LM 13600 based filters

Kimmo Koli kimmo at clara.hut.fi
Tue Nov 7 08:24:40 CET 1995

I'm designing a synth of my own and I want some comments on different
VCF topologies and their sound.

I'm thinking of using LM 13600 dual transconductance amplifier in the 
filters and as you know there are two way to use this chip: with or 
without the linearizing diodes. 

When using the diodes similar nonlinearity cancellation as in the Moog 
ladder filter could be achieved (actually better nonlinearity 
cancellation due to accuracy of the matching on the same chip). Of cource 
when clipping occurs in this topology it is very hard clipping. At least
the Paia Fatman uses this kind of filters.

Without the diodes signal gets distorted earlier but still the distortion 
does not increase very rapidly with the signal (soft clipping). As the 
main difference between digital and analog filters is the distortion the 
soft clipping may not be such an bad idea after all. I think the newer Korg 
MS-20 models used this filter topology.

So has anyone compared the filter sound of these two synths or any other 
synths using these chips? Any comments on the sound compared to the 
Curtis and SSM chips. The problem in comparison may be that the 
LM13600-based filters tend to be only 12 dB/oct and the Curtis (and 
propably SSM) 24 dB/oct. Other problem: a bad VCA desing may destroy the 
sound of an otherwise good filter (at least I fear that with the MS-20).

Note: I'm only interested in the sound of these filters. I'm not 
interested in thermal stability because that I can easily handle: 
I'm autocalibrating on the fly .
  Kimmo Koli      			     Email: kimmo at clara.hut.fi 
  Helsinki University of Technology          URL:   http://clara.hut.fi/~kimmo
  Electronic Circuit Design Laboratory
  Otakaari 5 A 
  FIN-02150 Espoo     			     Tel:  +358 0 451 2273
  Finland      				     Fax:  +358 0 451 2269

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