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 Forum index » Clavia Nord Modular » Nord Modular G2 Discussion
DIY filters and stuff...
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Tim Kleinert



Joined: Mar 12, 2004
Posts: 1042
Location: Zürich, Switzerland
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PostPosted: Wed Mar 30, 2005 12:22 am    Post subject: DIY filters and stuff... Reply with quote  Mark this post and the followings unread

Hi folks,

Currently I'm messing around alot with various approaches to building filters or messing around with the existing ones -all in an attempt to explore different filter "flavors".

What I've been wondering: All DIY filters (ie. built from the basic LPF poles, with feedback path) seem to display a considerably higher passband loss at high resonance settings compared to the Nord- and ClassicFilter modules. So, when you make the resonance scream by tampering with the feedback path, it sounds nice and gritty, but always looses alot in the bass.

Any ideas how to combat this?

Also: I clearly remember g2ian once posting a Stilson filter design -I clearly remember liking the sound of the resonance peak of that one alot. But I can't find it anywhere. Somebody got it lying around on their HD? g2ian?

cheers,
tim

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mother misty



Joined: May 13, 2004
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PostPosted: Wed Mar 30, 2005 4:52 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi shoshin,

I think this is what you're looking for?


stilsonfilter.pch2
 Description:

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 Filename:  stilsonfilter.pch2
 Filesize:  2.09 KB
 Downloaded:  792 Time(s)

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Rob



Joined: Mar 29, 2004
Posts: 580
Location: The Hague/Netherlands/EC
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PostPosted: Fri Apr 01, 2005 12:54 pm    Post subject: Re: DIY filters and stuff... Reply with quote  Mark this post and the followings unread

shoshin wrote:
What I've been wondering: All DIY filters (ie. built from the basic LPF poles, with feedback path) seem to display a considerably higher passband loss at high resonance settings compared to the Nord- and ClassicFilter modules. So, when you make the resonance scream by tampering with the feedback path, it sounds nice and gritty, but always looses alot in the bass.


That is correct, with a 24dB filter the passband gets attenuated about 12 dB with high resonance settings. Has been there since Bob Moog's ladder filter. There is a reason: as the feedback is 'inverted' it will suppress the volume by the negative feedback. Why -12dB is a matter of math.

The most common solution is to add a little of the input signal before the knob that regulates the resonance feedback. This input signal must be inverted first. When resonance is raised the filter then gets input on both the regular input plus some extra input signal through the resonance feedback knob when it is openend. It is custom to set this extra amount of input in a way that the passband is attenuated about 6 dB, so the resonance peak will not clip. Which happens if you overdo it.

Folow the purple wire in the attached patch, that btw fully compensates the loss at the shown setting.

When the four poles are slightly detuned the amplitude of the resonance peak is somewhat attenuated, which allows for some extra 'signal injection'. But the resonance peak also gets detuned a bit. Which is pretty analogue behaviour, so not much to worry about.

Saturating distortion like odd harmonic distortion suppresses the peak more that the passband, also common on analog filters by adding some signal limiting diodes to the output of the filter or final VCA.

So, in the patch I added an extra harmonic distorter, just to get that 'you know what' sound.


FatBass.pch2
 Description:
Filter that compensates the signal loss at high resonance settings. Plus a harmonic distorter.

Download
 Filename:  FatBass.pch2
 Filesize:  5.13 KB
 Downloaded:  893 Time(s)

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Tim Kleinert



Joined: Mar 12, 2004
Posts: 1042
Location: Zürich, Switzerland
Audio files: 6
G2 patch files: 214

PostPosted: Sat Apr 02, 2005 1:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
The most common solution is to add a little of the input signal before the knob that regulates the resonance feedback. This input signal must be inverted first. When resonance is raised the filter then gets input on both the regular input plus some extra input signal through the resonance feedback knob when it is openend. It is custom to set this extra amount of input in a way that the passband is attenuated about 6 dB, so the resonance peak will not clip.


That's the fix I was looking for. I knew it had to do with injecting the original signal somewhere, but that's all I could remember. Thanks, Rob.

RE odd harmonic distortion: looking closer at the Stilson filter reveals that it does just that too -a subtle odd harmonic distortion in the feedback loop, taming the resonance just a bit.

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