Originally Posted by TwoEars
What's the mainline like compared to your modded crack? Or the zana if you've heard it?
Also - what's the main difference in sound character between OTL and SET really? I really haven't heard enough tube amps to know.
Hey TwoEars - sorry for the much delayed reply! First, just to briefly clarify "OTL" refers to a tube amp without output transformers (or - output transformer-less). SET refers to "single ended triode" tube amps, which means it is using a triode tube type and is not a differential or push-pull amplifier. Most of the commonly discussed OTL headphone amps are SET designs - such as the crack, woo 3, woo 2, dark voices, etc. These amps almost all use a 6as7(g) or 6080 tube, in series with high uf caps, as an alternative to output transformers. It is a pretty clever design where - in order to lower the output impedance enough to run high impedance headphones - the signal is actually pulled off the cathode of the tube, rather than the annode. The 6as7(g)/6080/7236/5998 family is really the only option if you want to accomplish this, so that's what all of the above-referenced amps use. Still, the resulting output impedance is a bit on the high side. When implemented well, OTL's tend to produce a full and pleasing sound (with high impedance phones - low impedance will sound anemic and flabby), but there is also going to be some bloom to the bass and likely roll off in the sub-bass.
The original zana deux was/is also an OTL, but of a different and costlier design with lower ouput impedance (and I would say objectively better sounding - very fast, detailed and dynamic). I greatly enjoyed listening to this amp at CanJam, and a family friend had brought an unblemished vinyl copy of HDS701 (see: http://www.tnt-audio.com/sorgenti/vta_method_e.html ) along that we played on craig's system. It was one of the best sounding setups I heard all day. Of the cheaper OTL's, crack comes the closest to an original zana to my ears, but contrary to what some others have suggested in the crack thread, I do not think even a hotrodded crack truly rivals a zana. The new zana deux se, by contrast, uses output transformers, and I have not heard that.
The mainline is a highly regulated, c4s loaded, single gain stage, SET amp, with output transformers, and a big step up in all respects from the crack to my ears - notwithstanding that crack is a good sounding amp in its own right with hd800's. In short, mainline is more refined and detailed, faster, more dynamic, transients much cleaner, and bass far tighter and with greater impact. Trebles are beautifully extended but never strident, which makes it a perfect match for hd800's to my ears - better than a Zana to me (which, like crack, can occasionally manifest some treble sharpness), but reasonable minds can certainly differ once you're in the realm of these sorts of flagship products. As to the trebles and general sound signature, failed engineer discusses the same thing in his post a dozen or so pages back in reference to the ECP L-2 which is very similar under the hood to the mainline. To the best of my recollection, there was nothing I heard at CanJam that I liked as well with hd800's as the mainline (including the zana, WA5, and the $7k TTVJ 307A). I confess I have not heard the zana se, balancing act, or DNA products. If you'd care to read more of my ramblings about the mainline, I've posted some impressions in post nos. 43 and 48 here: http://www.head-fi.org/t/662240/mainline-new-bottlehead-premium-headphone-amp/30 Loquah has also written an exhaustive comparison of his impressions of the bottlehead mainline, crack and s.e.x. here: http://www.head-fi.org/t/683012/bottlehead-amplifier-discussion-comparison-thread-crack-sex-mainline