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Hi ASR,
Wanted to share my build of my Bottlehead Crackatwoa headphone amplifier. This amp was built last year, so this is more of a retrospective, but I'm very proud of it and thought you might be interested
Background:
The Crackatwoa is an upgraded model of Bottlehead's popular DIY Crack OTL headphone amplifier with a larger power transformer, shunt-regulated power supply, multiple inputs, and optional addition of a Bottlehead's TwoQuiet stepped attenuator. The original Crack headphone amp is a high output Z, cathode follower, OTL topology that has garnered much praise in the headphone community for its sound quality and reasonable price, particularly when paired with Sennheiser's 300 Ohm HD6XX headphone lineup. The original comes in a base form, or for an additional cost, can be upgraded with a Camille Cascode Constant Current Source (C4S) known as the "Speedball"; the Crackatwoa comes with the Speedball C4S board included at stock. The amp has a 120 Ohm output impedance, so is only recommended with high Z dynamic headphones (Sennheiser, Beyerdynamic, ZMF).
A large modding community developed around the original Crack, with builders adding upgraded volume pots, stepped attenuators, film output capacitors, bypass capacitors, filter chokes, cree rectifier diodes, among many others. Doc B. at Bottlehead, after watching this community for some time, thought customers' money would be better spent on an improved overall circuit rather than boutique components or small circuit modifications, and thus the Crackatwoa with its upgraded power supply was born. It takes a 12AU7 input tube, 6080/6AS7 output tube, and 6AQ5 tubes in the shunt regulator.
I was the owner of the original Crack + Speedball and a sizable tube collection to match. I decided I wanted a Crack on...crack, so to speak, and planned a "statement" amplifier build around the Crackatwoa, featuring a few choice component upgrades, a stepped attenuator, and hopefully, excellent aesthetics.
Product link: https://bottlehead.com/product/crack-a-two-a/
Additional components:
Here is a list of components I planned to add after completing the stock circuit.
Japanese Azuma gold-plated ceramic sockets
Audio Note silver-plated RCA jacks, for aesthetics.
Mundorf MCap EVO Aluminum Foil and Oil output capacitors.
Mundorf SUPREME EVO Silver and Gold RC filter capacitor on driver C4S board.
Audio Note 25mm Polished Chrome volume knobs. The plan was for the top plate of the amp to be sanded and polished to a mirror finish, so I thought having matching chrome knobs would give the amp a real premium look
Cardas Golden Cuboids with Dayton Audio spiked feet. For the sake of convenience, I picked up these myrtle wood cubes from Cardas to add my spiked feet for that premium audio component "look".
Base and feet:
After playing around with some test-staining, I decided on a General Finishes Walnut Gel stain and gel topcoat. The base was assembled and sanded, the hardware for the spiked feet was applied to the Cardas blocks, which were subsequently glued to the interior of the base, and they were stained together. I finished the gel topcoat at a later date after the circuit was completed and used a buffing wheel to bring the finish to a gloss (not pictured until final build).
Top plate and hardware:
The aesthetics of the amplifier were built around my plan to polish the aluminum top plate to a mirror finish, which predictably was no easy task. It was accomplished by sanding the plate to 800 grit via detail sander, doing a hard cut via buffing wheel and White Diamond buffing compound, and finishing it off with Blue Magic metal polish and an orbital buffing tool. I am a masochist, so I also chose to replace the included Philips-head hardware with button cap nickel-plated hardware. I individually polished each screw with a Dremel tool to match the mirror finish of the top plate. Here is the amplifier with the hardware applied prior to circuit assembly.
Stock circuit:
Assembly of the stock circuit was done over one week, using my Weller soldering station and Cardas quad eutectic silver solder. It was uneventful, as Bottlehead includes excellent documentation and instructions, including extensive safety precautions. Here is the completed stock circuit.
Upgrades:
After spending a little time with the stock amplifier, it was time to add my upgraded film capacitors as well as the TwoQuiet stepped attenuator. The stock potentiometer and balance adjustment were replaced with a thirty-six, 1.5dB step attenuator with course/fine adjustment developed by Bottlehead. The stock Dayton Audio RC filter caps on the C4S board as well as the Nichicon 100uF electrolytic output capacitors were replaced with premium Mundorf film capacitors. Knowing that builders were undoubtedly going to add these enormous film output caps, the guys at Bottlehead left a generous amount of space in this area of the amp. Even so, I had to adjust the orientation of the left attenuator terminal strip to fit them in. Here is the finalized circuit.
12AU7 to 6SN7 adapter:
I found in my time with the original Crack that I preferred the sound of 6SN7 input tubes over the 12AU7s and their equivalents. I purchased a 6SN7 to 12AU7 adapter from Garage1217, but did not like that the metal leads of the adapter were exposed. So, I purchased a 6SN7 tube base and fashioned a shell. It required some grinding of the ceramic via Dremel to fit the base around the adapter, then some thin cyanoacrylate adhesive to bond them. Very happy with how it turned out.
Final build:
So that's pretty much it! At this point, I am enjoying this headphone amplifier on a daily basis. I have several input and output tubes and enjoy mixing and matching to play with the sound. Headphones I use are the Sennheiser HD600, Sennheiser HD650, ZMF Atticus, and ZMF Auteur, all 300 Ohm impedance. Here are some photos of the amp in its current state, with a RFT 68HC/6SN7 input, MOV A1834 output, and Telefunken EL90 shunt regulator tubes.
Thanks for checking out my build!
