Bottlehead Crack

Plenty for the headphones it is designed for, very little for anything else. With 300-ohm Sennheisers owners rarely turn the dial past nine or ten o'clock, and 600-ohm Beyerdynamics are comfortably driven; Amir at ASR found it had more usable level and bass with the HD 650 than his RME. Into low impedance it collapses: a 25-ohm planar was a 'total disaster' on the bench, reviewers call planars and low-impedance cans a waste of it, and one reviewer found the 600-ohm DT880 needed the pot well up and ran short of deep bass next to a transformer-coupled tube amp.

“almost infinite amount of power with warmer/nicer amount of bass”

Amir · Audio Science Review

“It was a good thing I did not start testing with Drop Ether CX as it was a total disaster: almost no output and what was there was severely distorted.”

Amir · Audio Science Review

“Speaking of power, the Bottlehead has oodles of it should your ears be amenable.”

Timothy Smith · 6moons

“The Crack really needs to be cranked to drive the DT880.”

WaveTheory · Head-Fi

Sources split, and the split tracks the individual build and the house wiring. Several reviewers and owners describe a dead-quiet amp with no hum even at full volume. ASR's bench unit had mains noise that grounding could not remove, one reviewer heard hum modulate with desk knocks (microphonics), and the owner threads are full of hum and buzz hunts — cured variously by a diode mod Bottlehead's own forum recommends, shielding, re-soldering and tube swaps.

Sources split on how much it colours the sound, and the measurements side with the larger camp. Most hear — happily or not — a warm, smooth, slightly 'wet' tube presentation with extra bass weight on the HD 650, less resolving than solid-state rivals at the price. A smaller camp calls it clean, detailed and close to neutral, and owners say the Speedball and certain tubes move it in that direction. Both camps are mostly describing it on HD 600/650-class headphones.

Minimal by design: one RCA input, one headphone jack, no gain switch, no preamp output. What owners value instead is the tinkering — it uses common, cheap tube types that are easy to roll, and there is a well-trodden upgrade path (the Speedball, film output caps, a better pot) that owners say audibly tightens the bass.

“easy tube rolling with common tube types”

WaveTheory · Head-Fi

“Only needing one power tube and one input tube, tube rolling is one of the most fun things to do with the Bottlehead Crack, and you can easily tune the sound to your liking.”

Lieven · Headfonia

“Not many features, only for high impedance cans”

MikeP1985 · Head-Fi

“The bass comes across tighter and noticeably more controlled.”

JAnonymous5150 · r/headphones (on the Speedball)

Workable but quirky. The stock volume pot has too much gain packed into its first quarter-turn and shows channel imbalance at low settings (measured by ASR; some owners replace it, others never notice). All connectors, including the power cord, enter from the top plate, the plate simply rests on an open-bottomed wooden box, and the finish around the headphone jack wears with use.

“The volume pot is probably one of the few weak parts.”

Timothy Smith · 6moons

“I liked the feel of the volume control although as you will see later there is significant channel imbalance.”

Amir · Audio Science Review

“That can be unhandy when you don’t have a lot of space or when your interconnects and power cable are rather unflexible.”

Lieven · Headfonia

“I haven't noticed any channel imbalance in mine except for at the very, very bottom of the volume range - lower that I would ever listen - which I understand to be mainly due to the volume pot.”

Severian · Audio Science Review forum

Sources split on the kit experience, not the parts. Reviewers and owners praise the manual as exceptionally clear and say a carefully built Crack runs reliably for years. Just as many describe first builds that didn't work — a silent channel, a dead Speedball channel, a batch of counterfeit transistors Bottlehead replaced, weeks of hum troubleshooting — and one reviewer paid to send his back for repair. Bottlehead's repair service and forum support get credit throughout.

Sources split along the familiar electronics line. Reviewers and owners overwhelmingly call it the best-value entry into tube headphone amps and a match for amps several times its price with the right headphones; one early reviewer noted a pre-built Crack costs close to finished rivals. The measurement camp counters that what people like is a small bass lift from the output impedance, which EQ and a clean solid-state amp deliver for less — and at least one builder sold his for that reason.