lm13600/700/ne5517-SVF-Problem

[modlar at gmx.net]

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).

Is it the LM ?
Any help ? (Perhaps this is a simple beginner question)

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