Modern analog transistor ladder filter

Invented by Robert Moog and used to this day in Moog synthesizers ladder filter uses a chain of differential transistor pairs connected to capacitors for 4 pole analog filtering. Such filter has its unique rich sound very suiltable for subtractive synthesis.

Our ladder filter uses SSI2144 dedicated ladder filter IC where all the matched transistor chains, exponential converter and resonance circuit  are packed into monolithic silicon block. SSI2144 is a modern version of SSM2044 filter chip designed by famous Prophet 5 synthesizer engineer Dave Rossum.

Subtractive synthesis

Legendary SSI2144 analog ladder filter IC

SSI2144 considered by many to be the best-sounding analog synthesis filter IC ever produced, known for its rich, creamy, and buttery sonic characteristics very suitable for subtractive synthesis.

The SSI2144 preserves the original sound of the SSM2044 while incorporating modern process technology for improved performance, including lower noise, better control rejection, and more consistent unit-to-unit performance.

Compact 8HP dual filter and dual VCA Eurorack module

Due to small footprint of SSI2144 IC we managed to fit two separate filters and two VCAs into small 8HP Eurorack module format. When paired with our voltage controlled oscillators this module allows having two separate synthesizer voices without the need of any extra modules.

Our Dual VCO

Integrated with dual voltage controlled amplifiers

Each of two filters in the module are connected to dedicated voltage controlled amplifiers based on low noise SSI2164 VCA ICs designed by the same engineer.

Each VCA in our module has its own linear/exponential gain switch as well as dedicated CV input to control the gain with control voltage from 0 to 5V. When nothing is patched to these CV inputs – VCAs remain open giving the option to use the filter without the integrated voltage controlled amplifier if needed.

This gives the opportunity to patch a more complex synthesizer voice by adding distortion and other effects then sending the resulting output to dedicated VCA module.

Our quad VCA module

Frequency Modulation (FM) filter CV

FM CV inputs are passed through a dedicated FM potentiometers to adjust frequency modulation strength.

FM CV input is usually patched to envelope generator with fast attack and decay for a fast frequency 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 module
• Env. follower

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

Our mixer module

Voltage controlled resonance

Resonance boosts the gain of signal frequencies around the cutoff point by sending a portion of filtered signal back to the input. Our dual filter Eurorack module offers both resonance potentiometer and resonance CV input for each channel that responds to voltages from 0 to 5V.

On high settings ladder filters start self oscillating producing very pure sine waves that can be used as sine wave oscillators.

Temperature compensated exponential converter

We have integrator high end thermistors placed very close to filter IC to make the linear CV to exponential frequency converters within the SSI2144 filter IC temperature stable.

Filter modules has a dedicated knob for cut frequency,  pitch CV inputs with standard 1 volt per octave tacking as well as dedicated FM inputs.

Module built using high quality components

This compact dual ladder filter Eurorack module is built on two 4 layer PCBs using 220 high qualiy electronic components: 20 audio grade  Burr Brown OpAmps, precision voltage reference for cutoff control, 0.1% precision resistors and 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.