The Bi-Amp mixer is located on the left side of your teacher workstation. This module will probably be the top module in this rack. Channel 1 controls the amount of audio output. If this is turned to high, other sites might get an echo.
Dial Settings: This part of your audio may have a plastic cover on it. The only channel settings should be 1 at 9 o'clock and the master at about noon. If it is causing an echo, you turn the number 1 dial couterclockwise.