OTA based VCF questions

[Jules Ryckebusch]

> 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

LM13600 good down to the 10's of nanoamp range. I used it in the 
FatMan as a low pass VCF my starting point was the data sheet VCF 
with some Simonton exponential conversion etc. Go For It!!

> 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? current mirrors are bad assed and can easily work with
4-6 "mirrors" coming off the first transistor

> 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 either capacitor couple the audio through or not worry about 
it. I like the "Thump" from a litle CV carry through on some filters.

> 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, and
> should we make some sort of protection (a limiter or something)?

Set your current mirror or source correctly and you won't have a 
problem. the chip will not be damaged if you go higher than 2ma, you 
just don't get any more control range (or not much useful range)

Good Luck!!

Jules