[sdiy] Instability in an OTA-C LP cell

[Olivier Gillet]

Hi all,

Today I observed something strange with an OTA-C low-pass cell I was
playing with (half LM13700 + half TL072 as a buffer ; 220/10k at the
input ; 1nF ; powered by +/- 5V). For large inputs (2.5V pp, -> 55mV
pp at the OTA input), and high cutoff values (> 10kHz), the filter
"crashes" in a state where the output is either the positive or
negative clipping value of the op-amp (a bit less than 4V for the
TL072). There's no resonance - only one 1-pole cell.

I accidentally found something that appears to fix the problem without
impacting the sound - a 47k series resistor at the non-inverting input
of the op-amp (values below 20k only increase the cutoff frequency at
which the problem starts to occur):
http://dl.dropbox.com/u/612135/ota_buffer.png

Has anybody any idea about what's going on? Why would the fix work (or
appear to work)?

Thanks!
Olivier