[sdiy] OB-X VCF mods for BP and HP outputs

[john mahoney]

(Previously posted to AH, since I had unknowingly been unsubscribed from
SDIY -- I thought this list was broken! Peter Grenader had mentioned this
"outage" on AH, so apparently I am not the only one who was mysteriously
unsubscribed.)


I'm soon getting a pair of Oberheim OB-X voice cards which I will be turning
into standalone, monophonic voice modules, along the lines of the SEM. The
filters in the OB-X and the SEM are very similar, with 2 stages (if that's
the right term) that each use a 3080 OTA, a 741 op amp, and some Rs & Cs.

The OB-X uses another 3080 to provide VC resonance, but we can ignore that
for purposes of this discussion.

The SEM filter provides LP, BP, HP and Notch outputs (an additional 741, pot
and switch are used for the Notch output). The OB-X only has LP. I can see
on the schematic where to tap in for BP and HP outputs, which would be great
to have! However, I am not sure if this is feasible because I don't know if
this feature of the SEM filter is provided by the biggest difference in the
2 circuits. And that is...

The SEM filter has a JFET (I think) after each 3080. Is that JFET providing
some kind of buffering that allows simultaneous use of the LP, BP and HP
outputs? Or, can I ignore the OB-X filter's lack of those transistors?

Thanks for any & all advice.

The SEM schematic is at the Synthfool site (thanks, Kevin):
    http://www.synthfool.com/schematics/sem.gif
The OB-X schematics are in the Service Manual (thanks, Cary):
    http://www.retrosynth.com/docs/obx/index.html

--
john