2-pole LM13700 lowpass filter working?
I’m not sure. Did a lot of prototyping and these are the results.
Since I couldn’t figure out a proper way to calculate the cutoff frequency when using different current and resistor combination, I decided to use the default lowpass filter circuit from the LM13700 datasheet.
Hooked my expo converter up to pin 1 (100nA < Iabc < 1.1mA) and started testing. The Vout of this pole is the Vin of the second pole. Unfortunately I only had 1 LM13700 working left, since I blew up all my other’s ;). I experimented a lot with different capacitor and resistor combination and the only ok sounding combination is a 1nF capacitor with 10K big and 220Ohm small resistors. Check the following sound files for the results:
Oh, just for fun: a pic of the breadboard in action
left bottom: BA6110 based VCA. Right top: dual 2N3906 based expo converter. Right bottom: Only remaining LM13700 configured as a 2-pole lowpass filter.