Crack;Bottlehead OTL
Mar 9, 2010 at 3:56 AM Thread Starter Post #1 of 12,127


500+ Head-Fier
Jun 1, 2003
Maybe the subject heading is a little confusing for folks that aren't used to Doc Bottlehead's tendency for naming his DIY kits a little differently than the run of the mill audio company.

Doc's just announced a new OTL headphone kit called "Crack". If you've ever been interested in developing some soldering skills and learn about what's going on inside of a tube amp, Bottlehead kits are a great place to start.
The fantastic part about this all tube design is the whole kit just costs $200!

While I haven't heard this amp yet, I do know Doc pretty well and nothing that isn't pretty special gets past the prototype stage.

Here's a link to the details.

Crack OTL Headphone Amplifier Kit
Mar 9, 2010 at 4:54 AM Post #2 of 12,127
Dang you. I already have a S.E.X. amp that is HEAVEN with my HF2s. I bought it used. It'd be interesting to build my own...

Too bad we didn't have one at the last meet in Burlingame to check out.
Mar 9, 2010 at 7:42 AM Post #4 of 12,127
I know what you mean Uncle Eric! I need another tube headphone amp like another hole in the head but it's such a nice simple design that it begs to be modified.

One thing I really like about Doc's kits are the instruction manuals. Not only are the easy to follow but they usually explain a lot of the operating principles.
The Bottlehead forum is killer too since they have a forum for each current production product so you can share ideas and the results of any mods.

Bottlehead Forum - Index
Mar 9, 2010 at 4:47 PM Post #6 of 12,127
Looks great. I might be ordering one soon!
Mar 10, 2010 at 1:19 AM Post #7 of 12,127
Man, that is an enticing amp. But like some of you I need another amp like a hole in the head!
Mar 10, 2010 at 1:47 AM Post #8 of 12,127
Same here - just can't do it, but this is VERY cool, and a great place to point people who want to build an amp for cheap.

Interesting he says he "thinks" you can use a 6AS7 - the 6080 and 6AS7 are electrically identical, so you CAN use a 6AS7. The others are in fact different to varying degrees from the 6080/6AS7, so indeed maybe they would not work. I've owned a lot of 6AS7-based amps, and all of them have tolerated the 5998, since it actually draws slightly less current than a 6AS7G. The 6528, OTOH, draws twice the heater current of a 6080/6AS7G, so that might be a no-no here. But IME it's a crappy sounding tube anyway.

Sorry for the tube digression. This looks like a cool offering, though!
Mar 10, 2010 at 5:10 PM Post #10 of 12,127
Hi Skylab,

Yes of course you are right, the 6AS7 and 6080 are interchangeable. The context of that sentence in my product description is a little shaky, isn't it? And you are correct that the 6528A won't work, my flub. I gotta remember to wear my readers when I look at those tube manuals!

This puppy was done partly as an exercise to see just how quickly and efficiently we could get a project from the hand waving phase to the production prototype phase, and consequently there just hasn't been enough time to try everything in terms of tube types and circuit mods that we would like to try.

I'm happy to answer questions anyone might have about how we're running the prototype, but I don't want to wear out my welcome here with what might be perceived by some as a bunch of unsolicited marketing hype. I'll document the upgrades we develop as they happen over on the Bottlehead Forum. IME Head-Fi'ers are a super bunch and you are all most welcome to come visit.
Mar 10, 2010 at 5:16 PM Post #11 of 12,127
Hi Doc

Any thoughts on cutting the top panel for an octal socket and shipping with one of those little adapters & noval sockets like on the foreplay?

I have not seen internal pics, are they up anywhere?

Edited a second time:
The photos are up on the bottlehead site :facepalm: Great news, there is room for smallish OPT's
Mar 10, 2010 at 5:30 PM Post #12 of 12,127
This is perfect...I'd been really wanting a Bottlehead kit, but couldn't justify springing for the SEX yet. And was sitting on a pile of nice 12AU7's since I sold my EF1. And have several high-impedance headphones.
Mar 10, 2010 at 7:08 PM Post #13 of 12,127

Originally Posted by nikongod /img/forum/go_quote.gif
Hi Doc

Any thoughts on cutting the top panel for an octal socket and shipping with one of those little adapters & noval sockets like on the foreplay?

I would like to have added that potential for mods. A good part of the reason we can put this kit out for a reasonable price is that we were able to employ an existing power transformer (designed by Bottlehead's chief engineer, Paul Joppa, and made in the US) that we have made for us in quantity. That big ol' 6080 heater sucks up a lot of current and the power trans is running pretty much at the design limit current-wise with the addition of the 300mA heater draw of the 12AU7. We felt that the additional 300mA current draw of the most likely octal candidate, 6SN7, would tax it.

Also the addition of the adapter would raise the kit price and, though it doesn't always make sense, sometimes one is at the mercy of marketing psychology when it comes to choosing a price point. In the current economic climate I have been reminded of Bottlehead's roots as a maker of bang for the buck stuff.

That said, a person could probably figure out a way to enlarge the existing hole if they had an octal candidate in mind with a 300mA heater current draw. We have a CNC machine here in the lab and we might be able to rework a few of the laser cut plates, but the machine is so busy running as an engraver for our other business that I don't foresee being able to reconfigure it anytime soon. Another option is to have a custom plate made by Front Panel Express. We use them for all of our prototypes.
Mar 10, 2010 at 9:03 PM Post #14 of 12,127
Based on the output explanations I assume this otl design not very suitable for low impedance phones like k702, isn't it?
Mar 10, 2010 at 9:12 PM Post #15 of 12,127
There are a couple of threads going at once about this kit in different forums. Forgive me if this seems redundant to anyone following both threads - in the other one I mentioned that yes, this is really intended for high impedance cans, but it will work with low impedance cans though not optimally. I also mention that we have another kit with a low output impedance that works very well with low impedance headphones, so I felt we could afford to do this second, more focused design for the high impedance phone users.

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