Korg Polyphonic Ensemble (was: Re: [sdiy] Secrets of Dan Brown's "The Buchla Code" Revealed

[JH.]

Romeo: not the Resonator. (This is a set of BP filters.)

Grant: the Vactrol-based Traveler is here:
http://home.debitel.net/user/jhaible/pe1000_2.gif

Caps are selected to match the tolerance of vactrols. (Yes, Hamamatsu.)

The VCA/Filter is here:
http://home.debitel.net/user/jhaible/pe1000_1.gif

That's in interesting circuit, in many respects.

One transistor acts as VC resistor for VCF (controlled via 1M from
envelope generator), and the other one works as VCA (controlled via
4M7 from the same ENV)
The capacitor for VCF (well, for a soft 6dB roloff which _might_
be similar to Buchla's in-between mode) is switched to GND via
a diode, when "Soft Expand" sees a positive voltage. And it's
left floating (almost ... 100k remaining) when "Soft Expand" is 0V.

That's a fucking brilliant circuit. Dynamic filter function and VCA built
from 2 transistors. (The 3rd transistor is just a summing amplifier
for several VCAs.)
And it foreshadows hwat would become a full-fledged resonant VCF
and VCA with a few more transistors in the PS-3100.

JH.



> I think you're referring to the PS3100 resonator:
> 
> http://machines.hyperreal.org/manufacturers/Korg/PS-synths/schematics/
> 
> 
> On May 20, 2006, at 1:12 PM, Grant Richter wrote:
> 
> > Hi Jurgen,
> >
> > Hey, I could be completely full of it.
> >
> > I recall seeing a center tapped Vactrol in a Sallen-Key setup with 
> > like 4700 pF and 1000 pF values used in some Korg design? There was no 
> > 220 to ground.
> >
> > The Vactrol was a Hamamatsu and not a Perkin-Elmer.
> >
> > Yes, they added gain to the feedback amp. Why Buchla never did is a 
> > real mystery?
> >
> > Do you have these published any place? I sorta think I saw them on 
> > your site at some time in the last 10 years.
> >
> > Korg is my favorite MI company. They have made so many good keyboards.
> >
> >
> > On May 19, 2006, at 7:28 PM, JH. wrote:
> >
> >>>> Also part of the filter for the Korg Polyphonic Ensemble keyboard 
> >>>> was
> >>>> lifted whole heartedly from the Buchla design (pretty sure the Korg
> >>>> drawings have latter dates).
> >>
> >> Which part?
> >>
> >> The traveler is just a S&K VCF (no VCA switching as Buchla), with 
> >> added
> >> variable resonance (unlike Buchla).
> >>
> >> And the individual voices, yes that's LPF and VCA combined, but look
> >> at this clever implementation, with simple BJT instead of expensive
> >> Vactrols !!
> >>
> >> JH.
> >
> >
> 
>