The alternating uniform magnetic field is useful for the biomagnetic study with magnetic nanoparticles. However, for the coil system driven by the magnetic field generator with a conventional low-voltage power source, it is challenging to achieve the desired field strength with a large coil due to the large impedance at higher frequencies. This article proposed a Helmholtz coil system that can generate the dc and ac magnetic field up to 1.3 kHz with low distortion. In order to overcome the inductive load, a computer-controlled parallel capacitor array was designed and built to tune the series coil-capacitor circuit to the resonant frequency. Experimental results show that the coil system has a bandwidth over 1 kHz with a 1% uniform region covering the 10 × 10 mm sample area. The proposed system is promising for the application in biomagnetic assay using the tunnel magnetoresistance sensors.