[sdiy] problem with 13700 flter upper range

[Simon Brouwer]

Dan,

Smaller values of C4 and C5, will give higher cutoff. So if you change
them go for smaller not bigger. You could try 330 pF for example.

Also, the cutoff will be proportional with the current into pin 1 and pin
16 of IC4. In this circuit this current will ever reach less than 0.2mA
(assuming AR and LFO voltage will be at most +9V) while the LM13700 can
handle considerably more (absolute maximum value 2mA, but better not go
higher than 1mA).

So the other thing you could try is to halve the values of R28, R31 and R36.

Dan Snazelle schreef:
>
> OK
>
> i have gone through a bunch of filters on the breadboard this week and at
> first was leaning toward a simple sallen and key with a vactrol for CV
> control. however i didnt like having to have the cutoff knob go through
> the vactrol...so i moved toward lm13700 based filters.
>
> looking at this one in particular ( for low parts count)
>
> http://www.musicfromouterspace.com/analogsynth/SOUNDLABMINISYNTH/images/synthkit_vcf_vca_001.gif
>
> which is the soundlab filter.
>
> right off the bat though i ran into problems, the first being that it's
> highest cutoff seems to be lower than 12k. whatever i put into it (drums
> mainly) gets a ton of high end chopped off.
>
>
> I have reduced the input resistors, tried messing with the filter caps
> (using 820 pf polystyrene), etc. BUT i dont know enough about filters to
> figure out what might be wrong with my top frequency.
>
> I have tried with both CV ins AND the cutoff pot.
> I dont want to fry the LM13700 so i am afraid to mess with the resistors
> on the + and - rails of the cutoff pot too much. (i assume plugging the +
> end of the pot straight into +v instead of using the resistor will bring
> up the High end a little bit,  in face i tested it for a moment. but it
> sill wasnt as high as my dry signal.
>
> SO what can i do?? is it just a (feature) of this filter? should i try the
> wasp filter instead?
>
> or go back to my lowly sallen key?
>
> thanks guys!!!
>
> --------------------------------------------
> check out various dan music at:
>
> http://www.myspace.com/lossnyc
>
>
> http://www.soundclick.com/lossnyc.htm
>
>
> http://www.indie911.com/dan-snazelle
> (or for techno) http://www.myspace.com/snazelle
>
>
>
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-- 
Vriendelijke groet,

Simon Brouwer
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