[sdiy] Another CV breakthrough question

[jh.]

> 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.


Hi Tony,

I never thought about it - but now that you mention it: yes, mismatch of any
diode drop voltage will cause this problem. It's basically two stacks of
diodes which give a certain level shift upwards, and if there is a mismatch
this will result in a differential signal as the ladder current changes.

JH.