Fosi Audio K7: power output and what it will drive

The Fosi Audio K7 is a desktop DAC/amp at $199. It delivers 600 mW into 32Ω and 64 mW into 300Ω, which ranks it 66 of 156 for raw power among the sources in this database.

Measured at 100 dB SPL it has comfortable headroom on 391 of the 396 dynamic and planar headphones catalogued here and enough but not generous on 5.

The manufacturer publishes 600 mW into 32Ω. 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: 2.1 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: fosiaudio.com, fosiaudio.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 2.5 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 22 dB SPL, which you will hear between tracks and in quiet passages.

The manufacturer does not publish an output impedance for this source. That is the one number that decides whether it leaves your headphones alone or acts as a tone control, so its absence matters more than it looks: without it, nothing here can tell you how this source will behave with a multi-driver in-ear.

If you find a measured figure you trust, the checker will work out the consequence for any headphone.

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