[sdiy] SEM filter questions

[Harry Bissell]

All you would need to do is get an old SEM module and trace the schematic yourself.

If there is patent protection, its expired... and copyright only covers the actual factory schematic
and the circuit board.

Of course if you say the word "Oberheim" you are (probably) infinging a registered trademark and you could be
in a lot of trouble. Likewise you might not be able to say "SEM" clone...

You could say "Inspired by the legendary "synthesizer expansion module" of the early 70's that formed the heart of some of the
first and most respected polyphonic synthesizer systems"...

BTW I'm not super-impressed with the SEM filter overall, its good but not special IMHO. I don't think the FETs add
much 'character'.  The most obvious thing that could affect the sound is the resonance limiting circuit.

The state variable filter is pretty much fair-game imho

H^) harry

----- Original Message -----
From: Neil Johnson <neil.johnson97 at ntlworld.com>
To: David G Dixon <dixon at mail.ubc.ca>
Cc: synth-diy at dropmix.xs4all.nl
Sent: Thu, 19 Apr 2012 10:02:19 -0400 (EDT)
Subject: Re: [sdiy] SEM filter questions

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
-- 
Modules and more: http://www.cesyg.com
Homepage: http://www.njohnson.co.uk
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-- 
Harry Bissell & Nora Abdullah 4eva