OT vs OTL

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.