Astell&Kern SR35: power output and what it will drive

The Astell&Kern SR35 is a portable player at $800. It delivers 300 mW into 32Ω and 32 mW into 300Ω, which ranks it 131 of 280 for raw power among the sources in this database.

Measured at 100 dB SPL it has comfortable headroom on 547 of the 567 dynamic and planar headphones catalogued here, enough but not generous on 16, only marginal on 2 and not enough power for 2. The way into A&K. Higher output impedance than most rivals here.

The manufacturer publishes a maximum output of 3.1 V but no power rating at any load. Every figure above is therefore a ceiling: the real output into a low impedance will be lower, possibly much lower.

The balanced output is rated separately: 1.1 W into 32Ω through the 4.4mm and 2.5mm 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 output noise shown above did not turn up in any manufacturer document during this check. It is carried over from the material this database was first built from, and is marked unverified rather than quietly presented as sourced.

Source: astellnkern.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 4 µV of output noise, here is where that noise lands in the five most revealing in-ears in the database.

That noise figure has not been traced to a document from the maker, so the levels below are as good as the number they start from.

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 1.1Ω output impedance, the one-eighth rule says this source suits headphones of 8.8Ω and above. 28 of the 567 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.

Of the 280 sources here, 61 do everything the Astell&Kern SR35 does as a headphone output: reach 100 dB on every headphone it reaches and stay inside the one-eighth rule wherever it does. The cheapest of them is the FiiO KA11 at $29, against this one’s $800 — $771 less, and a USB dongle rather than a portable player.

The cheaper one also publishes a noise figure, which the Astell&Kern SR35 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 Astell&Kern SP3000 · vs Cayin N3 Ultra · vs FiiO M15S · vs HiBy R6 Pro II · vs HiBy RS6 · vs iBasso DX260 · vs iBasso DX270. 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 Astell&Kern source in the database, and how much of what this site says about them came from Astell&Kern’s own documents.