Dual Filter

The SoundForce Dual Filter is a 2-stage cascaded OTA design based on the late MS-20. The design has been fine-tuned with extended resonance control as well as a resonance CV input. Each filter can be either a 12dB/oct low-pass or 6dB/oct high-pass. To keep the module HPs on the low side, the filter mode setting is located on the back of the module. The module was designed to be a very affordable full-featured modern filter.

Features

The SoundForce Dual Filter features 2 identical filter sections, one on the left and one on the right. Each filter has 2 audio inputs (one of which has a level control pot IN1), a manual cut off frequency pot and a manual resonance pot. There are 2 CV inputs for the cut off frequency both with their attenuverters and 1 CV input for the resonance with an attenuator.

Highly efficient design

This design attracted me very particularly due to its beautiful simplicity. With only 2 poles, it uses a total of 3 OTA ICs and 4 OP-AMP ICs for both filters even with the added resonance voltage control. Early on, efficiency and part counts were a focus during testing and development. In combination with automated manufacturing and a very short supply chain, the module has a very high quality standard at a very affordable price. The module uses only A-brand ICs and passive and no costs were cut during purchasing.

Resonance fine tuning

At the back of the module you can find trimmers to fine-tune the amount of resonance. By default, the trimmers are set so the filter starts to self-resonate when the resonance knob is at 4 o’clock. Turning the trimmer clockwise will create a more exaggerate resonance and it will start to self resonates earlier. On the other hand turning the trimmer counterclockwise will tame the resonance range. The factory value is marked on the pcb with a sharpie so feel free to experiment and go back to the default if necessary.

Filter modes settings

Each filter has 3 jumpers that sets the mode: lowpass or highpass. You will find those on the back of the PCB with clear labels for the positions of both modes. The jumpers are color coded, one filter is black, the other is white.