The “Auto Cal” button is designed to automatically match the gains between the two channels, by raising or lowering the gain of the rear channel in the plug-in. For this to work, first the microphone’s “Cal” switch should be turned on. Then set the preamp front gain level appropriately, and then set the rear gain to approximately the same level as the front. Then press the Auto Cal button to match the gain to compensate for any remaining discrepancies.