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February 15, 2011

The Bottlehead Crack OTL Headphone Amplifier

Bottlehead has been known to me many years before I purchased my first kit from them. Doc and Queen have been in the business of selling tube-based kits for quite some time and have earned an excellent reputation for providing easily accessible and excellent sounding kits. All of the kits follow a rather unique naming structure: S.E.X., Foreplay, Seduction, Quickie ... you get the idea ;)

The Bottlehead Crack is a very inexpensive kit, for only $219 (as of Feb 2011) you get all of this:
The Bottlehead Crack Kit

The amp runs on a single 9 pin 12AU7 and an 8 pin 6080 / 6AS7. I'd recommend a little bit of tube rolling on this little guy as it can sound very nice with the right compliment. The manual is included on the CD in PDF format with step by step instructions. The kit includes an aluminum top plate that comes pre-drilled for the stock parts. Since it's aluminum, it's a piece of cake to drill out any additional holes or enlarge existing ones if you decide you'd like some different parts in there. I used a Hammerite spraypaint to add a bit of personality to the top plate and transformer bell cover. A Northwest Alder wood base is also included, you glue it together and finish in the color of your choice. I used a combination of MinWax products to get the wood to a deep red finish and a Polyurethane finish to preserve it.

Below are a few of the initial build photos:

After a number of steps into the build, with DACT mounted

Fitting the large Solen 100uF Film Caps

I decided that I wanted a little more than the standard Bottlehead square emblem adorning the wood base. I managed to find a promotional products supplier that fashioned metal badges. I designed a vintage looking logo in Adobe Illustrator and fired it off to them, the results turned out well.

Finished Wood Base with EAR Feet and Custom Emblem

Bottlehead offers an excellent upgrade to the Crack called the Speedball... again with the crazy names. This, more specifically, is the Camille Cascode Constant Current Source upgrade. Bottlehead states on their page: 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.

Speedball Upgrade Fitted
Final Build with PSU bypass cap and Pilot Light

Custom made 10 gauge power cord for the Crack

Obbligato Bypass caps peaking out from the vent holes

The Final Build

As always, a few tweaks here and there are always welcome. This Crack features the following upgrades:
  • Cardas Rhodium RCAs
  • DACT 100k stepped attenuator
  • Kimber TCSS wiring
  • Kiwame Resistors
  • Neutrik Locking Plug
  • Obbligato 1uF Copper coupling caps (hidden under Speedball board)
  • Solen 100uF Capacitors
  • Teflon Tube Sockets
  • Woo Audio Tube Socket PCBs
  • Auricap Bypass
  • Vintage Pilot Light

The Crack, with Speedball, is an excellent sounding headphone amp for high impedance headphones (think ~300 ohm like Sennheiser's higher end cans) and a great value for the price. It can be a little bright with certain tube compliments in my humble opinion, so be sure to get a few to roll. 

UPDATE (5/1/11): It's been a little while since I worked on the Crack, so I figured I would give it a fresh upgrade :) I managed to squeeze in a Solen 220uF film cap in the final electrolytic power cap location, sitting on a pair of standoffs. I also took the time to bypass the other two power caps with a pair of low-cost Audiophiler 2.2uF film caps.

All these parts just narrowly fit in the stock case

Leads were cut and longer Kimber TCSS wire was run

2.2uF Audiophiler bypass caps on left and right
UPDATE (5/18/11): Since I can't leave well enough alone for longer than 5 minutes, I opted to add a choke to the power supply for even further refinement. I selected a Triad C7X with a rating of 270ohms to take the place of the final resistor in the power supply (also 270 ohms). Since pretty much all the real estate has been used up on the plate, I opted to add two 2" aluminum standoffs to raise the choke above the capacitor near the RCA inputs. This required adding 1" spiked feet to the bottom of the wood base for adequate clearance, just like in the Bottlehead S.E.X. build. I think this baby is finally done.

Triad C7X choke right above the power supply cap

Spiked feet for added clearance and ventilation

The Fine Print:
Please remember that building circuits and performing circuit modifications can be dangerous to you and/or your surroundings and should only be performed by a certified technician. The owner of this blog and all associated parties can not / will not be held responsible if you attempt a build or modification posted above and cause physical harm to yourself or your surroundings. Many electronics contain high voltages that can kill, and mods, if performed improperly, can be a fire hazard. Please keep this in mind. 


  1. brashgordon1@msn.comApril 2, 2011 at 6:10 AM

    So what was your final cost for this build?

  2. Hello HF9,
    If I remember correctly you owned a Woo Audio 3. Just wondering how the hot-rodded Crack compares to it.

  3. I did own the Woo a while ago. I feel that with the right tube compliment the "hot rodded" Crack is the better amp. It provides a stronger, more resolving sound.

