[sdiy] problem with 13700 flter upper range

[David G. Dixon]

Harry,

Forgive my ignorance, but when one applies a current to the I_abc input of
LM13700 using a voltage source and a resistor, what is the target voltage of
the input?  Is it virtual ground or the negative rail?  Looking at the
circuit diagram for the chip, assuming that Q1 is in full conduction (?), it
looks like it would be the negative rail.

David G. Dixon
Professor
Department of Materials Engineering
University of British Columbia
309-6350 Stores Road
Vancouver, B.C. V6T 1Z4
Canada
 
Tel 1-604-822-3679
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> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
> bounces at dropmix.xs4all.nl] On Behalf Of harrybissell at wowway.com
> Sent: Tuesday, January 27, 2009 9:12 AM
> To: simon.oo.o at xs4all.nl; synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] problem with 13700 flter upper range
> 
> You really want to drive the OTA Gm inputs with a current source rather
> than voltage. Try looking for the EFM VCF1 that was adapted from the
> LM13700 data sheet... that would do it.
> 
> Eventually you run out of supply voltage when you use the scheme in Ray's
> circuit. You could parallel inputs (drive the CV into two inputs)  to
> double the current.
> 
> H^) harry
> 
> 
> 
> On Tue, 27 Jan 2009 16:47:55 +0100 (CET), Simon Brouwer wrote
> > 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/sy
> nthkit_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
> > >
> > >
> > >
> > > _______________________________________________
> > > Synth-diy mailing list
> > > Synth-diy at dropmix.xs4all.nl
> > > http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
> > >
> > >
> >
> > --
> > Vriendelijke groet,
> >
> > Simon Brouwer
> > -*- nl.openoffice.org -*- http://www.opentaal.org -*-
> >
> > _______________________________________________
> > Synth-diy mailing list
> > Synth-diy at dropmix.xs4all.nl
> > http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
> 
> 
> Harry Bissell & Nora Abdullah 4eva
> 
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