CFM - SPT (Switching Pole-Topology) Filter

Exciting Filter with superb build quality – based on a classic design (CEM 3320) with added Roland filter (TB 303 etc.) inspired twists.

CFM’s SPT Filter is an (up to) 24db/4-Pole filter with maximum flexibility. Each of the four poles has a seperate output and each of the poles can be switched between High- and Low-Pass. This means you can get a ton of filter-types out of this unit: 1-4 Pole HP, 1-4 LP and any combinations thereof – like bandpass and more unusual types. With an external attenuverting mixer or (quad) bipolar VCA with mix out, things can get even more interesting by blending, inverting and morphing the outs with CV.

Sound wise this is can also be a classic filter. The used chip is a reissue of the CEM 3320 found in synths like the Oberheim OB8, Sequential Circuit Prophet 5/Pro-One & Elka Synthex.

However, there are some added, special extras in the form of switches. You can find these just above the Pole Topology switches. They are labeled Pole Centre-Frequency Shift. They spread or drop the center frequency of the associated pole by changing the capacitor value of it. The inspiration for this was taken from the unusual filter design Roland did for the TB 303 (and other Roland synths), where one pole had a different capacitor value than the other ones. With the CFM SPT Filter you can experiment with this “trick” on all poles.

In order to get even more acid-ey there is also a switch acting on the resonance input of the chip and driving it into wild, sometimes unstable territory. So: turn on, tune in and drop ...the poles.