“tuned” the LPF

Yesterday I tuned the LPF using my trusty old board o’ potentiometers…

Best sounding values came out like this:

which I, after some thinking, recognized as the original values like in the JUNO6! Very nice! Just had to replace the 100pF capacitor with a 240pF capacitor like in the original. Only thing missing is the “mystery resistor” on the + input of the OTA to ground. For now I just left this res out.

A fitting frequency range for the filter should be achieved by a good control current. According to the IR3109 specifications, transconductance should range from 1µƱ – 10mƱ. The expo converter from the previous post delivers a current ranging from 70nA to 1.65mA which translate to a transconductance of 1.34µƱ to 31mƱ. Ok for now.

I made a mockup of the JUNO6 board and the IR3109 ic: