Bells and noise

Noise Bells is a percussion and drone synth module based on ideas presented in the 2015 Hackaday article “Logic noise: More CMOS cowbell!” by Elliot Williams. The PCB and front panel are Kosmo format but the same circuit could be implemented in Eurorack format, or other ±12V formats, with no changes other than PCB, panel, and jack dimensions.

The module consists of two separate sections. One is based on the “cowbell” section of the Hackaday article. Two square wave oscillators (frequencies manually controlled with panel pots) feed into an XOR gate, which, if the oscillator frequencies are not simply related, produces a complex sound with many frequencies that are sums and differences of the oscillator frequencies and their multiples. The XOR output goes to a “VCA” — in quotes because it’s just an analog switch and a resistor which function like a VCA for binary inputs only — controlled by a simple envelope generator (fast attack, slower decay time controlled by a pot). The result is bell-like percussive sounds. Aside from a few component value changes, notably a larger capacitor allowing longer decay times, there are a couple of significant modifications to Williams’s design:

The other section uses four oscillators combined using three XOR gates. From there it’s identical to the first section. It lies sort of halfway between the article’s “cowbell” and “hi hat” designs and was designed to use most of the IC functionality left over from the “cowbell” side. Whether it leans more toward bell or cymbal sounds I leave for you to decide.

It’s a simple module, and an easy build. The noises it makes are a bit strange and somewhat hard to control. If you get a sound you like, don’t touch the knobs, you may never find it again! If you’re looking for a really killer cowbell sound look elsewhere, but this is fun to play with. I might even like the drones better than the percussion.

For BOM, schematics, and PCB layout see the Github repository.

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