FiiO FX15: how much amplifier do you need?

The FiiO FX15 is 24Ω with a sensitivity of 103.0 dB/mW, which is 119.2 dB/V. To reach 100 dB SPL — genuinely loud, louder than most people listen — it needs 0.50 mW, or 0.11 V across the drivers.

That puts it in the easy category. A phone, a laptop headphone jack or the cheapest USB dongle will drive it to painful levels. Of the 280 sources in the database, 280 have comfortable headroom. The cheapest portable source that does the job is the Apple USB-C dongle (US) at $9.

Sensitivity here is in dB per milliwatt. Manufacturers often quote dB per volt instead and leave the unit off, which is the single most common reason online calculators give answers that are 10–15 dB wrong. The converter explains the difference.

FiiO publishes this directly as 103 dB/mW (@1kHz), per milliwatt, so nothing was converted.

FiiO prints the impedance as 24Ω@1kHz, not as a bare number. Everything on this page is calculated with the nominal 24Ω, and the rest of that line is quoted here rather than tidied away, because how a figure is qualified is part of the specification too.

Source: fiio.com. How this data was built.

Most people listen between 75 and 85 dB. The 100 dB row is the one worth planning around: it gives you headroom for dynamic peaks without designing your system for a volume that would damage your hearing in under fifteen minutes. The safe listening calculator puts numbers on that.

Every source in the database, judged at 100 dB SPL into 24Ω. “Headroom” is how much power is left over above what the headphone needs — 10 dB means ten times the required power, which is what you want for dynamic peaks.

Hiss. At 119.2 dB/V these are sensitive enough to reveal a source's own noise floor. The noisiest source in the database, the Schiit Vali 2+, would put its hiss at roughly 27 dB SPL in these headphones — clearly audible in quiet passages. The hiss calculator lets you check your own amplifier's noise figure.

Output impedance. At 24Ω these want a source output impedance below 3.00Ω to satisfy the one-eighth rule. 35 of the 280 sources here miss it, the worst being the Bottlehead Crack (OTL kit) at 120.0Ω. Above that threshold the source starts shaping the frequency response. Check yours here.

Or open the comparison tool to put any two models side by side, or browse all 578. Or see every FiiO headphone in the database, and how much of what this site says about them came from FiiO’s own documents.