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.