tapped 4pole filter

[WeAreAs1 at aol.com]

Hello Harry,

The Oberheim OBxa and OBsx both had 2-pole filters built around the 3320.
They used a rather unusual method of connecting the resonance loop.  The OBxa
actually had two 3320's per voice - one for 2-pole and one for 4-pole response
(I guess they were cheap at the time), and a single analog swith to select
between their outputs.  The OBsx just had 2-pole filters (maybe they were
trying to make sure it sounded worse than the OBxa to justify the xa's higher
cost??).  The sx filters are exactly like the 2-pole filters from the xa.

Interestingly, the extra diodes, resistors, and extra trimpot that they put in
their 2-pole feedback loop look a lot like what they had in the feedback loop
of their old SEM state-variable filter (built around two 3080's).  I don't
quite get it, but I'm sure some of the smarter list members would.

The OB-8 also had 2-pole/4-pole switching, but accomplished it with a
byzantine maze of analog switches built around its 3320 (seven separate
switching elements!), which basically allowed them to reconfigure it into
their OBxa 2-pole mode.  I don't have the drawings near me at this time, but
I'll bring them home later tonight and send you a diagram of their OBxa 2-pole
filter.  I don't remember whether the two-pole mode allowed the filter to go
all the way into oscillation, but I do remember that it didn't sound so great
to me, except on string patches.

Michael Bacich