In depth build writeup and upgrade writeup.

[TravAndAlex]

Hi all.  In a bit of an attempt to give back some of the info I’ve picked up from all the knowledgeable folks around here, I recently wrote a build article https://www.headphonesty.com/2019/03/building-the-bottlehead-crack-headphone-amplifier-a-comprehensive-guide/and an upgrade article https://www.headphonesty.com/2019/04/upgrading-bottlehead-crack-headphone-amplifier/for Headphonesty. Admittedly it all reads as a bit of a love letter to the Crack. A friend asked me to put a Crack kit together for them, so it seemed like the perfect opportunity. I intended the articles to be fairly comprehensive guides and reference points for builders and modders.

Anyways, if anyone feels I’ve missed anything important, please just let me know.  Thanks and cheers!

[Paul Birkeland]

The UF-4007 going from 19 to 21 looks like it could short against terminal 20.  The setup in the manual has that diode on the other side of the terminal strip to avoid that potential issue. 

"While it can provide high voltage, the combination of the Crack’s high output impedance and the headphone impedance limits the amount of current it can provide. This creates a couple of compromises:"

The limit of available current is a function of being an OTL amp.  You can only run so much current through a single triode, and if you fix the amount of current, then you make significantly more power into a 300 ohm load compared to a 30 ohm load.  The output transformer steps down voltage and current increases in the process.  With many other design issues set aside, we could double the operating current of the 6080 and barely decrease the output impedance.  To decrease the output impedance would require more transconductance in the output stage, which is why you sometimes see OTL amps with two 6080s, as this drops the output impedance substantially. 

On the upgrades page, you have installed a counterfeit Alps Blue Velvet pot (Alps RK27 pots only come with solder pins).

I like those black box film caps you've selected.  It looks like it was sooo much easier to get them glued down to the chassis compared to a cylindrical cap.

[Doc B.]

Thanks for the very nice writeup. I will make one small suggestion - don't start with 60 grit sandpaper on the wood base! That will eat it alive. The bases are very well milled and I seldom start with anything more coarse than 220 grit.

It was neat to see the reddit post by JR about how the bases are milled. I was not aware of that post. It's funny that when we started using alder 20 years ago we got grief for it being "trash wood". Nowadays it's considered a great cabinet wood right up there with cherry. Need I even mention that Fender guitars are most often made of alder? I always thought red alder the perfect choice for a DIY kit because of the subtle grain and the ability to take just about any color stain you want to use. It looks great stained with mahogany, walnut, and even bright red or black stain. And it also looks nice with a clear finish, which develops a nice red undertone as the wood ages.

I'll also mention that JR can make you a custom base in lots of other wood types. We did a limited run in walnut a few years ago, that was a knockout.

[oguinn]

JR built a really nice 4U cabinet and a base for a switch I made at a totally reasonable cost. Would definitely recommend.

[TravAndAlex]

Thank you gents for the feedback! I will submit updates to the articles. 

[cddc]

nice writeup

[tim273]

I agree, nice write ups, I liked the upgrades one with the different capacitors and such.

I too purchased the power cord and have used it all along so I can't say whether or not it makes a difference, but it sure sounds good! Based on how you twist and braid the wires it seems similar Cat5/Cat6 networking cable where there's 4 twisted pairs each one twisted at a different rate (and then all the pairs are twisted).  The twisted pairs prevent crosstalk and interference for digital signals, but I can imagine it also helps with reducing power line noise and RFI. 

I'm sure there's more to it than that, but that's what it reminded me of.

[jtori]

Thanks for the very nice writeup. I will make one small suggestion - don't start with 60 grit sandpaper on the wood base! That will eat it alive. The bases are very well milled and I seldom start with anything more coarse than 220 grit.

It was neat to see the reddit post by JR about how the bases are milled. I was not aware of that post. It's funny that when we started using alder 20 years ago we got grief for it being "trash wood". Nowadays it's considered a great cabinet wood right up there with cherry. Need I even mention that Fender guitars are most often made of alder? I always thought red alder the perfect choice for a DIY kit because of the subtle grain and the ability to take just about any color stain you want to use. It looks great stained with mahogany, walnut, and even bright red or black stain. And it also looks nice with a clear finish, which develops a nice red undertone as the wood ages.

I'll also mention that JR can make you a custom base in lots of other wood types. We did a limited run in walnut a few years ago, that was a knockout.

This last bit about JR being able to make custom bases is great info.  I've contemplated asking, Doc, whether you might consider offering different wood options for the bases.  Thanks for sharing this info.

[tvr2500m]

Very nice. I read through it all. I went light with the color palate for my Crack, and I also quite liked the dark palette - darker woodwork, dark chassis plate, dark transformer.

Northwestern Alder a junk wood? That's too bad. So many types of wood, so many lovely rich characters of each. I simply oiled and waxed mine. It's darkening it's hue and grain beautifully as it's aging. Yes, a bit like cherry.

In the upgraded build, the Cree diode upgrade is installed on its PCB. Is the PCB available somewhere? Partsconnexion had them, but in-stock/available now? Yes, this could be easily breadboarded, but I feel the PCB looks more tidy.

[ianp]

Partsconnexion had them, but in-stock/available now?

Available now - https://www.partsconnexion.com/PCB-77727.html - images of the top and bottom of the PCB.