CEM3310 in Oberheim OB-8

[jhaible]

Hi,

just a short reply, 'cause I'm in an extreme hurry and I don't know if I'm
near my computer the next days.

As far as I got it from fast reading, everything you describe in your mail
is right. It took me some time to figure this out, a few years ago. 
In a later revision, they even threw out the complete CEM3360, and 
replaced the VCA with a 3080. (One half of the 3360 was for filter
contour amount, th eother half was the VCA)
So later OB-8's would be closer to the OB-Xa in *that* point.

BTW, the fact that Oberheim got more precision out of the 3310
even with decreased reference voltages for peak and sustain than
with an external VCA that would always see the full ENV
amplitude at its inputs is very flattering for the Curtis Envelope
chip, isn't it ?

I'm always astonished when I think of the continuous development
and improovement of Oberheim synths from the beginning to the 
OB-8. IMO, the OB-8 is the real summit of Oberheim's synth
developments. After that, things went downhill. (Slightly with
the Xpander and M12, and faster with what came after that.)

JH.

----------
> Von: WeAreAs1 at aol.com
> An: synth-diy at mailhost.bpa.nl
> Betreff: CEM3310 in Oberheim OB-8
> Datum: Donnerstag, 4. März 1999 10:02
> 
> Hello list,
> 
> I was looking at the service manual for the Oberheim OB-8 and I noticed
> something that I had not noticed until now.  The OB-8 uses Curtis
CEM3310's
> for its envelope generators.  In early versions of the OB-8 (software
version
> A-3 or less), the output of each filter envelope (Ux11) is routed through
a
> CEM3360 VCA for filter envelope depth control.  This 3360 is controlled
by the
> signal named "VCFM".  In later versions (software A-4 or later), this
3360 is
> bypassed and the unattenuated envelope output is directly connected to
the
> filter CV summing bus.  Additionally, the filter envelope amount control
> signal "VCFM" is re-routed to PIN 3 of the each CEM3310.  These are the
only
> changes to the hardware.  The service manual states "The 3360 VCA is
bypassed
> and the 3310 in conjuction with the software modulates the amplitude of
the
> envelope.  A-4 or later software must be installed.  This improves
temperature
> stability and tracking of the filter envelope modulation."  These changes
are
> described in ECO405, dated 12/12/82.
> 
> After looking again at the CEM3310 datasheet, I find this to be a very
unusual
> way of connecting it.  In fact, I have never seen PIN 3 even connected on
any
> other application of the 3310.  The data sheet refers to PIN 3 as a "Peak
> Threshold Voltage Pin".  The internal circuit diagram shows this pin as a
> connection to the minus input of comparator.  The plus input of the
comparator
> is internally connected to PIN 2 (the Envelope output).  
> 
> Although the data sheet makes no specific mention of it, this OB-8
circuitry
> indicates that it is possible to set a peak envelope amplitude by
introducing
> a voltage into PIN 3.  The data sheet does mention that "a Sustain
control
> voltage greater than the Peak Threshold Voltage will result in a 'jump'
to the
> sustain level".  They show that a positive feedback loop (with an opamp
> buffer) from the Threshold pin to the Sustain CV pin will "prevent the
Sustain
> level from rising above the envelope peak", eliminating the aforemention
> undesirable 'jump'.
> 
> Interestingly, the OB-8 does not incorporate this feedback loop at all. 
I am
> guessing that the A-4 software version somehow compensates by scaling the
> Sustain CV so that it never goes above the "VCFM" CV that is now being
input
> at PIN 3.
> 
> Not having access to an OB-8, I was hoping that some list member who has
an
> OB-8 with software version A-4 or higher (maybe Juergen H.?) might take a
few
> moments to measure the voltages found at PIN 3 (the Peak Threshold) and
PIN 9
> (the Sustain CV) of the CEM3310 at various settings of the Filter Sustain
> Level and Filter Envelope Modulation Amount.  Is anyone game?
> 
> This would be useful information for anyone who wants to use the 3310 in
new
> designs.  (yes, some of us have a few of the chips..)
> 
> BTW, the CEM3310 data sheet that can be downloaded from Paul Schreiber's
site
> is not complete, and does not contain any information about using PIN 3. 
The
> complete 4-page data sheet can be found in the Prophet V and Pro-One, and
> Memorymoog service manuals.
> 
> Best regards,
> Michael Bacich
> 
>