We describe a system for the compensation of time-dependent stray magnetic fields using a dual channel scalar magnetometer based on nonlinear Faraday rotation in synchronously optically pumped Cs vapor. We detail the active control strategy, with an emphasis on the electronic circuitry, based on a simple phase-locked-loop integrated circuit. The performance and limits of the system developed are tested and discussed. The system was applied to significantly improve the detection of free induction decay signals from protons of remotely magnetized water precessing in an ultralow magnetic field.