High efficiency and high power density are critical in megawatt-class mechanical-to-electrical energy conversion systems that operate within a limited speed range, as in wind- and gas-turbine-driven generators. Generator output is connected to an ac-to-dc conversion system for processing and controlling power flow to an electric grid. Conventional high-power ac-to-dc conversion architectures rely heavily on active rectifiers, which consist of fully-controlled power-electronic switches, making the system bulky, lossy, and less reliable. An alternative approach is proposed: integrating a multi-port permanent-magnet synchronous generator (PMSG) with series-stacked active and passive rectifiers. Majority of the power is processed by diode-based passive rectifiers. An active rectifier processes the remaining power while regulating the dc bus. The architecture allows a substantial increase in overall efficiency, power density, and reliability. The theoretical analysis is validated using an experimental setup. Non-idealities of the generator (inductance and resistance) affects the optimization of the number of PMSG ac ports. In the experimental setup, using an off-the-shelf generator, the conversion loss is reduced by 33%.