video upload by cirtcele
"I'm still learning how to use this thing but here are some excerpts from a patch with Tinkle and FM OP via Product wavefolder both fed into the Spectrum Shaper."
via nonlinear circuits
"36 channel voltage controlled comb filter aka string filter.This module has some history. In 2018, I was part of a team putting on a show with CellF at the Todays Art festival in Den Haag. I popped down to Rotterdam to visit Dennis (aka Mono-Poly) and his studio of vintage lab gear. Dennis had an incredible patch going centred around a Bruel & Kjaer 36 channel Spectrum Shaper. Listen to his album Testlab to hear how it sounds. I just knew I had to make something like it one day. This type of filter is often called a ‘String filter’, used to emulate stringed instruments tho don’t feel limited by that.
Fast forward to 2026, I read an AES paper by R. Burhans describing his 36 channel comb filter. It reminded me there were some designs in Electronotes, so I looked them up and discovered they were both inspired by and heavily influenced by Burhans, particularly in terms of the comb filter frequencies and spacing. One was a 40 channel version by Ian Fritz using transistor based filters. He had an interesting mod, originally suggested by Lester Ludwig, where each set of 6 filters had a feedback pot. Some screenshots from these articles are at the end of this build guide. The relevant Electronotes issues 107, 115 and 116 can be found online.
The Bernie Hutchins design used 39 regular op amp based filters, along with Ian’s feedback per 6 filters mod and added an inverter so the feedback could be positive or negative (so, centre off). Negative feedback ‘flattens’ the peaks somewhat and smears the sound, positive feedback makes the peaks narrower and higher (if tuned for it, you can make them self-oscillate, see notes). Bernie also added a noise gate on the output, summing 39 channels is always going to be noisy.
