The Marvelously Addictive Bottlehead Crack
Bottlehead Crack OTL DIY Headphone Amp ($279)
Over the years, I've stopped by the Bottlehead booth at shows to say "Hi!" to Doc B. numerous times, and each time I make a special point to have a listen to the Bottlehead Crack. This sweet little OTL amp never fails to impress.
It uses a 12AU7A as its input stage, and a 6080 in a cathode follower configuration as its output stage. The output is capacitively coupled (the only cap in the audio path), and has an output impedance of about 120 Ohms making it an amp suited mainly for higher impedance headphones. (I'd suggest 300 Ohms and up, but some will find it fine for cans down to 100 Ohms.) The power supply is a simple unregulated supply, with a full-wave solid-state diode bridge rectifier, and a C-R-C-R-C supply filter. It really doesn't get much simpler than this.
Additionally available is the Speedball upgrade kit ($125), which is a constant current source for the plate circuit of the 6080. I've got the kit, but won't be installing it until I've put the Crack through the full measurement routine. Once measured, I'll install the Speedball, and then measure again. After that, it'll be "game on" for further upgrades, tweaks, and tube rolling. Because it's so simple, the Crack is the perfect platform for novice DIYers to play around with and listen to the sound changing as various upgrade parts are explored. And expert DIYers will experience a delightful walk in the audio park. Just great fun!
Building the Crack
What a joy! I really couldn't believe how simple this amp was to construct. The main chassis is a laser cut aluminum plate and all components are mounted to this plate. All the parts for the build come in plastic bags, including a few Hershey's Kisses to make the process even sweeter. (Thanks Doc!) I found no missing parts in my kit. The wire used was 20 gauge solid core wire, which stripped and soldered easily.
The manual for the kit comes in .pdf form on a CD, and I found it to be thorough and extremely detailed. It included a picture and a clear explanation for every step. Wire lengths specified were all just slightly long, which is good just in case you cut the conductor while stripping by mistake. Not really a problem, though, as I had more than a foot of each color wire remaining when done. I found no outright errors in the build instructions, but I do have a couple of notes:
- Page 8) Desoldering braid (Solder Wick) and socket wrenches are listed as optional tools. I think you'll find need for them.
- Page 11) When inserting the power switch, I just pressed it into place. Unfortunately, the plastic is a little brittle and one of the mounting tabs broke. I suggest you gently squeeze the tabs together when inserting this switch to reduce the stress on the tabs during installation.
- Page 12) Safety Ground. It says: "Insert a #8 screw through the hole..." I would add "Insert the #8 slotted screw" as there are a couple of #8 screws that might get confused.
- Page 23) Last step. It says: "Cut a 2 inch piece of red wire ..." I used white wire as it appears to be part of the left channel connection between the plate of the 12AU7 and the grid of the 6080. White wire has been a properly used convention in the kit elsewhere for the left channel.
- Page 38) The manual didn't tell me to do this, but I used some clip leads to attach my volt meter to the supply rail to monitor the voltage when I turned the Crack on for the first time. I expected about 170 Volts but when I flipped the switch I saw the meter whoosh past 170 VDC and blow through 200 Volts. I immediately turned off the unit to see if something was wrong. Unfortunately, I didn't have the presence of mind to realize that an unregulated supply was going to ramp up quickly but once the tubes started to conduct would relax back down to its normal voltage. Phew!
Rather than write a lot of words about how to strip wire, solder, and de-solder parts, I've done a little video on the next page with fairly complete instructions. You'll also find my comments on the sweet sound of this addictive little amp.