For anyone interested in completely subjective testimony on the so-called "18db/24db" VCF mod, I can now offer some, after having installed it.
I first replaced the Rainbow Fish caps I initially used with ceramics. I just wanted to see if I could squeeze a bit more stability out the circuit-- it just sounded a little thin and wonky with the rainbow fish. The new caps had the exact effect I was hoping for. It's not a dramatic change, but it sounds smoother and more linear now with the update.
So, then I installed the 100ohm resistor and 0.1uf cap for the filter mod; again, I'm happy to say it seems to have bolstered the overall "stability" of the VCF. It's not dramatic, but the filter sounds slightly deeper, darker and smoother now-- to borrow a cliche, I'd say it's slightly more Moogy.
The differences are subtle, but I think the sound is improved. It's also slightly less nasal, and I could see some people preferring the more band-pass like unmodified sound, but for most uses, I think I'll prefer the modified sound.
Anyone using Derek Revell's PCBs, the modification couldn't be easier to implement as there are dedicated plated thru holes already in place for it on the Y board b/t diodes 16 and 23.
Despite its name, the mod actually ads a 5th pole to what was already designed as a 4-pole 24dB LPF, effectively bumping it up to a 30dBs. The additional pole, aka the mod, was standard on Synthi's manufactured after 1974. Anyway you can read all about it on most of the Synthi sites.
Graham comment this with: Swapping caps with 10% or 20% tolerances will move the centre frequencies of each stage around. Arguing that different types change the sound is only valid if they are matched for value, if you replace one that is -20% its nominal value with one that is +20% of course it will sound slightly different.
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