What does the trimpot do in the CP-9 circuit? I'm looking at the Tone Pad version. Can that be replaced by a fixed resistor? Is there any reason to add a real pot and allow me to tweak it when playing?
Thanks.
It's there so that the output of the 13600 (pin 12) can be trimmed so that it's exactly at Vsupply/2 (i.e. there's zero voltage across the 150k resistor at the 13600's output). If the output isn't central then changes in the gain control current can modulate the 13600's output in a not-so-good way. You *may well* get away connecting the bottom of the resistor that connects to the trimmer's wiper to Vsupply/2 instead of fitting the trimmer, but if you already have the trimmer then imo you might as well put it in, set it to it's centre position and measure the OTA's output voltage as I described.
Thanks. I'll keep it in there.