Crack 1.1 OTL Headphone Amplifier Kit

The Bottlehead Crack is a transformerless (OTL) headphone amplifier kit designed specifically for high-impedance headphones like Sennheiser, AKG, and Beyerdynamic models. With a straightforward, minimalist circuit that’s perfect for first-time builders, this Skill Level 1 kit can be assembled in just an evening or two, making it a favorite entry point into DIY tube audio.

At its core, the Crack uses a Bottlehead PT-10 universal power transformer and a proven C-R-C-R-C power supply layout originally made famous in the Foreplay preamp. This configuration—featuring ultrafast rectifiers and generous filtering—delivers a quiet, stable power source with ample headroom. The signal path is equally elegant: a 12AU7 dual triode directly drives a 6080 tube configured as a cathode follower, creating a clean, dynamic sound free of output transformers.

To keep the kit affordable, high-quality electrolytic capacitors are used for output coupling, but the design is highly tweakable. Builders can experiment with premium capacitors, alternative tube variants, and the optional Speedball Upgrade to elevate performance even further. Whether you’re chasing better tone or just enjoy hands-on experimentation, Crack offers endless potential for customization.

Affordable, approachable, and endlessly upgradeable—the Bottlehead Crack is a modern classic in DIY headphone amplification.

“For the HD650, this amp rivals pretty much anything else I’ve heard” 

“I auditioned the crack for a day with HD650s and was blown away.”

“All parameters checked out right off and it’s making the sweetest music I have ever heard and that’s just with a pair of ATH A700 phones.”

“I just got the 600 Ohm DT880 yesterday and can’t tell you enough what a perfect match these headphones are with the crack.”

“Although the headphones I’ve been using with the Crack (Sony MDR-SA5000, 70 ohm) are not an ideal match, they still show off the incredible sound quality of this little amp!”

“None of my other amps has done justice to the AKG K501s like the little Crack. It’s provided a richness of sound which nicely expands on the AKG’s strengths of soundstage and midrange performance.” 

“With “Nowhere Boy,” a new John Lennon biopic, the Bottlehead Crack amplifier and my old Sennheiser HD-580 headphones kept me totally absorbed in the film.”
“For those of you wondering about the new Crack amplifier, this is also very sweet and enticing set up. I think I heard the version with the extra add-on circuit. That was lovely with a variety of headphones, including the HD-800s. Ahhh.”

“the amp really sings now with the Etys, and with AKG K701s”

“This is now my favorite amp for my Sennheiser HD800s.”

“As for the sound, with my Beyer dynamic DT880’s (600_ohm)….Super sweet!! highs are smooth airy & detailed, ….midrange is very organic, detailed and transparent,…… bass extends deep and detailed (no wanting for any additional bass with the DT880’s….BTW)… and the soundstage extends well outside my head….”

“What an amazing amp! Dead quiet, the bass has incredible slam and definition. Brushes against a drumhead are stunning. It does a better job with my HD-650’s than my K-702’s but not by much” 

“The Bottlehead Crack is a superb introduction into the world of DIY audio. The parts quality is very good, the documentation is truly excellent, the Bottlehead forums provide fantastic peer-to-peer assistance, and construction is very easy. The Bottlehead Crack is an outstanding first project for the novice
audio DIYer. The big win, however, is that once you’ve finished your build, you’ll have a fantastic sounding amp for your high impedance headphones. I simply can’t recommend the Bottlehead Crack highly enough, if you’ve got a yen to try a little soldering, this is the way to go. You’ll love it!”

Tyll Hertsens, innerfidelity  (Here is a marvelous video from Tyll of how to solder your Crack kit. Anyone can Here is a nice review on Head-Fi, from Bill Way