Bottlehead Crack: OTL Addiction


Both me and Lieven love output transformer-less amps, often known as OTL amps. On amplifier with output transformers (OTs), the transformer at the output end of your amplifier is often likened to having an extremely and excessively long headphone cable to connect your headphone to your amp. We all know that the longer the path that the signal has to travel, the greater the degradation (imagine the length of the wire needed to build one transformer). This is why good output transformers are extremely expensive (think thousands of dollars for top grade OTs), because they are extremely crucial to the overall sound quality.

Despite their limitations, short signal paths and simple designs tend to be very good at things like soundstage and three dimensionality. Even with high end output transformers, it’s still an extremely long signal path, and an OTL design is always going to yield a cleaner sound. Of course, the limitations of OTL amps are their low power output, and with speakers that’s always a problem because people want to drive big speakers to fill up big rooms. But with headphones, especially high impedance headphones like the  Senns, OTL designs produce plenty of power.

The Perfect Amp for the 300Ω Senns

In the past, people tend to say that the 300Ω Senns and the 600Ω Beyers are extremely difficult to drive and that they require high power amps to shine. I don’t think this is true. Yes if you’re trying to drive them from your portable devices which only goes so loud. But pair them with a simple Cmoy amp, and you realize that these headphones are very easy to drive (high impedance headphones need high gain, and high gain is easy to achieve. low impedance headphones need high current, and that’s more difficult to do properly).

The Crack is an OTL design, and me and L had high hopes for it for our Senns. I was not disappointed. With both the HD650 and the HD800, the Crack delivers one of the cleanest sound, the blackest background, the widest and deepest soundstage I’ve heard on any amp below the $1K mark. Yes, that includes the famous WooAudio WA6 or its SE variant. Of course, the Crack probably can’t handle low impedance headphones as well as the OT-equipped WA6 or WA6SE, but that was never the design objective with the Crack anyway. In a way the Crack is similar to the $2,300 Zana Deux amp, being both of OTL designs and relatively limited to high impedance Senns (though both the Crack and the Zana also very well with Grados). They can’t handle the big heavy orthodynamics, but pair them with Senns and you’ve got a combo that’s really hard to beat.

The Crack sounds extremely clean that it’s just sublime to listen to the HD800 through it. It’s cleaner than any solid state amp I know, and being tube-based, it has that liquid smooth sound that you just can’t get with solid state. One of my reservations with the WooAudio 6 is that despite its very smooth sound, the soundstage only goes wide but has very little depth. The Crack is slightly less wide than the Woo 6, but it’s depth is far superior, resulting in the best three dimensionality I’ve heard under $1,000. The background is very black, every instruments very distinct. Excellent dynamics, though a bit short on the micro details.

The Crack doesn’t stop there. It takes that superb technicalities and combine it with a mildly dark, full mids and full lows sound signature, and I think I’ve just found one of the best amps for the Sennheiser HD600/650/800 series. And I’m only listening to the stock build version with no boutique parts or Speedball upgrades. My friend Drew, on the other hand, builds his Crack with premium parts, a DACT stepped attenuator, complete with the Speedball upgrade and some really fancy tubes. I don’t think I want to listen to it. I enjoy my Crack just as it is, and I don’t want to be tempted.

Other Headphone Pairings

What else can I say? The Crack is also beautiful with AKG’s K550, Alessandro’s MS-Pro, Sennheiser HD598, and ATH’s M-50. So I don’t think you have to be paranoid with low impedance cans, as long as they are dynamic drivers with relatively high efficiency. It doesn’t drive any of the Hifiman orthodynamics I tried it with (HE-400, HE-500, HE-6). Well it does to some moderately low volume level, any higher and distortions will come. It’s also beautiful with IEMs, provided you don’t listen at low volume levels because the gain level is quite high on this amp. I enjoy my Shure SE215 with it. I think the micro dynamic driver on the Shure is one of the most resolving drivers around, and when you pair it with something so clean and spacious like the Crack, you get pure bliss. With something like an Etymotics ER4-S and a good high quality recording and DAC, I don’t even want to tell you how it’s going to sound.

