After being sick as hell for few days, I managed to violate some neurons and get them back to work.
When I need to create some “drum” pattern on the modular system, it necessitates few essential modules (vca, noise, attack generator, and such…) which is a pitty considering that my system is (for now) rather small. I’ve built a TR808 thingy (which needs additional gain to match the modular standard) but it is quite large. I want something smaller.
Basically I need a drum bass (boom boom boom), a kind of snare (tac tac tac) and a striking white noise (tchiiiiinnnn) to simulate hi-hat or Crash. This post is all about the latter :
A white noise source into a variable high pass and going into a VCA which is controlled by a crude enveloppe generator. OTAs are always a bit of pain, they want very small signal to avoid distortion and create offset. But on the other hand they’re quite handy and reduce the component count nicely. Some AC coupling caps remove the offsets anyway.
I’m not really happy with IC3B and IC1B which buffer and offset the modulating signal to act on the OTA gain. That’s a bit ugly and R27 might interfere with the the discharge of C3.
however, it works very well as it, R40 is a trimmer (but will be replaced with a voltage divider) that brings an offset to the modulating signal and moves it toward the negative rail (to close the VCA properly).
I found the high pass filter more interesting that the traditional low pass for such application.
Some values are still to be determined hence their absence on the schematic.
One drawback remains, you can clearly hear a “poc” sound with the enveloppe being triggered. I though it was an issue on my design but all my VCAs do it with fast enveloppe (but they’re all made of OTAs…).
The snare and Drum could be indentical structurally, juste replace the white noise source with a sinus oscillator. Althought it could be more fun to do something else.
and I’m still looking for (cheap) reliable front panels (maybe acrylic but my experiences with it weren’t great).