A graphene-based coherent terahertz emitter which is voltage-tunable, was discussed recently by Sergey A. Mikhailov [ Phys. Rev. B 87, 115405 (2013)]. It is proposed to employ a multilayer graphene/boron-nitride heterostructure, which is able to emit radiation in the range 0.1 to 30 THz with a power density of up to 0.5 W/cm² at room temperature.