[sdiy] Filters

[Linium]

On Vendredi 2 Janvier 2004 14:50, jbv wrote:
> > >Is there a (easy) way to modify this VC-BPF design :
> > >http://www.cgs.synth.net/modules/cgs35_syntha_vcf.html
> > >to add VC-Q to it ?
> >
> > Replace the Q pot with a LDR+LED (and 1k resistor parallel to the LDR) ?
>
> Mmh... I guess there my be a slighly more hi-tech, OTA-based
> solution...

I have this filter and i am wishing the same feature. Unfortunatly i don't 
know much about OTAs, still deciphering the tuts at 
http://www.uni-bonn.de/~uzs159/index.html 

It would be interesting to join our interest to find a solution.

What would be the principle of the operations ? I see that there are two 1K 
resistors on the feedback loop the SteinerParker, the second of them is the 
variable resistor that set the resonnance.

The goal is to use an OTA to replace this 1k pot, will a CA3080 do the job ?

How it will be used ? Can we do something as simple as connecting the path of 
the filter to Iabc of the OTA with the current ouput going back in the 
circuit ? 
There are 2 differential inputs on the OTA. I guess that in our case, one (-) 
will be set to 0V, the other would be the CV input (?).

Could we connect the filter straigth to Iabc, or does it need a resistor to 
stay in the limit of the OTA ? (how much bias can it get).

What would be the voltage required at V+ to get a gain ranging from 1 to 
something equal to 1K if it were a resistor ?

Comments appreciated... 

Linium