End Words

With the recent price increase, and even if you pay Bottlehead to build one for you, I think the price is still relatively good. In fact I don’t think you can find an amp with a better technicality than the Crack for less than $500. If you build one yourself, that means only $279, and the Crack is a steal for that price. If you happened to order the amp before the price increase (like me and L did, plus the 10% bulk-buy discount), consider you’re practically robbing Bottlehead for the sound that you get.



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Bottlehead Crack: OTL Addiction
3.86 (77.14%) 7 votes


Lieven is living in Europe and he's the leader of the gang. Coming from a musical family he's always been interested in good sound. Unlike his family members the only musical instruments he plays are amps and DACs. He loves playing with old tubes and discovering new products while staying faithful to the good old Sennheiser HD650.

  • where i can get this if i’m from indonesia?

    • L.

      You could ask Analog Head in Jakarta, I think they were planning on selling it. If not it will have to be directly from Bottlehead in the USA

      • thanx L..i will wait till analog have it to sell it

  • where i can get this amp at indonesia?

    • ryan

      Probably you could ask mike to build one for you 🙂

      • @ryan79:disqus no no no…

        • ryan

          What if the money is good? 🙂

          • L.

            I doubt Mike can be persuaded 🙂

            • ryan

              Bottlehead charges $150 if I remember correctly. Mike should charge $300

              • L.

                Bottlehead stopped doing it

                • ryan

                  Another reason why Mike should build it for $300

                • Trent_D

                  Actually, I think they will still do it, but I believe that they would be charging an hourly shop rate of around $65. So, even if they build it in an extremely swift four hours, that would still be $260.

                  • L.

                    I’m sure there are some DIY’ers in Jakarta that want to build one 😉

                    • I really enjoy the building process actually… it’s just that I’m not sure I can commit to a reasonable time frame, unless he would be content with a 12 months timeframe. 😉

  • Steve

    Can anyone recommend me a DAC for the Bottlehead Crack with speedball and HD650’s?

    • Steve,
      The HRT MS2+ or the DAcport LX works very well for $300 level DACs.

    • L.

      I usually use the Fiio E10 when I use the Crack + HD650

      • Bradford

        Is this still your goto? The Crack + HD650’s are a lot of $$ together. Can I get by with the E10? Or must I splurge on the MS2+? I see the Modi has good reviews for around $100. The Audioengine D1 Premium is also under $200.

        • I don’t know about the Modi but the Fiio E10 or E7K works pretty good for the HD650. The Crack however is on a totally different level.

  • Best review I read on the cracks..Thank you

    • You’re welcome!

      Sent from my mobile device. Sorry for any mistype.

      • I couldn’t just decide whether I could build it or if the stock crack would have been “good enough”. Well, speaking of the Stock version it is very enjoyable and I bought one after reading this and Tyll’s review. Are you planning to do a review of the Crack with the Speedball upgrade soon?

        • L.

          We do have a guest review coming up that will cover the crack with and without the speedball (in combination with the HD650) But I can’t tell you how soon we will get this

          • Trent_D

            As long as the voltage checks go fine, I suck with a multimeter, the speedball will be added this weekend, so I would like to say two weeks on the review. Detailed DIY work doesn’t become me with my big sloppy butcher’s hands.

    • L.


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  • Hariadi Mastoyo

    I like the sound of my PS1000 just the way they are but I don’t mind adding a bit of a warm tone to them. I am tempted to build this amp myself but I wonder the quite low impedance (32ohm) of my Grado might not suitable with Crack. I also considering ALO PAN am. Which one is better match? I really appreciate some suggestion here. Thanks a lot guys!

    • It’ll be good if you can come and test things out really. The PS1000 is not a very common headphone and so I have very little experience pairing amplifiers with it.

