Fosi Audio SK02: power output and what it will drive

The Fosi Audio SK02 is a desktop amplifier at $100. It delivers 190 mW into 32Ω and 20 mW into 300Ω, which ranks it 163 of 280 for raw power among the sources in this database.

Measured at 100 dB SPL it has comfortable headroom on 521 of the 567 dynamic and planar headphones catalogued here, enough but not generous on 38, only marginal on 5 and not enough power for 3. Power is published at 32Ω; the rest of the table is derived from those figures. Output impedance is not published. Output noise is not published, so the hiss estimate cannot be run.

The manufacturer publishes 190 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: 755 mW 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.

The noise figure above is the maker’s own, but it is not in the specification table. Fosi Audio prints it in the product description: “a low noise floor of only 3.7μV”. Nothing is calculated here — the microvolts are theirs. What the sentence does not say is which output it describes, or which gain setting, and the table beside it distinguishes both. It does agree with their own table. The same page prints a 116 dB signal-to-noise ratio for the 3.5 mm output at low gain, and 190 mW into 32 ohms for that output, which is 2.466 volts; 2.466 volts against a 116 dB ratio is 3.90 microvolts, half a decibel from the 3.7 in the sentence. That is the reading used here: the single-ended output at low gain.

Source: 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 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 3.7 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 32 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.

Of the 280 sources here, 5 do everything the Fosi Audio SK02 does as a headphone output: reach 100 dB on every headphone it reaches and stay under the audible-hiss threshold wherever it does. The cheapest of them is the FiiO KA11 at $29, against this one’s $100 — $71 less, and a USB dongle rather than a desktop amplifier.

The cheaper one also publishes an output impedance, which the Fosi Audio SK02 does not — so on the paperwork it is ahead, not merely level.

This counts a headphone output and nothing else. Inputs, converters, microphone preamps, balanced outputs, the volume control, the case and the warranty are all outside it, and any of them can be the reason to buy the dearer box. What the count does say is that the three numbers a maker can be held to do not separate them.

Worked out in advance against its closest rivals: vs JDS Labs Atom+ (2nd gen) · vs Schiit Magni Heresy · vs Schiit Magni+ · vs SMSL HO100 · vs Topping L30 II. Each of those pages lists the headphones that tell the two apart.

Or put any two side by side in the source comparison tool, or run your own headphones through the amplifier matcher. Or see every Fosi Audio source in the database, and how much of what this site says about them came from Fosi Audio’s own documents.