Bottlehead Crackatwoa OTL tube headphone amplifier build

[L0rdGwyn]

Well, technically, objectively, it is, if the numbers you suggest are true. You might happen to like that distortion and coloration, and there is nothing wrong with that, but by the criteria of accurate reproduction of the original signal and "high fidelity", it would be a poor showing.

It's possible they are better, the 0.5% is based on the Crack plus Speedball. In that case, what point is there in measuring? The best case scenario is a review concluding with some variation of "the distortion is too high, the output impedance is too high, but it sounds good subjectively."

Who knows, maybe we could be surprised.

 

[L0rdGwyn]

I completely understand your point of view. I used to have a Bottlehead Crack and really liked it. I am fascinated with the whole objectivist vs subjectivist debate and my personal opinion is that there is a use for both neutral and slightly euphonic amps. My personal preference is for slightly euphonic amps (Lyr 3, Valhalla 2, Bottlehead Crack) for pure enjoyment and more neutral amps (JDS Atom, Magni 3) for music recording and gaming.

Why I am interested in measurements of the Bottlehead Crack and any other tube amp is to see if it is possible to figure out what specific distortion gives them their specific sound profile. I am against the whole "it must suck if it does not measure well" approach as there are many amps that do not measure fantastic but sound great - after all, musical and sound tastes are quite subjective. Ultimately, while we can measure a lot of sonic properties at this point in time, it is impossible to argue that we have exhausted all the possible things that can be measured when it comes to being able to realistically record and reproduce a recorded sound. If this was the case, we would not be able to tell the difference between a live performance and a recording of it reproduced through a transparent chain.

This was going to be a project of mine, to build a modest at-home measurement setup and characterize the distortion profiles of the sound that I enjoy most. I still just might, although I am short on free time these days, perhaps in a few months.

 

[L0rdGwyn]

This is a beautiful amp and project! I hope I'll have the skills to make something like this in the future. So the polish surface isn't that durable and prone to scratches. What would you do differently if you're making a new one? Have you considered the Bottlehead S.E.X.? Am intrigued but the cost of importing a Crackotwoa is a bit too high as I'm in Canada. P.s.: Please ignore the naysayers the hobby is about fun not about who has the best measuring amp imo.

If I were to do it again, I would likely investigate having a shop chrome the top plate rather than sanding/polishing/buffing myself. I think the chrome would be more durable, but otherwise I am happy with how it turned out. I am aware of the S.E.X., although I have not heard it. My primary amplifier these days is a "Glenn OTL" or "GOTL", which will only be familiar to those who spend any time on Head-Fi. I am planning to have the same designer/builder of that amp build me a SET amp in the near future.

 

[L0rdGwyn]

As a fan of science and statistics, I want to see everything measured. With things like the Bottlehead, it's more of a hobby and aesthetics and steampunk looks. If someone sold you a black Chinese box with the exact specs and sound, nobody would buy it. It's the Ikea effect, a cognitive phenomenon that anything you spend time with, engage and interact with, gains love and affection to the user. You built it, you customized it, it sounds amazing.

But that doesn't make it objectively good to someone else, and it's not something you can sell to someone that's made up their mind already based on other factors. That being said, I sure as hell want to see it measured just to see what it's truly capable of.

It is great that you are a fan of the sciences and that you have found a way to channel that excitement into your hobby. I would suggest you not be so antagonistic, nothing positive comes from it.

I think if you heard this amp or one similar to it in quality, you would be very surprised at how transparent they can sound with the right tubes, despite the measurements. Or who knows, maybe because of them? That is yet to be seen.

 

[L0rdGwyn]

I have some other other tubes to do some rolling with, but at the moment I am using a Russian Winged C 6H13C and USA CONN marked 12AU7A.

The 6H13C no doubt is a great performer for the price. I am sure you have heard, but the Tung-Sol 5998 is one of the best power tubes for the amp. You can also use 6SN7 tubes in the 12AU7 slot with an adapter, which I highly recommend trying.

 

[bunkbail]

I think you are making some pretty agressive and hostile assumptions there. How about an open mind? Yes, if the measurements turn out poor, it probably will be ridiculed, but are you assuming it will?

Dude, it's an OTL tube amp, it's obviously going to be measure like shit.

 

[SIY]

Dude, it's an OTL tube amp, it's obviously going to be measure like shit.

Possible, but not certain. One can engineer an OTL tube amp with excellent performance.

 

[bunkbail]

Possible, but not certain. One can engineer an OTL tube amp with excellent performance.

Can you point me to an OTL amp that boasts a distortion figure lower than 0.001%?

 

[Veri]

The 6H13C no doubt is a great performer for the price. I am sure you have heard, but the Tung-Sol 5998 is one of the best power tubes for the amp. You can also use 6SN7 tubes in the 12AU7 slot with an adapter, which I highly recommend trying.

Back when I had a BH crack I had a pristine 5998 and it noticeably affected the bass, which is odd since I'd not expect much from a power tube swap. It was too expensive a tube to keep though so I sold it for a neat price

 

[SIY]

Can you point me to an OTL amp that boasts a distortion figure lower than 0.001%?

Can you point me to a headphone that boasts a distortion figure lower than 0.001%? In my mind, that kind of number is a fun thing to play with from an engineering standpoint, but five or ten times higher is still more than good enough in terms of actual excellent performance.

