[sdiy] IR3109 (OTA-based) filter question

[>>>marjan<<<]

I've been tinkering with my JX3P's filters with the IR chips, and 
it's possible to convert it to Xpander style multimode. Basically
I've just copied the schematic, with different buffers/inverters
(IR has buffered stages CEM does not, or at least that's what 
we got at the end at the list), one TL074, one 4051, 1/3 4053
per channel (vcf) and bunch of resistors and few caps, plus some
control logic (counter) to advance thru the modes. Neat stuff.
I still have to implement it on all voices thou. Try it, it should
work (you'd have similar mess if you've modified it just for lp/hp/bp).
12/24 mode is lot easier, that's right. This is the option I've choosen
for Polysix, as it's ssm2044 could't be modified in Xpander mode
that easily (I think, as it has internal structure more like ladder than
four ota in series) but also single vco isn't justified cause for all
that work.
2/4 pole is just enough. My P6 has some nice mods, but all of them just
reroute and expand modulations, but recently I tried this mod and for
the first time my polysix sounds different (thinner/buzzier?). It also
needs noise and midi to be installed...


marjan

> > The Roland JP-6 has a multi-mode HP/LP/BP VCF which apparently (I don't
> >  have the service manual) uses the IR3109. The IR3109 is  used in the
> >  JP-8 VCF which has selectable 2-pole and 4-pole LP modes. The Roland
> >  (Boss) RPH-10 phaser uses two IR3109 for the allpass filters. I have the
> >  RPH-10 schematic and can send it if you wish.
> 
> I've figured out the 2-pole/4-pole switch, and thanks to an earlier reference
> to the CEM3320, I believe I've figured out the HP/LP/BP switching as well...
> 
> I'm currently trying to decide on the best method if implementing the
> switching, since I'll be making/breaking several connections simultaneously.
> I'm probably going to use 4016/4066 analog switches unless someone has a
> better idea