>
> I'm not that familiar with electronics really. but I would imagine that the
> trimpots control the CVs controlling the cutoff and resonance, and the way
> it's normally set up it probably adjusts it from whatever amount to ground,
> and the alternative way would adjust it from a positive voltage to ground,
> providing a more pronounced effect. So instead of only attentuating the
> control voltages it boosts them as well.
Here's some tech overview of what happens in there:
for resonance you have current input to the chip, hence 10k res and 10k
trimpot
fed by CV. Old mod used pot instead pot and all you got is lowering
maximum
resonance to some level (as 10k is left in).
Atom's mod take different approach, it attenuates incoming res CV and
feeds
old 10k res. So you get the same max resonance as before but now you can
fully
shut resonance, old mod took it to some level but here you can get 0
resoance.
My mod is somewhat different. I installed the switch to choose between
original
line (10k trimpot and 10k res) and new one (100k lin pot, 10k trimpot,
1k res),
that way I got higher resonance (set by trimpot and res, pot to 0), and
it goes
almost to 0 res (pot at the max). Pot is configured as variable
resistor.
Cutoff is bit different. I suspect it's OTA input as it's fed by again
current from +V, lower leg with res and cap is for stabilisation.
Atom's mod substitutes it for voltage divider and feeds current input
with voltage. In theory that would be a problem. Can't say more. I'd
toss in resistor inbetween pot wiper and chip input, 1k or so.
Haven't tried it. Now, if you look at the borad you'll see silkscreened
short link for bypassing trimpot to +V. Hmm, so maybe it really works.
You might even not need 470ohm res he suggested.
> I guess it would be possible to
> blow the filter chip but probably not likely. I don't know the specs of the
> chip so I don't know the maximum voltages but the mod doesn't seem too
> extreme. As long as it's not exceeding the power supply voltages I wouldn't
> see any reason to worry.
>
you can blow it even in normal voltages
> So the end result of it is the knobs have a higher range, and the filter can
> self-oscillate. BUT the problem with self oscillation on the Poly 800 is
> the way the voice architecture is set up. Since there's only one filter, it
> goes like DCO > DCA > VCF > VCA.
rather DCO1 > DCA >
DCO2 > DCA > VCMIXER > VCF > VCA
NOISE> VCA >
Chip's onboard VCA is used to control noise level. VC mixer sets
individual
levels of each DCO. DCA is performed in oscillator chip (BTW these are
real
DCOs, while those in Junos and JX's are DCOs with analog
core-integrator)
After the chip is only chorus circuit and left/right mixing opamps.
> The DCO and DCA are on the oscillator
> chip, and the VCF and VCA are on the filter chip. I'm just theorizing here
> really but that's what I think is happening. The final VCA seems to open
> instantly when you press a key but shuts off abruptly after the release
> cycle of the envelope is finished. So the end result of this is that if you
> really crank the resonance up, it will self-oscillate but it will continue
> to oscillate at full blast until the final VCA shuts off.
>
What really happens is that you don't have VCA after VCF, that's why you
get that effect.
> At least that's what was going on with mine, or what I thought was going on.
> I did the "normal" mod but mine still got to painful self-oscillation.
> One interesting note about it though is that if the oscillators and noise
> were off, it wouldn't do it, the noise had to be cut on 1 or something for
> it to work.
>
Strange, mine oscillates witout any input, no problem. And it's quite
nice
chip, I like selfocs resonance. Sounds much better than CEM VCF chip
IMO.
--
marjan
me : Marjan Urekar
e-mail:
urekar.m@...
s-diy :
http://surf.to/marjansystems
music :
http://go.to/forcemajeure