[sdiy] Another CV breakthrough question

[Tony Allgood]

Hi all,

I have a SH-2000, and the filter used is a five pole diode ladder
similar to that in the Sys-101/102. Now I like the sound but it does
suffer from a few problems. Firstly it was noisy, so I changed the diff
amp to something better. The original used a single op-amp, like the
SH-3a, but I am now using the three op-amp version, by a little add on
board with a single TL072 and a few resistors. Much quieter and better
sound too. I guess the older design unbalanced the ladder due to its
differential input resistance.

The second problem is bad CV breakthrough. On fast envelopes, you get
the usual audible thump. You can trim this out, with the VCF balance
trimmer, but it only works over a limited range of cut-off frequency. So
it sounds to me that this is a matching problem. I bet Vbe was only
matched at one Ic. So I changed the original hand matched transistors in
the top and bottom of the ladder to integrated matched pairs to see.
Nope, it didn't make a bit of difference. So I am wondering that in a
'diode' ladder, do all the transistors/diodes need to be matched in
pairs? I guess in a Moog ladder you are setting the DC point as you go
up the ladder, but the diode ladder relies on a steady increase due to
Vbe only.

Regards,

Tony Allgood  Penrith, Cumbria, England

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