This is a group buy for the acclaimed Bottlehead Crack Headphone Amplifier kit + Speedball Upgrade package. This is widely considered to be one of the best OTL Headphone amps available under $500 and is often compared to amps priced above the $1000 mark.
Reviewers have been shouting praise since its release. Here's one from Bottlehead's website:
"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
The initial goal was to come up with an easy to build, great sounding design that would harken back to Bottlehead's early days. Crack is Bottlehead's version of classic OTL headphone amp designs of several years ago that were hailed as some of the best sounding of their day.
This is what we call a Skill Level 1 kit, perfect for first time builders. The circuit is so simple you can build it in an evening or two. A simple circuit also means a low parts count, which keeps the price low, too. Crack is an output transformerless amp designed specifically for running higher impedance (200 ohms or higher) headphones like the many Sennheiser, AKG and Beyer designs.
A 12AU7 configured just like the "front half" of the Foreplay III is direct coupled to a 6080 tube configured as a cathode follower. To keep the kit price manageable (the kit is about 1/4 of the price of some past OTL amps) we use surprisingly nice sounding Panasonic electrolytic capacitors as the output coupling capacitors, but of course our kits often induce tweaking - in fact we imagine Crack may become the biggest cause of tweaking yet. You could spend more on different large 100uF film capacitors than you than you did on the kit and spend hours putting in different caps and comparing them. And of course you can tweak to your hearts content tube rolling different 12AU7 and 6080/6AS7 variants.
A single pair of gold RCA jacks, a stereo potentiomer volume control and a Neutrik 1/4" TRS headphone jack complete the signal path, and the kit comes with an IEC power entry socket.
It comes in a standard Bottlehead "gravity mount" cabinet configuration where all components mount on a 10" x 6" laser cut aluminum chassis plate that sits on a lovely Northwest alder wood base that you assemble and finish yourself. The manual is in PDF format and you can blow up the high res photos to see each assembly step in detail.
What yields an even more intense experience than Crack? The most hardcore users say it's a Speedball. Speedball is our Camille Cascode Constant Current Source upgrade for the Crack headphone amplifier. The 22k1 ohm plate loads of the 12AU7 input triodes are replaced by C4S loads, as are the 3K ohm cathode load resistors of the 6080 triodes. The PC board for the loads on the 6080 tube has very ample heat sinks to assure high reliability. We consider this a skill level 2 kit, meaning that it is intended for someone who has construction of a kit under their belt (like the basic Crack kit). The new skills you might acquire will be those of stuffing and soldering a PC board, and removing a few components to allow installation of the upgrade. Assembly and installation is an easy one evening session, requiring only the removal of four resistors from the previously assembled Crack kit.
So what does it do for the sound? The most immediate thing you will notice is a better sense of clarity. Things get tighter and quicker, bass and midrange get cleaner and more dynamic sounding, and the background gets more quiet. As with all of our upgrade kits, the improvement is easily heard.
Some of you will ask if you can build the Speedball kit into the Crack right from the start. Yes, you can, you just leave the aforementioned resistors out of the Crack assembly and then build and install the Speedball. HOWEVER - if you are not familiar with building and then possibly troubleshooting circuits, we strongly suggest that you build the Crack kit in stock form first and make sure it is assembled correctly and passes the resistance and voltage checks. The basic circuit is simple to sort out and fix if you goofed up a connection or two. From there it takes just an extra minute or two to snip out the four resistors you need to remove to add the Speedball, and you have the advantage of knowing that everything around your new Speedball upgrade parts are working perfectly.
All orders will be shipped by Massdrop.
Estimated ship date is Dec 11, 2014 PT.
After the drop ends, payment will be collected. To deliver the best value to the group the request will then be submitted to the vendor up front, making all sales final. Be sure to check the discussion page for updates.