[sdiy] Roland System 100 - Circuit details.

[Tony Allgood]

>First, the VCF.  Looking at the schematic it's obvious why I like it.
It is a roland ladder (diode) filter design.

Yes, its a nice sounding filter... but with real limitations. Its
suffers from terrible and I mean the worst CV breakthrough I have ever
heard in a commercial filter. If you sweep it fast and deep, its truly
horrible. Its almost the same as the one on the SH2000 later editions,
and sounds very similar.

I spent quite some time on improving the performance of the filter in my
SH2000 and the improvement in the diff amps were worthwhile. The Sys-101
has a better diff amp anyway, so its not worth improving this. But the
CV breakthrough is still bad. I thought it may have something to do with
the voltage points of the ladder, but the Sys-101 is just the same with
different operation points.

I now have matched pairs all the way up the ladder, and that didn't make
a smidge of difference. I used a bunch 2SC3381 which according to the
data sheet are matched to worse case 9mV... which actually isn't that
good. But I am unsure of the mechanism that is going on here but it is a
DC imbalance between each side of the ladder.

The Moog ladder is inherently more forgiving in this respect, presumably
due to the setting of the fixed DC points as you go up the ladder.

I think the TB303 would suffer CV breakthrough if you sould sweep the
filter over a bigger range.