FiiO M33 R2R: power output and what it will drive

The FiiO M33 R2R is a portable player at $730. It delivers 470 mW into 32Ω and 60 mW into 300Ω, which ranks it 113 of 280 for raw power among the sources in this database.

Measured at 100 dB SPL it has comfortable headroom on 559 of the 571 dynamic and planar headphones catalogued here, enough but not generous on 11 and not enough power for 1. Power is published at 16Ω, 32Ω, 300Ω; the rest of the table is derived from those figures.

The manufacturer publishes 540 mW into 16Ω, 470 mW into 32Ω, 60 mW into 300Ω. Everything else in the table above is worked out from those figures, and this table says which is which.

The balanced output is rated separately: 1.1 W into 32Ω through the 4.4mm jack.

Derivation holds output voltage steady between and beyond the published loads rather than output power, because that is what an amplifier actually does. The full method is written up here.

Source: fiio.com, fiio.com.

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 5.5 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 33 dB SPL, which you will hear between tracks and in quiet passages.

At 0.8 Ω output impedance, the one-eighth rule says this source suits headphones of 6.4Ω and above. 12 of the 571 models here fall below that. Most of them are multi-driver in-ears, and the mismatch tilts the frequency response audibly rather than subtly.

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. Or see every FiiO source in the database, and how much of what this site says about them came from FiiO’s own documents.