[sdiy] 2164 SVF sound/noise?

[David G. Dixon]

> I assume this is just a straight CA3080 type filter cell. What about the
> difference using the diodes in LM13700s or CA3280s and doing offset
> trimming. Is the noise still considerably more than the SSM2164.

I wouldn't know, as I've never done it.  However, I would imagine that noise
would be much less in the 13700 if the diodes were used, because one
wouldn't then have to use a 500:1 (or so) voltage reduction at the OTA
input.  This severe dividing-down of the input voltage must detract
enormously from the signal-to-noise ratio.  This is a non-issue with 2164.

However, for me one of the main benefits of using 2164 is that it avoids the
necessity of using an expo converter.  The main headache is that it must
feed a virtual ground, but this is actually not an issue at all, if you know
what to do (my little document goes into this in great detail, with many
examples of circuits with equivalent transfer functions using both OTAs +
buffers and VCAs + integrators).

> I haven't done any real work on looking at the SSM yet and I'm wondering
> how topologies dealing with the virtual ground outputs of the SSM will
> effect sound. I've found 4P LP and 2P SV topologies out there but nothing
> like the cells used for the SSM, CEM and OTA for HP and AP. Yes you can
> chain a couple of SV filters together for 4P HP, 2P BP and 2P notch
> filters but the sound will probably be different than the old, 4 cell
> topologies. Any comments, ideas or suggestions here?

The virtual ground is not an issue.  It simply represents a slightly
different route for current into the filter cap.