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 Forum index » DIY Hardware and Software
Odd Filter Oscillation Issue
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Cynosure
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PostPosted: Sun Nov 10, 2019 5:38 am    Post subject: Odd Filter Oscillation Issue Reply with quote  Mark this post and the followings unread

I am working on adding a filter to the output of a synth. I have the filter on breadboard and it works great except for one odd behaviour. The output gets very loud whenever the cutoff is near the fundamental of the input sound. It sounds almost like feedback, but only does it near the fundamental. Turning down the resonance helps, but doesn't completely stop it from happening.

Any idea what is going on to cause it? How can I stop it from happening?


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JovianPyx



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PostPosted: Tue Nov 12, 2019 12:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

Maybe it's the way the schematic is drawn, but it looks like the feedback being controlled for resonance is the wrong feedback path. My understanding is that the state variable filter low pass resonance control is a path from the second integrator output to the first integrator input. What I see there is a VCA controlled feedback path from the first integrator's output to it's input.
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Cynosure
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PostPosted: Tue Nov 12, 2019 4:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

It all works now. I was overdriving the input on the resonance control. I increased the input resistor and lowered the one on the output. All seems good now Smile

I have seen many SVF's that feedback from the first integrator (Wasp for example).

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PostPosted: Tue Nov 12, 2019 5:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

Right, in fact that feedback is called the band pass feedback. The one from 2nd integrator to input of 1st integrator is called the low pass feedback. So it depends on what the filter is doing.
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