 

[L0rdGwyn]

Back when I had a BH crack I had a pristine 5998 and it noticeably affected the bass, which is odd since I'd not expect much from a power tube swap. It was too expensive a tube to keep though so I sold it for a neat price

The only other legitimate measurements I have seen of the Crack used an Audio Precision SYS2722 from a Bottlehead forum member and showed the distortion essentially halved when using a 5998 vs the stock 6080 power tube. This was on the Crack + SB though, not the Crackatwoa.

I had planned to send Amir the stock tubes as well as a set of better tubes, including a Tung-Sol 5998 since it is a more popular choice and will dramatically drop the distortion. Unsure if he will want to measure both, otherwise I will advise him to use the 5998. The subjective performance increase is significant.

I had also thought of sending him some uber rare 6SN7 tubes, just not sure how into tube rolling Amir is

 

[Veri]

The only other legitimate measurements I have seen of the Crack used an Audio Precision SYS2722 from a Bottlehead forum member and showed the distortion essentially halved when using a 5998 vs the stock 6080 power tube. This was on the Crack + SB though, not the Crackatwoa.

I had planned to send Amir the stock tubes as well as a set of better tubes, including a Tung-Sol 5998 since it is a more popular choice and will dramatically drop the distortion. Unsure if he will want to measure both, otherwise I will advise him to use the 5998. The subjective performance increase is significant.

I had also thought of sending him some uber rare 6SN7 tubes, just not sure how into tube rolling Amir is

Oh wow so that explains what I heard awesome.

 

[SIY]

I had also thought of sending him some uber rare 6SN7 tubes, just not sure how into tube rolling Amir is

You may want to save a bit of the effort- see Chapter 3 of Morgan Jones's "Valve Amplifiers." I've repeated some of those measurements using a somewhat different test setup and gotten similar results. I've also gotten data on the 5692 variants (which he didn't have). Bottom line- except for a couple of carbonized variants (like the CV1988), there's not a large amount of variation from type to type, a dB or two here and there. The carbonized ones show significantly lower 2nd harmonic by 6 or 7 dB.

 

[L0rdGwyn]

You may want to save a bit of the effort- see Chapter 3 of Morgan Jones's "Valve Amplifiers." I've repeated some of those measurements using a somewhat different test setup and gotten similar results. I've also gotten data on the 5692 variants (which he didn't have). Bottom line- except for a couple of carbonized variants (like the CV1988), there's not a large amount of variation from type to type, a dB or two here and there. The carbonized ones show significantly lower 2nd harmonic by 6 or 7 dB.

Thanks, I will read into it. I do have a very nice pair of carbonized Brimar CV1988, I'll ask Amir if he is interested at all in measuring one. I will most likely send him the quietest 12AU7 I have on hand that I think sounds the least distorted and subjectively "best". That would probably be a Brimar CV4034. The stock tubes are rarely used in these amplifiers, so I don't necessarily think they have to be what are measured, but I will ask his preference. Don't know how keen he will be on measuring multiple tube configurations.

 

[bunkbail]

Can you point me to a headphone that boasts a distortion figure lower than 0.001%? In my mind, that kind of number is a fun thing to play with from an engineering standpoint, but five or ten times higher is still more than good enough in terms of actual excellent performance.

Now you're moving the goal post smh.

 

[SIY]

Thanks, I will read into it.

Worthwhile. You'll also see his measurements on 12AU7 variants in the same chapter- they are largely... not good, by which I mean "an order of magnitude worse distortion."

 

[L0rdGwyn]

Worthwhile. You'll also see his measurements on 12AU7 variants in the same chapter- they are largely... not good, by which I mean "an order of magnitude worse distortion."

I don't doubt it, that would align with my listening impressions. I can send an example of each, but it will be up to Amir which he would like to utilize in the review. I don't necessarily think measuring the optimal performance of the amp is a bad thing, but the circuit was not intended for 6SN7 originally. Heck, I could send him an adapter to use a pair of 6J5 as well

 

[SIY]

I don't doubt it, that would align with my listening impressions. I can send an example of each, but it will be up to Amir which he would like to utilize in the review. I don't necessarily think measuring the optimal performance of the amp is a bad thing, but the circuit was not intended for 6SN7 originally. Heck, I could send him an adapter to use a pair of 6J5 as well

I find it interesting that a year or two after I publish something, key aspects of it appear in these products. Expect to see something with the SYclotron circuit topology any day now.

 

[Simas87]

Hi @L0rdGwyn , I have noticed Feliks Audio Espressivo Mark 2 in the background? How does it compare to Crack-a-Two-a?
I have Beyerdynamic T1.2 and need an amp to go with it. Kinda not sure which route to take - Beyerdynamic A2, Feliks Audio Espressivo Mk2, Icon Audio HP8 Mk2, or to build Bottlehead kit. The best option would be to listen to all of them however, there are none within 1-2 drive away from where I live.
P.S. I like what you did your amp, finish looks awesome, well done!

 

[L0rdGwyn]

Hi @L0rdGwyn , I have noticed Feliks Audio Espressivo Mark 2 in the background? How does it compare to Crack-a-Two-a?
I have Beyerdynamic T1.2 and need an amp to go with it. Kinda not sure which route to take - Beyerdynamic A2, Feliks Audio Espressivo Mk2, Icon Audio HP8 Mk2, or to build Bottlehead kit. The best option would be to listen to all of them however, there are none within 1-2 drive away from where I live.
P.S. I like what you did your amp, finish looks awesome, well done!

View attachment 94340

Hi there! IMO, the Espressivo was pretty disappointing from a sound standpoint. Very nice build, but it was outshined sonically by the Bottlehead Crack + SB as well as the Crackatwoa. I am a proponent of DIY, so I would say build a Bottlehead kit