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 53 of 88 for raw power among the sources in this database.

Measured at 100 dB SPL it has comfortable headroom on 306 of the 310 dynamic and planar headphones catalogued here and enough but not generous on 4. 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 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.

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 120.0 Ω output impedance, the one-eighth rule says this source suits headphones of 960.0Ω and above. 310 of the 310 models here fall below that, mostly multi-driver in-ears, where 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.

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