[sdiy] Another CV breakthrough question

[harry]

HEY now I remember... I had this problem on a Moog Ladder....
it did NOT have matched trannies...

I added an LED across the large (220uF) bias AC "ground" capacitor... so that
the voltage at that point was stabilized.  It worked for me. Maybe I've
screwed
up something sublte... but it sounds much better than thump...

H^) harry

Magnus Danielson wrote:

> From: "Tony Allgood" <oakley at techrepairs.freeserve.co.uk>
> Subject: [sdiy] Another CV breakthrough question
> Date: Sat, 19 May 2001 10:05:17 +0100
>
> > Hi all,
>
> Hi Tony,
>
> > I have a SH-2000, and the filter used is a five pole diode ladder
> > similar to that in the Sys-101/102.
>
> 5-pole? Oh, I've never heard of something like it... I'd love to have
> those schematics... sounds reasnoble thought, given the known
> properties of the 4-pole diode ladder...
>
> > 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.
>
> For what I can see, it seems feasable that you should do the matching
> that way. You could also consider the trick of interchaning the diodes
> in a pair to see if you can make each diode-rail better matched to the
> other. This can help you in perfecting the balance.
>
> Good luck with removing that CV bleedthrough!
>
> Cheers,
> Magnus