SMSL SP400: power output and what it will drive

The SMSL SP400 is a desktop amplifier at $529. It delivers 3.0 W into 32Ω and 440 mW into 300Ω, which ranks it 21 of 280 for raw power among the sources in this database.

Measured at 100 dB SPL it has comfortable headroom on 567 of the 567 dynamic and planar headphones catalogued here. THX AAA topology with a proper split between balanced and single-ended ratings.

The shop listings these figures stand on quote 6.0 W into 16Ω, 3.0 W into 32Ω, 440 mW into 300Ω, 220 mW into 600Ω. 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: 6.0 W into 32Ω through the XLR4 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.

One caveat about the source. None of the addresses below is a SMSL document. The figures here stand on Audiophonics, a French retailer, which is a transcription of the factory table rather than the table itself. A transcription is usually faithful and may well be exact here — but it can carry an old revision, and nothing on this site would notice if it did. Find these numbers on SMSL's own site and send the link; this paragraph comes off.

Source: audiophonics.fr.

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 1.9 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 27 dB SPL, which you will hear between tracks and in quiet passages.

At 0Ω output impedance this source satisfies the one-eighth rule with every headphone in the database, down to the 3Ω 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.

Of the 280 sources here, 7 do everything the SMSL SP400 does as a headphone output: reach 100 dB on every headphone it reaches, stay inside the one-eighth rule wherever it does and stay under the audible-hiss threshold wherever it does. The cheapest of them is the Schiit Magni Unity at $119, against this one’s $529 — $410 less.

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 Bottlehead Crack (OTL kit) · vs Schiit Valhalla 3 · vs SMSL HO100 · vs SMSL HO200 · vs Topping A90. 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 SMSL source in the database, and how much of what this site says about them came from SMSL’s own documents.