9v EMS 4-pole diode ladder VCF

[caress]

has anyone built one of these?  i made one today and i'm getting very odd responses from all of the controls...  i double checked all of my connections on the board and everything seems to be in order.  i've been fiddling with the INIT and BIAS controls to get the best range from the CUTOFF and RESONANCE controls, but i can't seem to get a proper lo-pass response.  i get filter-ish sounds with lots of strange oscillations, hisses, distortion, etc. which seem to be rather non-linear.  any ideas?

here's the schem:
http://www.modular.fonik.de/files/EFMforum/orgEFMfiles/vcf6-9v.pdf

edit:  the only modification i did to the schematic was to change the polarized cap in the RESONANCE feedback loop from a 4.7uf to a 100uf.  i tried both and there was only a subtle change... tried because of some reading i did at electro-music.com where it was stated that the 4.7uf may cause a slight hi-pass response.

[R.G.]

I always had problems with the diode ladder. The transistor ladders worked better for me.

[caress]

hmmm... ok well transistor ladder it is then.  the diode ladder has a slightly different sound which is why i was trying it out, though.  i'll certainly keep at it.

[Eb7+9]

the EMS diode ladder filter is an interesting exercise in NFB design ...

make sure to match the devices in the un-degenerated pair and observe the proper form of NFB in the resonant path
the DC NFB is required, it helps to the center the pair bias-wise ...
(the original EMS circuit is basically a low gain op-amp with LPF in the loop)
otherwise, with AC coupling, any serious offset between the devices is gonna slam it to one side and
cause the circuit to spit out weird noises

my EMS-adaptation circuit can easily be adapted again to +9 only operation ... but two batteries is simply less work

www.lynx.net/~jc/emsfilter.gif

I once combined this circuit with a modified S/H circuit running off a triangle LFO instead of noise source
this yielded interesting shifting filter sweeps
unfortunately, it wasn't really the kind of filter response I was looking for in a S/H filter ...