Wanted to share my build of my Bottlehead Crackatwoa headphone amplifier. This amp was built last year, so this is more of a retrospective, but I'm very proud of it and thought you might be interested
Background:
The Crackatwoa is an upgraded model of Bottlehead's popular DIY Crack OTL headphone amplifier with a larger power transformer, shunt-regulated power supply, multiple inputs, and optional addition of a Bottlehead's TwoQuiet stepped attenuator. The original Crack headphone amp is a high output Z, cathode follower, OTL topology that has garnered much praise in the headphone community for its sound quality and reasonable price, particularly when paired with Sennheiser's 300 Ohm HD6XX headphone lineup. The original comes in a base form, or for an additional cost, can be upgraded with a Camille Cascode Constant Current Source (C4S) known as the "Speedball"; the Crackatwoa comes with the Speedball C4S board included at stock. The amp has a 120 Ohm output impedance, so is only recommended with high Z dynamic headphones (Sennheiser, Beyerdynamic, ZMF).
A large modding community developed around the original Crack, with builders adding upgraded volume pots, stepped attenuators, film output capacitors, bypass capacitors, filter chokes, cree rectifier diodes, among many others. Doc B. at Bottlehead, after watching this community for some time, thought customers' money would be better spent on an improved overall circuit rather than boutique components or small circuit modifications, and thus the Crackatwoa with its upgraded power supply was born. It takes a 12AU7 input tube, 6080/6AS7 output tube, and 6AQ5 tubes in the shunt regulator.
I was the owner of the original Crack + Speedball and a sizable tube collection to match. I decided I wanted a Crack on...crack, so to speak, and planned a "statement" amplifier build around the Crackatwoa, featuring a few choice component upgrades, a stepped attenuator, and hopefully, excellent aesthetics.
Product link: https://bottlehead.com/product/crack-a-two-a/
Additional components:
Here is a list of components I planned to add after completing the stock circuit.
Japanese Azuma gold-plated ceramic sockets
Audio Note silver-plated RCA jacks, for aesthetics.
Mundorf MCap EVO Aluminum Foil and Oil output capacitors.
Mundorf SUPREME EVO Silver and Gold RC filter capacitor on driver C4S board.
Audio Note 25mm Polished Chrome volume knobs. The plan was for the top plate of the amp to be sanded and polished to a mirror finish, so I thought having matching chrome knobs would give the amp a real premium look
Cardas Golden Cuboids with Dayton Audio spiked feet. For the sake of convenience, I picked up these myrtle wood cubes from Cardas to add my spiked feet for that premium audio component "look".
Base and feet:
After playing around with some test-staining, I decided on a General Finishes Walnut Gel stain and gel topcoat. The base was assembled and sanded, the hardware for the spiked feet was applied to the Cardas blocks, which were subsequently glued to the interior of the base, and they were stained together. I finished the gel topcoat at a later date after the circuit was completed and used a buffing wheel to bring the finish to a gloss (not pictured until final build).
Top plate and hardware:
The aesthetics of the amplifier were built around my plan to polish the aluminum top plate to a mirror finish, which predictably was no easy task. It was accomplished by sanding the plate to 800 grit via detail sander, doing a hard cut via buffing wheel and White Diamond buffing compound, and finishing it off with Blue Magic metal polish and an orbital buffing tool. I am a masochist, so I also chose to replace the included Philips-head hardware with button cap nickel-plated hardware. I individually polished each screw with a Dremel tool to match the mirror finish of the top plate. Here is the amplifier with the hardware applied prior to circuit assembly.
Stock circuit:
Assembly of the stock circuit was done over one week, using my Weller soldering station and Cardas quad eutectic silver solder. It was uneventful, as Bottlehead includes excellent documentation and instructions, including extensive safety precautions. Here is the completed stock circuit.
Upgrades:
After spending a little time with the stock amplifier, it was time to add my upgraded film capacitors as well as the TwoQuiet stepped attenuator. The stock potentiometer and balance adjustment were replaced with a thirty-six, 1.5dB step attenuator with course/fine adjustment developed by Bottlehead. The stock Dayton Audio RC filter caps on the C4S board as well as the Nichicon 100uF electrolytic output capacitors were replaced with premium Mundorf film capacitors. Knowing that builders were undoubtedly going to add these enormous film output caps, the guys at Bottlehead left a generous amount of space in this area of the amp. Even so, I had to adjust the orientation of the left attenuator terminal strip to fit them in. Here is the finalized circuit.
12AU7 to 6SN7 adapter:
I found in my time with the original Crack that I preferred the sound of 6SN7 input tubes over the 12AU7s and their equivalents. I purchased a 6SN7 to 12AU7 adapter from Garage1217, but did not like that the metal leads of the adapter were exposed. So, I purchased a 6SN7 tube base and fashioned a shell. It required some grinding of the ceramic via Dremel to fit the base around the adapter, then some thin cyanoacrylate adhesive to bond them. Very happy with how it turned out.
Final build:
So that's pretty much it! At this point, I am enjoying this headphone amplifier on a daily basis. I have several input and output tubes and enjoy mixing and matching to play with the sound. Headphones I use are the Sennheiser HD600, Sennheiser HD650, ZMF Atticus, and ZMF Auteur, all 300 Ohm impedance. Here are some photos of the amp in its current state, with a RFT 68HC/6SN7 input, MOV A1834 output, and Telefunken EL90 shunt regulator tubes.
Thanks for checking out my build!
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