lm13600/700/ne5517-SVF-Problem

[Hairy Harry]

The old ARIES 317 SVF has an "unusual feedback" arrangement that
makes the bandpass output degenerate to sort of a 6db/oct LPF at low
Q settings. This was to get rid of some offset error problems... It uses 
CA3080 also.

Hmmm... maybe I can find the description in Dennis Colin's own
words.

So we are not the first.

BTW... how about some slew-rate limit for the CV in ???

H^) harry


>From: Magnus Danielson <cfmd at swipnet.se>
>To: modlar at gmx.net
>CC: synth-diy at node12b53.a2000.nl
>Subject: Re: lm13600/700/ne5517-SVF-Problem
>Date: Mon, 07 Aug 2000 22:27:47 +0200
>
>From: modlar at gmx.net
>Subject: lm13600/700/ne5517-SVF-Problem
>Date: Mon, 7 Aug 2000 14:40:34 +0200 (MEST)
>
> > Hi all !
> >
> > I have a general understanding problem with a special state var filter.
> > It is based on a LM13600 and looks very like the tomg vcf9 (Serge?).
> > The problem is, that the offset voltage of the lowpass output
> > (which should be zero) shows changes of 8 to 10 Volts if there is a
> > sudden change of the control voltage. After this sudden offset change
> > the output offset slowly decreases to the normal zero value (2-3 secs).
> > Principally, the filter works (HP,LP,BP, with and without resonance)
> > but for musical purpose, these offset changes are inacceptable. Imagine
> > a AR-generator (feeding voltage control of the filter ) with Attack time
> > set to 0 sec ! The output will change to 8 Volt, added to the filtered
> > sound -> overflow !
> >
> > I tried to replace the LM13600 buffers with OPAmp-buffers (071), I
> > changed the LM13600 to the 13700 and the NE5517, no changes.
> >
> > To my knowledge, this isn't a principle behaviour of a state var filter,
> > (my formant filter, who works with the 3080, does not show this
> > at all).
>
>Strangely enougth I recognice this behaviour from my Formant's 24 dB 
>filter!
>
> > Is it the LM ?
> > Any help ? (Perhaps this is a simple beginner question)
>
>Well, just to get the discussion up and kicking, I am just pondering about 
>the
>DC offset errors. In the ASM-1 VCF (a state-variable 2-pole filter) there 
>are
>trimmers at both the CA3080s for doing DC offset adjustment. One must 
>recall
>that directly following the OTAs in a state-variable design is the 
>integrators
>and I would not be supprised that the quick changes is so quick that the
>feedback loop is unable to compensate for the offset error while the change
>occurs, then we see a buildup and then it takes some time before the 
>feedback
>loop to compensate this out. Interesting experiments would be to see how 
>the
>speed sensitivity and dropout speed changes with diffrent frequency range 
>and
>Q settings.
>
>This is just me doing a quick pondering about the problem, but hopefully it
>will be enougth for some others to also ponder over it and we can have a
>discussion and possibly learn about it.
>
>Cheers,
>Magnus
>

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