Abstract: A combustion system for gas turbine engines incorporates a feedback loop which provides positive damping for the control of unstable combustion, particularly buzz in reheat systems, but also at least some cases of combustion rumble in main combustion systems. The feedback loop includes a sensor, a control unit and an exciter. The control unit monitors the strength of the sensor signal, which senses a cyclically varying parameter of the unstable combustion process, and adaptively varies the amount by which the signal is phase-shifted and amplified in order to obtain a sensor signal of minimum strength, this being achieved by controlling the exciter in accordance with the phase-shifted amplified signal to change the boundary conditions of the combustion process and thereby eliminate the unstable combustion.