  4. "Very nice implementation. Would you be so kind to entertain a few questions please? If so: Did you try the amp before adding the Speedball upgrade? If so, was there a noticeable difference? You have mentioned, in your blogging, that you “need the treble and upper-midrange to be a bit rolled off”. This is how I like it as well. I assume the Sylvania 12AU7, you are using, offers that characteristic. Can you please share which one that is? I have found a few online. Finally, you used Hammerlite paint on the top panel. I like the look of this a lot. Did you prime it first as the instructions indicate not to use it on bare aluminum. As it is not going to be in the elements, I wouldn’t see this being a problem. Thanks in advance, EggManDFW"

  5. Hi Eggman, I did try the amp before the Speedball CCS upgrade, the differences were not subtle. The Speedball really helped to clarify the treble and solidify the bass; everything seemed more dynamic. I did some tube rolling before I settled on a tube. I happened to have a couple of 12AU7s and 6AS7s to roll and settled on the Sylvanias in the picture. Not sure what type they are exactly, but they appear to be 1960s vintage.

    I did sand and prime the panel before spraying on the Hammerite. It also needs a nice long time to dry, otherwise it will come right off. I've moved on to powdercoating and would recommend that instead.

  6. I just got my Crack kit, and your post was my inspiration. I've already got some upgrade parts: Kimber TCSS wiring, Kimber RCAs, teflon tube sockets and PCBs, and a Neutrick locking 1/4" jack. The speedball, DACT, and caps are a ways off, but I think I'd like to upgrade the resistors before I start. I do have some questions for you...

    Were you able to find a good deal on the Kiwame resistors and what wattage did you use?
    I see that you have non-stock 10 watt resistors, what did you use for those and the 5 watt (I've only seen Kiwame in 1 and 2 watt)?
    What benefit did you get from the Triad C7X over the 270 ohm resistor?
    I get where you upgraded the Solen 100 and 220 uF caps, but how did you wire the Auricap and Audiophiller bypass caps, and what benefit does this give?
    Did you find it easier to solder the wires to the tube PCB before soldering the PCB to the socket?

    Great write up. Thanks in advance,

  7. Hi there Hellimeda. You should definitely pick up the Speedball, it makes a huge difference and I believe it's set to go up in price... today actually.

    The 2 watt Kiwames are sold as Koa Speers at Mouser. Mouser only has half or so of the available values, and they sell for ~24-44 cents each. The rest you will need to get from PartsConnexion or Percy Audio for ~ $1 each.

    I wouldn't bother replacing the 10W resistors with anything fancy as they get thrown out when the Speedball is installed. The 5W are just generic wirewound, they are a little nicer than the sandcast resistors that come with the kit imho but they don't really bring any sonic benefit that I noticed.

    Bypass caps are wired in parallel and a general rule of thumb is that they should be 1% or less of the value they are bypassing. There's plenty to read on Google about bypassing caps, so I won't get into it too much, but using a nicer quality bypass cap can potentially increase detail in coupling configurations. In the power supply, it can help reduce ripple. Some engineers, such as the ones at Bottlehead don't believe that bypassing is worthwhile for various reasons, but other engineers such as John Broskie known for his Aikido amplifiers bypass nearly every cap on his PCBs. As most things in Audio, it comes down to your own experimentation and taste.

    The Triad C7X seemed to add a little bit of smoothness to the presentation, but it could be placebo. It was quite subtle to these ears.

    1. Thanks so much for the great information and the "just-in-time" response. I picked up the Speed Ball before the price increase. I'll let you know how things turn out.

  8. Hi, very nice work!!! ;-)
    I have a couple of questions
    How much B+ and how much plate voltage for 6080 tube and 12au7 tube?
    Thank in advance.

  9. Hi Giorgio, I'm afraid I don't have that spec for you, I'd recommend checking with the gents on the Bottlehead.com forum.

  10. Nice Amp! I just ordered my kit from Bottlehead and have a couple of questions about your build since I'm planning to do some similar modifications.

    Which Caps are the Obbligitos bypassing? The Solen 100uF's?

    I've read that the choke can add noise to the amp if it is in the wrong position relative to the transformer. Did you notice any gain in noise with it in the position you mounted it in or did the amp stay quiet?

    Where did you get the 90° IEC plug? I can't seem to find them anywhere.

    Where did you connect the wires for the pilot light? Forgive my lack of knowledge on pilot lights, but is it on all the time or only when the amp is on?


  11. Hi Michael,
    Yes, the Obbligatos are bypassing the big Solens. You are correct that a choke can add magnetic flux interference if placed too close to another transformer. This was not a problem (I did not hear any noise coming from the new choke install). The right angle connector is a Schurter right angle IEC, Handmade carries them if I recall correctly. The pilot light was wired to the tube heaters.

    Hope that helps.

    1. Thanks for the reply! That was exactly the information I was looking for.


    2. One more question. I really like that custom Bottlehead Badge. Could I get a copy of the Illustrator file for it so I can have one made for myself? If not, could you at least tell me what font you used?


    3. Hi Michael,
      I actually don't have the design file anymore. From memory, I think it was a free retro font called Diner, you may want to try that.