Here we go – my first active* Eurorack Module design.
I’ve always had a thing for the Roland SH101 filter, so it was happy times when Alfa Par announced that they were going to reissue the IR3109 chip. Of course, Robert Keeble has an excellent implementation with his AM8101 VCF module, and Ericasynths have a variant as well (with VCA and resonance CV, but without diode clipping in the feedback path). And, if you’re into DIY, you can still source the Pharmasonic SH module files, which are based on the ace 3109 Shruti board developed back then by Emilie Gillet.
Why a maffez version? I guess I wanted a straightforward implementation that also harnesses my experiences with modifying Roland filter clones. So, this means in addition to a simple build (two ICs and some passives) you get an extra bandpass mode for yummy acid vibes! Listen to some impressions:
*First “active” simply means that this is my first design with ICs, Gerbers and the whole load, though it is not my first module design ever (look at the mysterious Demix Module here, something you will hear in the upcoming Mandalorian season, whoop!).
What’S even cooler than one Model F? Two! Listen how dual bandpass in series can sound…
Here’s a look at the beta version with some kinks ( ironed out in the version linked below).
In terms of controls, there are cutoff, resonance, modulation depth, audio input signal level, and filter mode. There is a second audio input with fixed level, an extra “key” modulation input that tracks your cutoff to 1V/octave, so you can play sine wave solos with the self oscillating filter. The inputs and output are buffered.
“Inside” that little thing you have two ICs, the AS3109 and a quad op-amp. The original circuit uses two transistors in the feedback path that do clever signal polarity tricks for level loss compensation (that’s when the passband loses level when you dial in resonance – we often also call this “bass drop”) and resonance itself. In my version I simplified this by means of an inverting op-amp buffer.
The second that’s a bit different is an added bandpass filter response. This basically just harnesses well known principles of filter pole mixing (check electricdruid’s excellent site and Gillet’s papern on that topic), yet in the 101 realm, that’s less common as it should be 🙂
Theoretically, you can create a whole lot of more filter responses, and some folks are working on a 12hp through hole version of the Model F, if I’m correctly informed. That one will also sport -6db, -12db outputs and such. For 8HP, however, a rotary switch would have taken up quite some real estate, so something had to give. Well, still a better love story than the stock 24db VCF… and if you don’t like the bandpass, you can easily rewire for -6db out or -12db out.
If you build the version linked below, put wire in place of C15, which increases low end stability on self-oscilalting resonance.
Input level/signal clarity ratio: Tom Noise and others did some A/B with their SH101 and found that you have to dial input down on the Model F to get the same signal clarity. Hence, if you want clear, use 200k for R12 (then also bump up R31 to 150k, so the filter is not too quiet). If brash does the trick, then use 120k as per schematic for R12 and R31.
Production Files and Schematic
So far, the module is something you have to have made for you or make yourself. All necessary files you need are linked right below. Schematic, BOM and GERBERS are all in line (Gerbers may have 1.4 version label but between V1.3 and current V1.5 there were only some small changes in component vales).
As mentioned, the Gerbers upped here are an updated version and I need to order these as PCBs first. Once they’re in, I’ll post pictures of the build process. Should you want to build one in the meantime, do as always – components with lowest profile first, then the more spacious ones at the end. Only solder in pots and jack sockets with the panel fitted unless you want uneven results.