual VCO with the envelope follower and gate-to-pulse setup also shows how much mileage you can get out of a compact patch. Curious to hear how it behaves with slower, more evolving CV sources as well.
Hi, thanks for your good comments and suggestion. Next week I’ll prepare a 5th demo with slow CV filter modulation and let you know when its ready.