Equalizer APO
Equalizer APO is a free and open-source parametric/graphic equalizer for Windows, built as an Audio Processing Object (APO) that operates system-wide rather than per-application. It supports an unlimited number of filters, multichannel audio, low-latency real-time processing, minimal CPU usage, and VST plugin integration. Originally created by Jonas Thedering, it is widely used for correcting speaker/headphone frequency response and general audio tuning.