The collective charge density wave (CDW) conduction is modulated by a transverse single-particle current in a transistor-like device. Nonequilibrium conditions in this geometry lead to an exponential reduction of the depinning threshold, allowing the CDWs to slide for much lower bias fields. The results are in excellent agreement with a recently proposed dynamical model in which wrinkles in the CDW wavefronts are ironed by the transverse current. The experiment might have important implications for other driven periodic media, such as moving vortex lattices or striped phases in high-Tc superconductors.