{"id":2757,"date":"2022-09-24T10:26:55","date_gmt":"2022-09-24T10:26:55","guid":{"rendered":"https:\/\/maffez.com\/?page_id=2757"},"modified":"2023-12-06T16:18:58","modified_gmt":"2023-12-06T16:18:58","slug":"model-f","status":"publish","type":"page","link":"https:\/\/maffez.com\/?page_id=2757","title":{"rendered":"Model F"},"content":{"rendered":"\n

Update Dec 2023<\/strong> – There’s a through-hole version now too (by Werner, based on my schems) that sports more filter responses (-24 + QCOMP, 18, -12, -6, BP) – scroll to bottom of page for gerbers and all. Cool contribution, Werner! THX!<\/p>\n\n\n\n

Here we go – my first active* Eurorack Module design.<\/p>\n\n\n\n

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<\/a>. Of course, Robert Keeble has an excellent implementation with his AM8101 VCF module<\/a>, and Ericasynths have a variant as well<\/a> (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. <\/p>\n\n\n\n

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:<\/p>\n\n\n\n

Saw and Sub Smack<\/figcaption><\/figure>\n\n\n\n
Characteristic level loss compensation and diode clipping in the filter resonance<\/figcaption><\/figure>\n\n\n\n
Bandpass through pole-mixing<\/figcaption><\/figure>\n\n\n\n

*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<\/a> here, something you will hear in the upcoming Mandalorian season, whoop!).<\/p>\n\n\n\n

What’S even cooler than one Model F? Two! Listen how dual bandpass in series can sound…<\/p>\n\n\n\n

model y into two model fs<\/figcaption><\/figure>\n\n\n\n

The Module<\/strong><\/h2>\n\n\n\n

Here’s a look at the beta version with some kinks ( ironed out in the version linked below).<\/p>\n\n\n

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Model F <\/figcaption><\/figure>\n<\/div>\n\n\n

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. <\/p>\n\n\n\n

self oscillating filter – octave tracking <\/figcaption><\/figure>\n\n\n
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first proto boards – worked well but still I wanted some things changed <\/figcaption><\/figure>\n<\/div>\n\n\n

“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.<\/p>\n\n\n\n