Re(2): [sdiy] Another CV breakthrough question

[Tony Allgood]

>Big surprise: A *transistor* ladder is shown, and it has almost the
same crappy single opamp differential amp as the Formant Pro. No wonder
when *this* version cannot be balanced with
matched transistors ...

Yes, thats right, but that horrible little diff amp was superceded in my
SH-2000 by me adding three op-amp diff amp instead. I used the original
one as the final stage with a gain of one, and added a TL072 with the
diff gain of... er, can't remember, but its the same as the single
op-amp in the original. Its on a little bit of Vero and fits very nicely
on top of the board. No track cuts either since it takes the inputs and
outputs from the removed tant caps' pads. Those of you with a SH3a will
benefit from this too. You'll get much better noise specs. But it did
nothing to the CV rejection of the diode ladder

And there were two versions of the filter board. I have schems with both
filter boards, but the only real difference is that one uses the moog
ladder and the other uses the diode/transistor ladder. The diode ladder
filter is the same as the Sys-101/102, but the diff amp is the same
horrible one opamp thing. They must have learnt about better diff amps
by the time the much better System-100 came out. The Sys-101 uses the
same discrete diff amp as the TB303 more or less.

Now the Sys-101 is one of my favourite synths, so I'm keeping the diode
ladder filter in my SH-2000. However, I think I am going to match the
transistors better in the ladder to get rid of the that CV breakthrough.

Regards,

Tony Allgood  Penrith, Cumbria, England

Oakley Modular Synth and TB3030:
www.techrepairs.freeserve.co.uk/projects.htm
My music: www.mp3.com/taklamakan