[sdiy] Ideas for Sequential Pro-One filter modifications please

[harrybissell at wowway.com]

I've done some mods to the filter...

I did a State-Variable filter from the CEM datasheet. I had LPF, 
BPF, and Notch outputs. The signal level handling was reduced, overdrive
was not real pleasant.  

I also di a two-pole, four-pole tap on the standard filter. Keep the
resonance around the full filter, and just switch the output buffer
between the second and fourth stage.

The drop of volume with resonance is a feature of this filter topology.
One fix (I have not tried) is to inject the signal through the
resonance path. That would reduce the volume drop.

H^) harry



On Wed, 18 Jun 2008 00:42:38 +0100, Tom Wiltshire wrote
> Hi folks,
> 
> I've owned a Sequential Pro-1 for many years, and I've never been  
> particularly happy with the filter. I'm not sure whether this is a  
> fault or a feature. It's possible that other Pro-1s sound better 
> than  mine. Let me describe the sound.
> 
> With the resonance low (below 2) the filter just acts as a filter,  
> without adding any particular coloration. Turning the resonance up a 
>  bit more brings out a certain bite and emphasis, but starts to 
> reduce  the overall volume. The sound is fairly thin, but very 
> cutting. It  makes for good lead noises, but has none of the liquidy 
> squelch of my  SH101. I rarely turn the resonance up much above 
> halfway as the sound  just becomes too thin, so unless I want a 
> resonant whistle, I don't  go there.
> 
> Now, it seems to me that this isn't actually far from the reported  
> behaviour of this filter, so I doubt there is anything serious up  
> with it. It certainly works as a filter - I just don't like it that  
> much.
> 
> What I'd like to know is;
> 
> Has anyone done any good mods to the Pro One filter?
> 
> I wondered about pulling the CEM3320 out and using it in a  
> replacement filter circuit. There are plenty of examples about: The  
> OB-Xa has two different CEM3320 filters(!), the OB-8 has a multimode 
>  design, and the Elka Synthex goes one better still. What do you think?
> 
> Alternatively, do you think replacing filter caps with modern  
> improvements (polypropolene or something) would help the sound?
> 
> Thanks,
> Tom
> 
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Harry Bissell & Nora Abdullah 4eva