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!


Checking the labeling of connections.

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 step-by-step instructions made construction a breeze.

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!


Final voltage testing of the Crack. REMEMBER! Use one hand only, keep the other in your pocket and away from ground. Wear shoes. Voltages inside a tube amp can be FATAL!

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.

9415 Coppertop Loop NE
Suite 101
Bainbridge Island, WA 98110
(206) 451- 4275

MacedonianHero's picture

Just one tiny thing...blowing on a solder joint (cooling it quickly) will not cause it to be a "cold solder" joint. What it will do is cause larger grain structures in the joint. This is not a good thing as cracks can more easily propagate through solder joints with large grain structures (over ones with smaller grain structures). Cold solder is not fully liquifying the solder during the soldering operation. Basically applying solder with both liquid and solid phases. If some readers are interested in soldering, the industry standard (if they're so inclined to look it up) is J-STD-001 by IPC. For under $300, this amp looks like it gives great value to performance. Cheers

Tyll Hertsens's picture

Thanks for the tid-bit. yes

Armaegis's picture

>>blowing on a solder joint (cooling it quickly) will not cause it to be a "cold solder" joint. What it will do is cause larger grain structures in the joint


I'm confused on your usage of the word "grain structures"... higher cooling rates almost always results in finer microstructure (smaller grains, thus more grain boundaries). Are we on the same page or am I misreading something here?

MacedonianHero's picture

For soldering, the ideal cooling profile is 1-2'C per second. Anything more will lead to larger grain structures and less reliable solder joints in the end. 

Armaegis's picture

By larger grain structures, you mean more grain boundaries correct?

drheadphone's picture

Thank you so much for the review, the video and the resourceful links. I bought this amp a few weeks ago to pair with my Sennheiser HD650/800 headphones. I have to admit I'm a little intimidated with initiating my first DIY project. At the very least, I feel inspired.

Tyll Hertsens's picture

.... if anything, I wanted to inspire people to give this a shot. Glad to have given you the nudge.

It was straight forward, and extremely rewarding.  yes

MacedonianHero's picture

Totally agreed on the rewarding part. When my kids are a bit older, I'll be getting back into DIYing as a hobby. 

gorboman's picture

I had an opportunity to try the Crack. You're right about matching it with a high impedance headphones. I liked it with the Beyer DT880PRO/600. It was full sounding, smooth, but only a little warm. The Crack with the DT880/250 sounded meh. There were no HD650 to try it with, but based on your review, I guess it's a match for 300 ohm headphones and above.

Other cans that I've tried with below approval results were LCD2 and T50RP Mad Dog. A friend told me that the Crack just won't work with ortho-dynamic headphones. I'll just hold on to his word since I know so little about electronics.

HammerSandwich's picture

Clearly, the tubes of the Crack did a great job of delivering detail and oomph while also softening the edges of the sound somewhat.

Are you certain that's because of the tubes & not the high output impedance?

Tyll Hertsens's picture

That could be too.

Draygonn's picture

Glad you are giving the amp some well deserved attention.  Great match for the HD650s and HD800s. 

Willakan's picture

I just want to see how high the second harmonic is :D!

13mh13's picture

Prob. the most neglected (de)soldering accessory. But I don't DIY (de)solder w/o 'em.

Several brands out there, but some leak, and can cause a mess. Kester makes the best one I've used. The felt tip is probab. the most useful:

n_maher's picture

Great article, Tyll. 

ryebread's picture


Please update this article once the speedball mod is complete!

earwaxxer's picture

I have the Quickie. Also very enamored by it. I'm trying to get Doc to get more of his gear reviewed! I think bottlehead stuff is fantastic.

GilSt's picture

Thanks for the article!

The speedball completely removed the "veil" off the HD650's for me.

uzi's picture

First off, where was that helpful video guide when I was looking to build one of these?

I hadn't soldered in years, but after hearing stuff like the Schiit Valhalla and reading up about the Bottlehead Crack, I decide that an OTL amp was worth having for my HD650... so I built a Crack with the Speedball upgrade.  An initial problem was that I was lacking most of the tooling needed, so I had to pick up a soldering iron, a dmm, etc. etc. ... which wasn't cheap, but stuff I wanted to have anyways.

So, I was pretty close to being a pure rookie at this, took a stab at it and built things up over a few nights.  Got to the initial stage of the Crack build and things seemed to be working decently.  Unfortunately, I shorted something while testing on the way to the Speedball upgrade and blew a fuse.  Bought some replacement fuses, completed the build and then... had decent volume on one ear and hardly any on the other.  Voltages were off for one channel and the works.  A botched build, somehow.

And then began the (longer than I'd like to admit) time of sitting on the project, tinkering here and there... testing all my solder points, etc.  Eventually I figured that I'd get this thing working *someday*, so I went ahead and put in an order for some extra tubes at  When those tubes arrived, I popped some in the sockets, replacing the ones that came with it, and *boom*, it worked!  It was the tubes!  (Kinda hard to know that that was the problem with only one set of tubes, you know.) And the tubes definitely have an effect on the sound of the unit.

So now I have this for some tubey goodness and a Burson Soloist for going solid state for my HD650. They both pretty much exude the qualities you'd expect.  The Crack+Speedball has a great, colored, tube sound to it while the Soloist is exact, precise, crisp and very detailed.  Which one I prefer depends on what I'm listening to.

donunus's picture

By the way Tyll, Was the WA3 barebones stock? The one currently available at the Woo site? I'm asking since there used to be a wa3, a wa3+, and of course tube rolling options.

Dudes place's picture

I being a newb, was wondering if one of these Bottleheads could be used as a preamp for a PA  between a D/A system to warm up the sound. Just mainly wondering about the impedance and output levels.

Tyll Hertsens's picture

...should work just fine. Just like a pre-amp.

Dudes place's picture

I figured it would be fine, was just wondering if impudence would be an issue with output.

thanks, great forum by the way!

neo's picture

Did you have had the Speedball upgrade yet? I'm using the crack with the 6080 stock tubes. Don't know whether to take the tube rolling route or install the speedball..Either way, I'm satisfied with the Stock, just curious about how better can it get.

Edmunds's picture

Thanks for the review.

Would you be able to describe your perceptions of how the Crackhead and WA3 compares to the WA2?

Tyll Hertsens's picture

...haven't heard the WA2.

Hariadi JW's picture

Will Crack do well with Grado PS 1000? Thanks for the advice

nunobartolomeu's picture

Hello from Portugal,

I am wondering if the Philips Fidelio X1 combines perfectly with the Bottlehead Crack OTL, because I have found on Amazon a Philips Fidelio X1 with a lower price than the Sennheiser HD600 and I cant decide wich one is the best buy.

Thanks in advance,

Nuno Bartolomeu

P.S Congratulations for the Website that help us decide the best article we can buy.

Tyll Hertsens's picture

The X1 will work fine on the Crack, but personally I'd prefer the HD600. X1 is more bass heavy, and possibly more neutral relative to the Harman target response curve.  I like them both, but do like the open airyness of the 600.

neo's picture

Isn't the X1 30 Ohms? The Crack doesn't sound very good with low impedance headphones

lenbell's picture

i have the hd650s and will use my laptop as my source for music...what is a good DAC that will work with both the bottlehead and a pair of adam x5 speakers..ideally i dont want to unplug one to listen to the other..thanks