[sdiy] Filters

[Linium]

On Samedi 3 Janvier 2004 02:21, harrybissell wrote:

> > Comments appreciated...
>
> I STRONGLY advise building a sucessful, stand alone VCA using the OTA to
> get the hang
> of it first... before you try and hook into the filter. Check out Joergen
> Bergfors site... he has
> a bunch of VCA circuits so you can see what would be involved.
>
> H^) harry

Thanks for your informative reply, it seems that i had a wrong idea wanting to 
feed the loop into Iabc  ! So the loop should go into V+ instead after proper 
levelling. I thought that the loop was a current rather than a voltage 
variation, because of the pot not mounted a divider. But it was a too quick 
reading of the schemo, well don't care i don't qualify to understand this 
filter yet.

So the CV-Q input is first converted as a variation of current and put in 
Iabc... As i look at the Tom Gamble schemos, he does the same kind of thing, 
the loop goes into V+ and CV-Q to Iabc of his LM13600... I guess he converts 
the CV into current with the dual trannies like on the VCF 1e for the 
resonnance CV.

Well i will try to understand more the filter and the indications you gave in 
your mail. I don't know if i will continue with the OTA on this filter since 
the photocell solution seems easy and i guess that its only drawback is a 
relative accuracy (let alone slowlyness?), but for CV-Q it should do ok. 

I intend to build a lot of VCA soon, one minimoog VCA (to test if it is really 
famous) from the Joergen Bergfors site and several LM13600 based n°2 of his 
comparatif which seems the best of the bunch (a better choice someone ?).

Linium