[sdiy] 2164 SVF sound/noise?

[David G. Dixon]

My additions to Tom's comments below:

> Others can tell you what they think, but here's my view after I have
> played with these chips a fair bit.
> 
> > 1. Do they avoid the HISSY nature of OTA SVF filters? (the self noise of
> an 13700 filter can be quite noticeable with no signal)
> 
> The SSM2164 is not as noisy as the LM13700. This isn't a carefully-
> measured fact (for that we have datasheets - yeah, right!) but just my
> perception. I've built a filter module with three of these filters in
> parallel and even that isn't noisy.

I concur.

> > 2. do they SOUND different? or can you swear you are working with an OTA
> SVF?
> 
> The SSM2164 sounds round and fruity. So yes, I'd say it stacks up well
> against other OTAs. Way better than the CA3080, which is noisy as hell and
> distorts like mad.

My 2164 4P LPF sounds every bit as good as my 13700 4P LPF, if not better.

> > 3. how is the resonance
> 
> Depends on the configuration. In the SVF, there are the much-discussed
> problems of getting it even across the audio range. Aside from this, it
> *sounds* good, even when it isn't even!
> I know some other people have built 4-pole LPFs with this chip, so maybe
> they can comment on those. I haven't tried that yet.

2164 and 4P cascade filters were made for each other!

> > maybe these questions are too general but it sure seems like these chips
> are all the rage
> 
> They're cheap, they're in current production, and they're widely
> available. What's not to like?!

...just don't ever reverse the power pins or power up one pin without
powering up the other.  They are not forgivable like opamps.

> One thing to note though; the CV response is exponential. This is an audio
> VCA, not an OTA. This is good for filter cutoff, and good for the
> resonance amp on an SVF, but not good for the resonance amp on a standard
> LPF, where a linear response (or even reverse-expo) would be more
> appropriate.

...however, there is a relatively easy way to get a reverse exponential
response from 2164 when using it as a resonance amp in an LPF.  I've done it
on my 4P LPF and it works a treat.  Email me privately for details.

> If you've got some in the drawer, get them out and play with 'em. They're
> great. I'll send you an SVF diagram when I'm done fiddling with it.

I can send full-blown schematics for 2164 4P LFP (built and tested), 2164 2P
SVF (simulated) and 2164 "Korgasmatron" 1P HPF/2P LPF (simulated) to anyone
interested, along with a document explaining the subtle differences between
OTAs + buffers and VCAs + integrators in filter applications, and my
approach to emulating the MS-20, among others.  Please respond privately.