  • netmask254

    Can some crack owner help telling the 3-dimension of the finished box? Posted on Bottlehead forum but no response. Thanks a lot!

  • Arya

    Mike, I like to build crack, but I’am green with any electonical and soldering. Is it really easy to do it? Oh, what kind of lead that you used for soldering your Crack? Is it the same as lead for ordinary electrical PCB?

    • Dave Ulrich

      If you go slow and follow directions, it isn’t that hard. If I can do it, most people should be able to manage.

      • Arya

        Okay. Thank you Dave.

      • True.

    • Soldering skills is a must have for building the crack.

      What lead? You mean the internal wire? They supplied their own solid core wire which is very good and easy to solder.

      • Arya

        Oh, I mean the copper you used to solder, Mike. I’m sorry for my previous word, my friend said “lead” instead of copper. Hahahaha

        • You mean the tin?

          I use whatever I have. We just finished a new built using Oyaide tin and it does sound better than my first build.

  • Andy Kurniawan

    Hi Mike and L,
    A bottlehead fan here, can u tell me what tube you prefer with the crack?
    I use speedball plus tungsram e80cc or valvo e80cc with tung sol 5998, iknow that doc prefers the mullards but i have no experience with it.

    • L.

      Wasn’t that mentioned. Stock power and 12AU7 RCA clear top for me 😉

      • Dave Ulrich

        I like the RCA 6AS7G for the power tube and the current production tung sol 12au7.

        • Andy Kurniawan

          I read about the new tung sol production in Russia. Basically they bought the brand name and stick the label on their own tubes. Are they any good?

          • L.

            I wouldn’t say they’re terrible but they’re not close to the NOS tubes

          • Dave Ulrich

            I have found this tube in particular to be excellent. It was recommended over on the bottlehead forums awhile back. With the RCA 6AS7G, I would take it over the Clear Top for sure (I have a clear top, as well).

    • I prefer STOCK tubes by far.

      So much I like the Stock tubes that when the stock tube broke I tried really hard to pry it from another Crack owner’s hands but they won’t sell me. Eventually I got another stock tube from Bottlehead.
      And I’ve tried LOTS of different tubes with the crack. None of them has the bass and the soundstage depth the stock tube has.

      • Andy Kurniawan

        Is it just me, because I noticed noise even when there’s no music playing with the stock driver. It’s gone when I switched to e80cc.
        I don’t even know what my stock tubes are, all the markings gone on my stock tubes.

        • L.

          Well that’s what tubes can do unfortunately

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  • Linuels

    Mike and L,

    I’m planning to purchase this or the S.E.X. 2.1 for my HD 650. However, I don’t have any soldering experience. Can you provide me other pre-built desktop amp that has the same performance/sound signature of the OTL crack even if it is more expensive?

    • L.

      Lafigaro 339 or 336, Woo Audio WA3, Woo Audio WA2

      • Linuels

        Thanks mate. I will look these amps.

  • Igor Sheykin

    Hello! Can you give short summary for the devices and source used to build Crack? Used sort of tin, length of wires, …
    I mean if you tried different variants, and one is definitely better than others.

    • L.

      Just use what you can get and follow the instructions. Should be good 🙂

      • Igor Sheykin

        Thanks. Just have to wait till it arrives to Germany.
        Here I have one more small question: if compare T90 and HD700 with Bottlehead Crack as amp, who would win? Who pairs better?

        • L.

          I dont think there is a better. They both perform exceptionally good with the Crack. Just choose the headphone which sound suits you most

      • Igor Sheykin

        Important question: what do I need to make it?
        soldering-iron, tin, pliers, wire cutters, ???

        • L.

          yes, all of that

  • Sergio Mejia

    Hi, I have two questions, could help me please

    What Fiio device would result is the best combo for HD700 + CRACK Bottlehead, basically fulfilling the function of a DAC?

    You better just be a DAC or ideally buy a Amp / DAC to also fulfill the function of portable amp?