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

[Tom Wiltshire]

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