Flexible voltage controlled filter Eurorack module

This compact 4HP voltage controlled analog filter (VCF) eurorack module is based on quality SSI2140 filter IC with four temperature compensated OTAs (operational transconductance amplifiers).  Each OTA paired with precision capacitor creates a filter pole with frequency slope of 6dB per octave. SSI2140 IC is used in famous Prophet 5 Synthesizer, it is an improved version of SSM2040 filter IC designed by Prophet 5 engineer Dave Rossum.

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

Each VCF block has a hardware switch for lowpass and highpass output enabling various filtering configurations.

Precision voltage controlled filter cutoff

All filter poles are controlled with precision exponential cutoff that can be controlled with dedicated Cut potentiometer, FM CV and 1 volt/octave pitch CV.

1 volt/octave pitch CV input

Usually same pitch CV is sent to both oscillator and filter so the filtering frequency would automatically move depending on the pitch of the note that is playing. This makes sure that all notes would sound similar. Without the pitch CV input filter would stay static and filter out the high notes too much making them less audible.

Our Dual VCO

Frequency Modulation (FM) CV

FM CV has a dedicated potentiometer to adjust frequency modulation strength.

FM input is usually patched to envelope generator with fast attack and decay for a fast jump at the beginning of the played note. Our envelope generator paired with gate to pulse converter can generate such envelope with very smooth analog exponential decay.

• Gate to pulse
• Env. follower

FM CV can also be used for control filter from music software using voltage CV. Both control voltage and envelope CV can be mixed to one control signal using analog mixer (adder).

Our mixer module

Music demos made using this filter Eurorack module

Listen to our audio demos produced using this filter module:

All sounds from in these demos are completely analog. Our dual VCO Eurorack module is generating tone for the filter using two oscillators; two envelopes, for VCA and Filter, are produced form a single gate using our Dual Envelope Follower paired with Mult with Gate to Pulse converter.

Drum section in all demos is produced using our to percussive modules:

• Dual Kicks
• Snare+Hats

Audio filtering modes and their uses

Filter module has two analog switches to select the filtering mode for each of the two multimode filter blocks.

12-24dB Lowpass filter for subtractive synthesis

When both VCF switches are set to LP module works as 12db and 24db (2 or 4 pole) lowpass voltage controlled filter. Such filter is the basis of subtractive synthesis where high frequencies are filtered out from the chosen cut point.

12db or 2 pole VCF response is used in famous Korg MS20 filter with bright sound. 24db response filters the signal more with a deeper sound similar to Moog and many other filters.

12-24dB Highpass filter

When both switches are set to HP module offers 12db and 24db highpass outputs. Highpass is the opposite of lowpass filtering low frequencies from the cut point.

Many synthesizers such as Korg MS20 and many others have two separate filters – one lowpass and one highpass. Highpass filter is often used to remove unwanted low frequencies from the synthesizer sound.

Boosting low frequencies and sidechaining

Apart of filtering low end – Highpass filter can be used to increase low frequencies at the cut point by increasing its resonance.

Highpass filter can also be used for sidechaining. For this you would have to patch kick to a envelope follower and then patch its output to highpass filter FM input. This way when the kick hits it increases the cutoff point filtering the low frequencies giving clean low end for the kick sound.

Bandpass filter

Switching one pole to lowpass and the other to highpass offers bandpass filtering where the signal is filtered from both sides around the cut frequency.

Due to physics bandpass output has 3db lower gain that can be easily compensated by increasing gain of the VCA.

Our quad VCA module

Eurorack module with discrete voltage controlled filter resonance

Voltage controlled VCF resonance

Resonance is a very important filter parameter, especially in subtractive synthesis. It boosts the gain of signal frequencies around the cutoff point.

Filter resonance is achieved by sending a portion of filtered signal back to the input.

Boost around cutoff filtering frequency

This resonance frequency boost gives the filter its characteristic sound. When resonance ins at its maximum level – filter starts to self resonate and can be used as sine wave oscillator.

Resonance feature based on discrete VCA

In our VCF Eurorack module resonance is implemented using discrete VCA based on matched transistor pair. VCA gives the resonance voltage control capability. Our filter Eurorack module offers both resonance potentiometer and resonance CV input that responds to voltages from 0 to 5V.

Maximized dynamic range

Filter resonance VCA is also configured in the way that when the resonance is increased it lowers filter input gain.

The feature is implemented to maximize the voltage running through the filter poles just below clipping. This increases dynamic range of the filter module lowering its noise floor.

Resonance distortion circuit

Our filter offers resonance with diode distortion. Distortion adds pleasant sounding special character to filter output when resonance is turned on. Diodes are also there to tame maximum resonance gain.

Module engineered using high end components

This compact Eurorack module is engineered on two 4 layer PCBs using 157 high end electronic components including 14 audio grade  Burr Brown OpAmps, precision voltage reference and resistors as well as very high quality temperature stable Japanese capacitors for analog filtering function.

All potentiometers are of Alpha variety. Module has knobs with soft touch for pleasant tactile feedback.