OTA based VCF questions

[Paul Perry]

At 10:49 PM 28/10/98 +0100, you wrote:
>Hello list,
>we are trying to make an OTA based VCF and so far, the Electronics
>Workbench tests have proved to be succesful. However those were done with
>some idealisations, so we are in need of some practical guidance. 
>1) In the LM13600 data sheet, it is noted that Iabc|max = 2mA. What is the
>lower margin for that current, practically? Is it zero, or some value above
>that? 
...........near zero

>2) In our model we used the lowpass VCF cell given in the datasheet, and
>connected four in cascade. Given the lower margin and the upper margin of
>2mA, is it possible to map them to cutoff frequencies of say, around 50Hz
>for the lower margin, and say, around 17 KHz for the upper one? 
............yes

Would some
>sort of exp converter be needed to achieve that goal?
.....very much so

 If so, which one
>would be applicable? Where do we locate the schemo for it?
....search for tomg's site, or look in Horowitz&Hill 'The Art of Electronics"

>3) In order to drive four VCF cells in cascade, should we use only one
>transistor and divide it's collector current to all the cells, or use some
>sort of a current mirror? Pros, cons?
.......your milage may vary.....

>4) There was some feedthrough for input signals of amplitude of about
>1-10mV, however, when we tested with input signals of greater amplitude, it
>was not noticeable. Should we try to eliminate it? If so, how?
.......you can't eliminate it at the low current end

>5) We would like to make the filter with a knob and a VC input also, and we
>are thinking of a resistor voltage divider (I'm not sure if this is the
>right term) taken from the power supply, for the knob, and feeding it to an
>OpAmp sumer, together with the CV Input at the other input of the sumer
>(I'm not sure this is the right term either, maybe it'd be better to call
>it mixer ). What we would like is that the both of these inputs be able to
>span the entire range of the VCF independently and also when they are
>together. When those are pliugged in together, if their sum (or the
>eqivalent thereof) goes over 2mA, is the LM in danger of some sort,
.....IT WILL DIE AS SOON AS YOU EVEN ***THINK*** OF DOING THIS

 and
>should we make some sort of protection (a limiter or something)?
.....you have to feed it thru a current convertor, which you put an
output resistor on of 18k (say) as a limiter

paulperry melbourne australia