Spectrum Shaper

An 80 HP, 36-channel voltage-controlled comb filter, a kind often called a string filter. Its maker met a Bruel & Kjaer 36-channel Spectrum Shaper in 2018 in a Rotterdam studio of vintage lab gear, and in 2026 read R. Burhans' AES paper on his 36-channel comb filter and the Electronotes designs it inspired: a 40-channel version by Ian Fritz with a feedback pot per six filters (a mod Lester Ludwig suggested), and Bernie Hutchins' 39-channel one, which added an inverter so the feedback goes positive or negative, centre off, and a noise gate on the output. This one uses 36 LM3900 filters as in Burhans' design, at his frequencies from 50 Hz to 9005 Hz, in six sections of six on SSI2190 VCA/mixer chips: every channel's amplitude is set by its slider and voltage controlled by its CV input, and each section has a Q pot for positive or negative feedback and its own output. Negative feedback flattens the peaks and smears the sound; positive feedback makes them narrower and higher, up to self-oscillation with the sliders up. Each section has its own vactrol noise gate, as does the main output; ALL 1 and ALL 2 are the main outputs. Its maker warns it is a great big build that can kill the joy of DIY.

Key features

• 36 channels from 50 Hz to 9005 Hz, each with its slider and its CV input
• Six sections of six, each with its Q pot (negative to positive feedback, centre off) and its output
• IN 1 and IN 2; the main outputs ALL 1 and ALL 2, with noise gates
• 80 HP; Nonlinearcircuits publishes no current draw, depth or assembled price (it sells the DIY PCB and panel), and ModularGrid lists none