Bottlehead Crack (OTL kit): power output and what it will drive

The Bottlehead Crack (OTL kit) is a desktop amplifier at $375. It delivers 271 mW into 32Ω and 333 mW into 300Ω, which ranks it 140 of 280 for raw power among the sources in this database.

Measured at 100 dB SPL it has comfortable headroom on 551 of the 567 dynamic and planar headphones catalogued here, enough but not generous on 11, only marginal on 3 and not enough power for 2. A DIY kit, and an output-transformerless design with a famously high output impedance. That impedance is the whole story: it suits 250–600Ω dynamics and nothing else.

The manufacturer publishes 333 mW into 300Ω. Everything else in the table above is worked out from those figures, and this table says which is which.

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: bottlehead.com, bottlehead.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.

The manufacturer does not publish an output noise figure for this source, so the hiss estimate that runs on every other page here cannot be run on this one. Rather than substitute a plausible number, this page leaves the question open.

If you know the noise figure from a measurement you trust, the hiss calculator takes it directly and tells you where that noise lands in your own in-ears.

At 120Ω output impedance, the one-eighth rule says this source suits headphones of 960.0Ω and above. 567 of the 567 models here fall below that. That is most of the database, not a niche — 222 of them 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, 34 do everything the Bottlehead Crack (OTL kit) 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 $375 — $346 less, and a USB dongle rather than a desktop amplifier.

The cheaper one also publishes a noise figure, which the Bottlehead Crack (OTL kit) 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 Geshelli Archel 3S Pro · vs iFi iCAN Pro · vs Lake People G103-S Mk II · vs Little Dot MK III SE · vs Schiit Asgard 3 · vs Schiit Jotunheim 2 · vs Schiit Lyr+ · vs SMSL H200 · vs SMSL H300 · vs SMSL H400 MK2 · vs SMSL HO200 · vs SMSL SH-9 · vs SMSL SP400 · vs Soncoz SGA1 · vs Topping A30 Pro · vs Topping A90 · vs Topping A90 Discrete. 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 Bottlehead source in the database, and how much of what this site says about them came from Bottlehead’s own documents.