[sdiy] need help with a filter

Nugent, Allison C. (NIH/NIMH) [E] nugenta at mail.nih.gov
Sat Dec 21 19:33:45 CET 2013


Hello,

I'm hoping someone can help me with a problem.  I'm trying to build a drone synthesizer.  I've built six simple two op-amp triangle/square oscillators, and a mixer.  That's all soldered onto protoboard and working fine.  

The problem is the low pass filter.  I'm trying to build the simple low pass filter from the MFOS noise toaster.  I'm not even building the voltage controlled part of the filter that receives input from an LFO/ARG.  I've built it about 12 times on a breadboard, and I can't get it to work.  It's showing some strange behavior.  As soon as I connect the non-inverting input to virtual ground (my device is working off a 9V single supply) the filter starts producing a square wave output, even with no input into the inverting input.  I was initially using an independent virtual ground on the breadboard, but when I tried to tie into the same virtual ground as the mixer, the oscillation fed back up to the mixer, and mixed with the input. 

I don't have a lot of experience building stuff like this, and problems that are obvious to more experienced folks are not terribly obvious to me.   But I really want to get this thing finished...  if you have any ideas, please let me know.  I'm happy to post a schematic, if that would be helpful.  

Thank you!!!

Allison



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