An apparatus for converting mechanical vibration energy into electrical energy includes a converter that, when subjected to vibrations, generates an electrical potential difference, and an electrical circuit connected to terminals of the converter. The circuit includes a first arm and a second arm parallel to the first warm. The first arm has a first variable-capacitance capacitor system. The second arm includes an electrical load to be powered. A second variable-capacitance capacitor is series-connected with the electrical load. A control circuit is configured to modify respective capacitance values of the first and second capacitor systems so as to modify the converter's mechanical resonance frequency.