[sdiy] problem with 13700 flter upper range

[Tom Bugs]

Hey Dan,
If you're using that MFOS circuit (which I'm sure will be fine) then it 
is C4 and C5 which are the main filter caps. If you're getting too 'low' 
a freq. range that is likely because you increased the value from 560pF 
to 820pF -- go the other way and it should operate fine at higher freqs 
-- eg. try 470p or maybe 330p (or did you at least try the spec'd 560p?)
All best,
Tom

Dan Snazelle wrote:
> 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!!!
>
> --------------------------------------------
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>
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>
>
> http://www.soundclick.com/lossnyc.htm
>
>
> http://www.indie911.com/dan-snazelle
> (or for techno) http://www.myspace.com/snazelle
>
>
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