AW: [sdiy] Roland System 100 - Circuit details.

[Czech Martin]

Ok, the transistors in the moog ladder are tied to
the voltage divider chain via base,  that way the internal
nodes (caps) will not float very much up and down
during sweeps. Diodes simply don't have a base
and will therefore fly up and down.

I think that is the reason that the Moog needs only 
top and bottom pair to be matched.
For better "common mode" suppression the complete
characteristic of the diode chains must match left
and right.

This could be the reason for your results.

m.c.

> -----Ursprüngliche Nachricht-----
> Von: Tony Allgood [mailto:oakley at techrepairs.freeserve.co.uk]
> Gesendet: Freitag, 7. September 2001 09:52
> An: Synth DIY
> Betreff: Re: [sdiy] Roland System 100 - Circuit details.
> 
> 
> >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.
> 
> 
>