[sdiy] SEM filter questions

[David G Dixon]

> > By the way, does anybody know of a good replacement for the 2N4302 JFET?
> > I'm tempted just to use the Darlingtons on the 13700, but if the JFET 
> > really does change the sound, I'd like to get close.
> 
> Considered using JFET op-amps?

Yes.  Indeed, my preference is to reconfigure this entire circuit with
SSM2164 VCAs and TL07x integrators.

> You should be able to get 2N4302 or similar easilly.

Well, no.  However, I found a document in Datasheet Archives which suggests
that the 2N5457 is a good replacement.

> Recall, this filter has been cloned commercially over and over again. 
> Don't forget to check out the ASM-1 VCF, which is one such clone.

I had a look at ASM-1.  It uses CA3080 OTAs with (10k|30pF)-20R dividers and
CA3140 (MOSFET input opamp) integrators with 330pF caps.  The original used
CA3080 OTAs with 100k-220R dividers and 2N3402 buffers with 300pF caps.
Hence, other than the OTAs, these two are actually quite different.  If the
"small things" are important for the sound (which I doubt they are) then the
ASM-1 VCF shouldn't be considered a true clone.  It does reproduce
(basically) the resonance feedback network of the original, though.

I will probably build several versions -- SSM2164 + TL07x integrator,
LM13700 + Darlington buffer, and LM13700 + 2N5457 buffer -- and try to
discern if there are in fact any sonic differences whatsoever.  The first
one really appeals to me, becuase it requires neither an expo converter nor
ac coupling capacitors.  If it sounds as good, then I'll go with that for
future builds.  Perhaps I will offer some sort of "2164 Uberheist SVF"
module after all!