Early models of the Korg MS-20 had main HP and LP filters implemented using custom "Korg 35" filter ICs. In later models these ICs were replaced with a daughterboard containing OTA-based filters. Small changes were made to the main circuit board to accomodate the daughterboard. Here are the schematics for that daughterboard, the KLM-307, and for the accompanying main circuit board, the KLM-127. randy@www.zombie.com