[sdiy] LM13700/LM13600-based filters

[Tom Wiltshire]

Hi All,

I've been looking into filters based around the LM13700  
transconductance op-amp. Not the world's greatest chip, but has the  
enormous advantages of being both cheap and available.

One thing that jumps out at me is that very few designs use the  
darlington buffers, mostly preferring to use a standard op-amp buffer  
instead (some examples below)

Can anyone tell me what the problem is with the on-chip buffers? Why  
not use them if they're there? Are they *that* awful?!

Thanks,
Tom

Scott Bernardi's:
http://home.comcast.net/~sbernardi/elec/og2/4pmultimode.pdf

MFOS 24dB LPF:
http://www.musicfromouterspace.com/analogsynth/VCFJAN2006/ 
VCFJAN2006.html

MFOS 12dB SVF:
http://www.musicfromouterspace.com/analogsynth/STATEVARVCFFEB2006/ 
STATEVARVCFFEB2006.html