there are 2 trimmers on the pcb, for zeroing out the bleed on each vca. you can get all scientific and measure the db. but I just do it by ear. insert a harmonically rich waveform like a saw or square into the audio input with the panel control/knob at full ccw(counter clockwise)and plug the out to whatever mixer/amp chain you work with(maybe turn your mixer/speakers up louder than usual) and then turn the trim on the pcb to the left/ccw until the vca is silent. If your vca is silent from the start then good, and move on and repeat for each channel.