The Marvelously Addictive Bottlehead Crack Page 2
Again, due to the 120 Ohm output impedance, this amp is really designed for high impedance cans. I'd recommend the Sennheiser HD 600 and HD 650 as perfect matches. The Sennheiser HD 800 is also a great phone for this amp, though it might be a little pricey to strongly recommend for this pairing. On the other hand, its amazing resolution will let you peer into every mod you make if you're thinking about the Crack as a long term tweaking platform. I'll also recommend the Beyerdynamic DT880 600 Ohm as a good match, though I don't find them as warm and comfy as the 600/650. I used the HD 800 and HD 650 for my listening tests.
I included the Woo Audio WA3 in my listening set-up for comparison during listening tests. It's also an OTL design that uses the 6080 output tube, but has two 6922 input tubes instead of the single 12AU7A tube of the Crack. My source was Amarra on a MacBook with optical digital out feeding the new Asus Xonar Essence One (which I find myself liking quite a bit). I used the unbalanced outs to splitters and then on to the two tube amps.
My initial impressions of the Crack prior to comparison testing was that it was surprisingly clean and tight...and dead silent. Yes, it did have some lushness, but mostly it just sounded good to my ears. With my 650 plugged into the Xonar (a solid-state amp), the sound was obviously tighter and more detailed, but had a harder nature. Clearly, the tubes of the Crack did a great job of delivering detail and oomph while also softening the edges of the sound somewhat.
The differences between the Crack and the Woo were far less noticeable than the jump from the solid-state Xonar. Both tube amps were significantly softer and lusher sounding. After a while of listening with both the HD 650 and HD 800 some trends began to appear. Generally, the Woo seemed a bit more relaxed and even in its upper mid-range to treble response, while the Crack seemed a bit more forward in the upper-mids to mid-treble, and then seemed to roll off the high-treble more than the Woo. I would say the Woo WA3 was a bit more refined sounding but lusher, with the Crack seeming a tad hard in comparison. On the other hand, the Woo sometimes seemed to gloss over some of the micro-dynamics of a tune, while the Crack seemed a bit more punchy and controlled.
The thing about tube amps, though, is that all the characteristics I noted could easily change with a little tube rolling. While I'd certainly play around with various tubes on the Woo, I wouldn't be nearly so apt to start hacking into the circuit with capacitor, resistor, and potentiometer changes as I would with a Crack I built with my own two hands. In fact, with the Crack, I'm certain I'll be back inside messing around with the innards.
The Bottlehead Crack is a superb introduction into the world of DIY audio. The parts quality is very good, the documentation is truly excellent, the Bottlehead forums provide fantastic peer-to-peer assistance, and construction is very easy. The Bottlehead Crack is an outstanding first project for the novice audio DIYer. The big win, however, is that once you've finished your build, you'll have a fantastic sounding amp for your high impedance headphones. I simply can't recommend the Bottlehead Crack highly enough, if you've got a yen to try a little soldering, this is the way to go. You'll love it!
Bottlehead web site and Crack product page.
Bottlehead forums are a spectacular resource, go to the Crack sub-forum for questions and assistance building this amp.
A Bottlehead forum threads on suitable tubes for rolling and output cap upgrades.
Bottlehead Crack thread on Head-Fi.
Another great resource is the diyAudio Foums.
A good place to get upgrade parts for your Crack is Parts Connexion and Michael Percy Audio.
A good place to buy tools is Stanley Supply.
Here's a really cool review of the Crack at Headfonia.com, don't miss the pictues of his beautiful build at the end of the article.
For resistor color codes check the Wiki page here and the chart below.