FiiO BTR7: power output and what it will drive

The FiiO BTR7 is a Bluetooth receiver at $200. It delivers 320 mW into 32Ω and 50 mW into 300Ω, which ranks it 34 of 53 for raw power among the sources in this database.

Measured at 100 dB SPL it has comfortable headroom on 304 of the 310 dynamic and planar headphones catalogued here and enough but not generous on 6. Balanced 4.4 mm, screen, wireless charging.

Ordered from easiest to hardest to drive. Headroom is measured at 100 dB SPL, which is louder than almost anyone listens — a small negative number means the source still plays loud, just without reserve for peaks.

These figures come from each manufacturer's own sensitivity rating. A handful of planars — the HiFiMAN Susvara and HE-6se and the Abyss AB-1266 in particular — are widely reported to need far more than their published sensitivity implies, so treat a bare pass on those as optimistic rather than settled.

At 3.5 µV of output noise, here is where that noise lands in the five most revealing in-ears in the database.

This one hisses. With the most sensitive in-ears in the database its own noise lands around 25 dB SPL, which you will hear between tracks and in quiet passages.

At 0.5Ω output impedance this source satisfies the one-eighth rule with every headphone in the database, down to the 8Ω in-ears. Nothing here will have its frequency response shaped by the source.

The rule is a rule of thumb, not physics: a source impedance below one eighth of the headphone's keeps the interaction under about 1 dB. The checker works it out for any pair.

Or put any two side by side in the source comparison tool, or run your own headphones through the amplifier matcher.