[sdiy] Oakley News

Oakley Sound oakleysound at btinternet.com
Tue Nov 20 14:23:13 CET 2018

 > It says on the webpage that the VCOs are ramp-core, so I take it 
these are discrete and not done with 3340s?

Here's a photo of the dual VCO core.


It's very straightforward. Standard NPN pair expo current sink with 
modified Rossum compensation. 1nF C0G capacitor is reset to 6V and 
dragged down to 0V by the expo. LM393 comparator does the zero voltage 
detection and directly drives a J112 FET to discharge the capacitor. It 
actually turned out to be not too dissimilar to the one used by Moog in 
the Prodigy and the Source. The pulse waveshaper is the same one I use 
in my big VCO module.

The EGs are AS3310s though. Just can't really beat them for squeezing a 
good analogue ADSR in such a small space. The AS3310 is not quite a 
CEM3310 though. The attack peak seems to sit around 5.4V rather than 5V. 
That's not a problem as one can increase the maximum sustain voltage to 
match in a new design - possibly more of a problem as a replacement for 
an original CEM3310 in a vintage synth though. The waveshape also gets a 
little 'bent' at very fast decay times which the CEM3310 doesn't do. I 
recommend sticking with the 22nF compensation cap rather than the 10nF 
that Alfa uses in their datasheet. Using 10nF does seem to 'bend' the 
waveshape still further.

 > What did you use for the SVF? Is it an original design or one of the 

I normally try not to clone too much as this inevitably ends up with 
comparisons. However, in this case, it is the Dennis Colin design from 
the SEM. Indeed, the whole module has a SEM vibe to it, which wasn't my 
original intention. It sort of just turned out that way.


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