Pas-Isel: A technical breakdown

    So now we've established why I designed Pas-Isel, here's how it happened.

    We begin, as all good things do, with the timeless patent for the transistor ladder filter by Dr. Bob Moog. However, we look at the not-so-legendary half - the horizonal highpass.

Bob Moog's highpass, from the seminal 1969 transistor ladder patent. (Source: Google Patents)

    This is indeed a highpass, not a lowpass... But that's easily changed. Simply put the highpass in a negative feedback loop! This trick was used by Don Buchla in his 100 series modules, with the 191 Sharp Cutoff Filter. Each filter pole of the Moog design, consisting of a capacitor, a pair of transistors and buffering amplifier, is put into a negative feedback loop, for a total of four stages of inverted single-pole highpass filters, resulting in a four-pole lowpass response at the end.

    A breakdown of the Buchla 191's operating principle, written and illustrated by the brilliant Osamu Hoshuyama, can be found here. 

    The Buchla 191 is a bit of a complicated and unconventional beast, though - It's highly discrete, incorporating differential transistor pairs for the negative feedback sections and JFETs for buffering, and as a result needs a lot of trimpots to get it to work. It's also non-resonant, although in recent years Verbos Electronics made a resonant (but not self-oscillating) version with their equally discrete take on it, the Verbos Dual Four Pole.

    Buchla is not the only company to try and take on the inverted highpass approach to making a lowpass filter. The Hammond Organ Co. patented a filter that used the principle, utilising a quad JFET array as the voltage control element, and went on to put it in their catchily-named 102100 and 102200 preset monosynths. It's a minimal parts count take on the idea, putting the entire four-pole highpass in a single op-amp negative feedback loop and removing the ability to adjust resonance.

Hammond's take on the inverted highpass lowpass. (Source: Google Patents)

    Pas-Isel takes the best of both the Buchla and Hammond designs, with a few twists of our own. We took the independent buffering/feedback per pole of the Buchla, the use of monolithic IC op-amps as buffering/feedback elements of the Hammond, and as a nod to previous attempts to get around Moog's patents, diodes as the voltage control elements. The control voltage processing circuitry is also derived from the diode ring CV driving circuit of the Korg MS50 filter, and the resonance feedback loop incorporates soft-clipping diodes like most classic Korg MS-series filters.

    Pas-Isel cleanly self-oscillates at maximum resonance. At attenuated input gain, it behaves politely and transparently like an 80s Roland filter, yet livens right up into an ARP-like bite at stronger input levels. Altogether, it's a very different beast from any VCF we've heard before. We hope you'll like using it too!