[sdiy] SEM filter questions

[Neil Johnson]

Hi David,

> A buddy has asked me to build an Oberheim SEM-inspired filter.

I guess if its just for personal use this would be OK, but for
commercial use I'd be a little careful now that Tom is back in the
game:

http://www.tomoberheim.com/

> I've been
> looking at a schematic I found on the web for something called "SEM-1A"
> which looks to me like a garden-variety SVF built around CA3080 OTAs and
> caps with 2N4302 JFET buffers.

Yep.

> Is this "the" SEM filter?  I was under the impression that it was based on a
> CEM chip.

Nah.  While Oberheim did use some CEM and/or SSM chips for things like
ADSRs and so on the filters were 2-pole SVFs based on CA3080s.

> Also, is there something special about this particular SVF which makes it
> sound unique?

Possibly.  Structurally its a standard 2-pole state variable.  Other
than the CA3080 OTAs, you've got 741s as output buffers, and an LM301
with 5pF compensation capacitor as the input mixer.   All the BJT
goodness is likely to add *some* colouration to the sound.  How much
though?

> I notice that the resonance circuit has a kind of interesting
> looking resistor network and a couple of anti-parallel diodes.

That's nothing unusual.  The ASM-1 VCF has the same (might even be
inspired by it).  I have a similar scheme in my 3-pole filter.  Many
other SVF schematics I've seen have something similar as well.  Its a
good place to put amplitude-limiting diodes, well away from the
high-impedance integrator nodes.

> Do the JFET buffers impart any special qualities to the filter?

Unlikely, but possible.  What does your Multisim say?  I'd think the
bipolar op-amps would have more of an influence.

Cheers,
Neil
-- 
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