The amplifier (AMP) is a 2 stage Class A discrete design using 3 pairs of hand matched transistors. Envelope is usually applied to ST1/1. ST1/2 goes thru an attenuverter and can be used for envelope or modulation. ST2/1 goes to the 2nd stage, and is typically used for volume control. ST2/2 goes thru an attenuverter and is typically used for modulation.