Even though the Botttlehead Crack is the best known tube headphone amp from Bottlehead, it has its limitatations by design, it was designed for higher impedance headphones 120 + ohms.
From Bottlehead’s site:
"Two Bottlehead headphone amps have left an indelible impression on the headphone industry over the past few years. The S.E.X. amp opened the way for someone with efficient speakers to make the transition to the ever growing range of great headphones available, having the best of both worlds in one Single Ended Triode amp. Meanwhile the Crack OTL (output transformerless) headphone amp has set a new standard for exceptional sonic performance from high impedance headphones at budget prices.
Last year we retired the premium Smack headphone amp kit. There were a lot of really sophisticated aspects to that kit- adjustable output impedance to handle a wide range of headphones, both single ended and balanced headphone output, a single gain stage circuit with active loading and hybrid shunt regulation, and very good transformers. And very good sound – Doc B. thought it was clearly our best sounding headphone amp at the time. Unfortunately for Smack we revamped the S.E.X. kit and in doing so came up with an amp that pretty closely matched Smack sonically and offered enough power (2W!) to run tough cans like LCDs, HEs and K1Ks, that had become very popular. Smack’s lower gain and 1/10 the power output was a compromise in the name of greater resolution and speed and more quiet operation. And we somewhat compromised the kit with a very functional but standard stereo volume pot to keep the price down."
After having the Crack and then adding the Speedball CCS update, I have always wanted to see what Bottlehead would do with a TOTL headphone amp…so after a few years and many headphone amps I bought a kit and wanted to share the attributes of this wonderful amp as well.
The price is now $1299 for version 1.1 and if you dont want to build it, for $800 or so Paul Birkland will build one for you.
I have used headphones from 32 ohms to 600 ohms and there is more power in SE or balanced output to drive them very well, to painful levels. It has 2 RCA source inputs, which makes it nice to flip a switch and test 2 different source inputs. The kit includes a nice stepped attenuator vs the analog potentiometer of lesser cost units. They use good quality headphone output jacks, and you can switch from balanced to un-balanced with a switch as well as HIGH or LO Impedance.
This kit is just a little more complex in design and building level. If you have built a Crack then you probably have the skills to build a Mainline. The manual is excellent and clear with many photos. If you pay attention to detail when cutting wire lengths and installing mechanical items its pretty easy. Its just not a kit that you will put together in one short evening. I took 4 days total and it worked first time!
Compared to the Crack, this is a clearer amp, still euphonic to me, but its more like a reference amp and doesnt add the softness found with the Crack. I found the stepped attenuator really easy to get used to, and have never found it difficult to find a volume level that I wanted for any particular song.
So from a comparitive level this takes what the Crack does to the next level for me. Having other tube amps, parafeed etc…here side by side the Mainline was as good as if not better in most cases in overall performance. Some other amps had better attributes in certain areas, but none that would would want me to buy them.
It uses a 6C45pi pair of tubes, which there are no real substitutes so tube rolling need not apply here. The tube was picked for its exceptional linearity over a wide set of operating voltages. The ones that came with my kit were Sovteks, EH has both gold pin and non gold pin versions.