[sdiy] wednesday's DIY

[ryan williams]

hi all,

I am planning a theremin that will have a VCF on board. It needed to be 
simple, single supply, and preferably be without opamps. I also wanted 
to build René Schmitz' MS20 filter. So, I have built a version using 
4069U inverters. It is the same filter structure, but with some changes 
since all amplifiers are inverting. CA3080s replaced with 13700. In the 
end, I will run it off +9V, but for now I have tested using +15V. With 3 
diodes in the feedback, the resonance works good until the signal level 
is about +-2V. Any more causes only the center part of the waveform to 
show resonance. This is fine for me since the theremin will have a 4069 
oscillator with similar output level.

so, does it sound like an MS20 filter? How would I know, I've never 
seen/heard an MS20. here's a sample at high resonance:

http://www.sdiy.org/destrukto/mp3/cmosed_ms20_filter.mp3

and the schematic:

http://www.sdiy.org/destrukto/files/cmosed_ms20_filter_11-02-2005.pdf

-ryan