The FiiO M15S features the same desktop/portable dual power supply modes found on the portable M17 desktop-grade player and the portable Q7 desktop-grade amp, all of which have been well-received by the audiophile community, which is why FiiO included technology on other products. With the implementation of the dual power supply modes onto the M15 and its more ergonomic design, the M15S is the perfect combination of portability and high performance. The M15S starts a new chapter in FiiO high-end players, sharing the same technologies as the portable desktop-grade M17 player and the portable desktop-grade amp Q7 that give you desktop-level performance in a portable package - including desktop mode, dual power supply modes, and five gain levels. When in the dedicated desktop mode and under USB fast charging, the M15S will work like a desktop device and functions completely without the battery. Under this mode the battery will remain the same, preventing premature battery degradation. The M15S is also equipped with an automatic "auxiliary DC power supply" so it can automatically recognize when to fast charge via USB. It also automatically switches to use the highest-voltage power supply, leading to an increase of 26.67% in power supply for even more power output. Even when not using the fast-charging adapter, the M15S can still automatically switch to a higher-voltage power supply that is enough to drive most headphones.