The FiiO FX25 is a seven-driver tribrid in-ear monitor for listeners who want electrostatic treble detail and a full, punchy low end from a single pair of earphones that will run happily from a phone, a portable player or a desktop amplifier. Each side combines a 10mm dynamic driver, two custom Knowles balanced armatures and four Sonion electrostatic drivers, the same electrostatic units used in FiiO's 13-driver flagship FX17, so it sits directly below the FX17 in FiiO's tribrid range. Hand-laminated carbon-fibre shells keep the weight to around 7g per earpiece, and the included cable swaps between 3.5mm, 4.4mm balanced and USB-C DAC plugs. It is supplied in a single finish, Dusk Brown.
Seven Drivers Per Side
The dynamic driver uses a 10mm lithium-magnesium alloy diaphragm with a density of only 1.6g/cm³, which FiiO states is lighter and more rigid than beryllium, paired with a flexible PU surround to reduce breakup and improve response speed. Two custom Knowles balanced armatures split the midrange: one handles the mid-high region and the transition into the electrostatic drivers, while the other has an acoustic vent in its back cavity that allows more air intake and greater diaphragm amplitude for a fuller mid-low range. The four Sonion EST65QB02 electrostatic drivers are carried over from the FX17 and extend the high-frequency response to 50kHz, giving the treble its airy, open character.
RLC Band-Stop Crossover
In place of the RC crossovers typically used in dynamic, armature and electrostatic hybrids, the FX25 uses an RLC second-order band-stop filter circuit. This filters out the resonance peaks caused by harmonic resonance in the drivers and housing, so the high-frequency curve transitions more naturally and the overall sound stays balanced rather than harsh over long listening sessions.
S.Turbo Acoustics and Carbon-Fibre Shells
The front cavity carries over FiiO's patented S.Turbo acoustic structure, a turbo-shaped sound-guide tube inspired by sports-car engine turbos that enhances low-frequency energy and impact while filtering redundant mid-high-frequency information from the dynamic driver. A precision resonant back cavity with a tiny vent balances the air pressure between the front and back of the dynamic driver, which further improves low-frequency extension and reduces masking of vocals by other frequency bands.
The housing is shaped from carbon fibre, a material more often found in flagship over-ear headphones. Multiple layers are bonded by hand before being shaped, which FiiO describes as a complex process with low yields and high costs. With a density of 1.58g/cm³ and high rigidity, carbon fibre delivers a shell that is both lightweight and resistant to drops, pressure and deformation. Because the carbon-fibre patterns occur randomly, each pair has its own appearance.
Cable and Interchangeable Plugs
The detachable cable uses 6N-grade monocrystalline copper of 99.99998% purity with silver plating for conductivity, and is given a deep cryogenic treatment at -192°C for more than 100 hours to release internal stress in the metal and reduce signal transmission loss. It connects to the earpieces with a standard 0.78mm 2-pin connector, so third-party upgrade cables can be fitted. Three twist-lock plugs are included: a 4.4mm balanced plug is pre-installed, with a 3.5mm single-ended plug and a USB-C DAC plug in the box. The USB-C DAC plug supports up to 32-bit/384kHz playback, 8-band parametric EQ through the FiiO Control app, and provides enough power for the FX25 to be driven directly from a phone or laptop.
In the Box
The FX25 is supplied with the cable and all three plugs, 22 pairs of ear tips covering three tuning styles (including FiiO HS20, HS18 and SpinFit CP360 tips, memory foam and double-flange tips), the NEST CUBE storage case in aluminium alloy and PU leather, a magnetic cable clip, a cleaning brush and a quick start guide. The FX25 is Hi-Res Audio certified.