[sdiy] SEM filter questions

[David G Dixon]

> > 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 no intention of replicating this as a product.  In any case, there is
already an SEM clone out in eurorack (the Bubblesound SEM/20).

> > 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?

It all probably adds more noise than colour.

> > 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.

Hmm.  I put zeners in the feedback loop of the first (BP) integrator in my
2164 SVF schematic, along with a 2164-based VCQ circuit and small amount of
fixed feedback for oscillation stability, and the simulations worked very
well.  I'm curious to see how this method compares.

> > 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.

Yeah, I haven't done a simulation yet.  Tonight, perhaps.