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

[rob]

Hi Tom,
Having owned a few Pro-1's my experience was that the CEM3320 filter was
excellent.... Cutting, resonant, no dead spots..great for making music with
the fast CEM3310's....yes volume goes down and the bass weakens. But it
still cooks !

So I when it came to cloning the filter with my own PCB I thought the
results were a foregone conclusion.

I was wrong. The CEM3320 needs abit more care and attention.........

Initially I used a Digisound design, but whilst this is more sophisticated
than a SCI Pro-1 it didn't sound right. (but has CV Reject and output Trim
circuit) The resonance just didn't happen...it was thin and failed to
resonante well. Back to the drawing board and the Pro-1 schematic, this has
a different Q feedback and sounds a whole lot better, there are larger
capacitor values in the audio chain. 2u2 rather than 1u, larger caps in the
filter, and I have used polystyrene 1%.  

Guidance Notes.
1) CEM3320 performance does vary across different chips. So if it sounds
crap, try a different CEM3320...
2) The Q feedback circuit is critical to the usabaility of the filter - as
Tom describes. Room to play here. Smaller caps in the feedback?
3) Using larger audio chain caps may improve bass response - aka Oberheim
4) Using 1% polystyrene filter caps does help the quality of the filtering,
but doesn't change the Q response  
5) Varying the filter cap value will give a different response...150pF
Pro-1, 300pF Oberheim.

Hope that helps...SSM2040's are by no means all the same either...

Regards
Rob
www.emulatorarchive.com


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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