OB-8 goes multimode ... maybe

Haible_Juergen#Tel2743 HJ2743 at denbgm3xm.scnn1.msmgate.m30x.nbg.scn.de
Wed Feb 7 02:28:00 CET 1996


At the moment, I am thinking of a new modification for my OB-8.
This is *NOT* done yet, but it should be possible, and I'd like to
discuss it here.

It all started with two things ...
(1) The 2-pole-filter of the OB-8 is a multimode filter; but unlike the
       old n-voice SEM modules, the bp, hp and notch outputs are not
(2) I found that in 2 years I have *never* seriously used the Noise
      in the signal path of the OB-8. Maybe that's due to the fact that
      it's just an on/off switch, and maybe it's because other synths are
      more effective for creating noise-based sounds - don't know.
Now it seems I have an unused switch on my Obie, and it's located
next to the 2p/4p switch on the frontpanel ... and it is a (storable) 
that I can find on the voice boards, readily demultiplexed ... what else
could I wish!
I think with the 4 combinations of the 2p/4p and Noise switches I could
easily control 4 filter modes instead of the original two ones ...
I could even use standard CD4052 2p4t multiplexers to switch between
the modes, as the filter outputs are in the 0 ... 15V range ...

What I am planning (if I will find the time) is this:
* Cutting out the CV trace to the cmos switch for the noise on each
   voice board
* Adding a 4052 (either for each voice or one for two voices) *after*
  the internal switch that selects 2pole or 4pole response.
* the 4052 should default to the original switch, i.e. with Noise = off
   everything should work as before.
*  one of the 4052's inputs would be connected to another output
   of the internal multimode filter (hp, preferably). The bit that changes
   between the default and the high pass output could be derived directly
   from the internal Noise CV. So with only 4 cheap chips and some
   rewiring I would have a highpass filter option !!
* There is still one one combination unused so far. Say if
   Noise = off  and 4pole = on     =>    4pole Low pass,
   Noise = off and  4pole = off     =>    2pole Low pass,
   Noise = on and  4pole = off     =>    2pole High pass (a la SEM),
   I still have the Noise = on and  4pole = on combination.
   I would *love* to use this combination for a 2pole Notch filter
   (or maybe band pass; but I prefer notch!) - but alas in this combination
   the filter engine isn't in the SVF configuration anymore (would
   be far more easy on an OB-Xa, btw).
   Ok, I could do some logic combination and cut the original 4p CV lines
   as well, but that would be too much work. After all, the concept of just
   adding 4 components to the whole synth to get 8 high pass filters is
   really tempting! Maybe I will try and find what I can get out of the 
   configuration ... 6dB filter with resonance, or 18dB filter come to mind.
Any other ideas / comments?
As I said, it's just an idea so far - so any comments to prevent me from
something stupid could reach me in time (;->)


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