We study the transmission properties of a nonlinear amplifying loop mirror that acts as an adiabatic amplifier of solitons. The adiabatic nature of the operation is quantified by evaluation of the energy and the Hamiltonian of dispersive-wave components contained in the output pulse. Near-adiabatic amplification is possible, and a chirp-free solition can be generated with an amplification ratio (compression ratio) of approximately 2 by a proper design of the device.