April 2011

Single Ended eXperimenter's Kit for Headphones And Efficient Speakers
Enjoying superb S.E.X.!
Article By Jeff Poth

Difficulty Level

  Yes indeed folks, I am a very SEXy man. I am the happy owner of a lovely Single Ended eXperimenter kit amplifier, from Bottlehead Corp. I built this some years ago, but it's still an active product and long overdue for a review. My particular unit uses Magnequest upgrade iron, and then sold as a standard package direct from Bottlehead but now you order separately from Bottlehead and Magnequest. Thank you VERY much to my Bottlehead friend in Jersey, who generously provided the kit some years ago.

What's a Bottlehead?
Tubes are affectionately referred to as "bottles" in some circles; hence a Bottlehead would be a tube aficionado. More specifically it refers to the Bottlehead Corp, a Washington based hi-fi kit supplier. They really made their name with the "Foreplay" preamplifier kit, a minimalist 12AU7 linestage that was initially a whopping $99 (now, the third iteration costs quite a bit more), and didn't even include a chassis, only a top plate (where all the hardware mounts, the rest of the chassis is just a platform for the plate). There were other early products, including the first iteration of the Single Ended eXperimenter's kit, but the Foreplay is really what made them a "mainstream" supplier in a specialized subset of hi-fi.

Oh, and the Foreplay is also one of the things that got me off my butt and behind a soldering iron. I learned a huge amount building, modifying, and adapting the Foreplay to my rig. This taught me a lot about layout, assembly, and minimizing noise. My early rig had this pre running into a sensitive Acurus amplifier with moderately (91ish) sensitive B&Ws. Being an idealist I didn't want to pad the output (the easy way to help a circuit like this with signal/noise), so chased it out of the circuit via the power supply and build details. Thanks to VoltSecond and the other prolific bottlehead types who helped me with some of these things.

The Good S.E.X.
The current iteration of the SEX amplifier kit is a 6DN7 based integrated amplifier, with a single input, a volume control, and a headphone jack. The 6DN7 is one of a family of octal socket dual triodes, with dissimilar sections. In the SEX one is used as a gain stage, and one as an output stage. They are cap-coupled. This arrangement is fairly minimalist, though the Parallel Feed (aka Parafeed in bottlehead circles) arrangement adds an extra choke and capacitor to the circuit. The upside of this is that you get to keep DC out of the OPT (OutPut Transformer), and thus can use an un-gapped OPT. A parallel feed OPT can have better performance in several key ways, including higher primary inductance and a smaller overall package for fewer parasitics.

As far as amplifiers go, circuits don't get much simpler than the SEX amp. It'd be typical to see similar circuits the driver stage for a larger tube, like an 845, rather than driving a loudspeaker via OPT. One of the major compromises of this simplicity (and the small size, and low cost) is that the amplifier is only good for just over a watt.

WATT?!?!
Yep, it's a baby. This is not going to drive Paradigms in a home theater, or make deep bass with nearly any loudspeaker. It will, however, drive the right loudspeaker to a very reasonable level, and headphones are no problem so far as power is concerned (with a few watt-sucking exceptions). Mid to high 90s efficiency are a requirement for even reasonable levels and rooms. Lucky for me, I'm constantly building and as such got to try this with a number of different arrangements. I used it with Rethm "The Second" cabinets, loaded with Fostex FF225k (a superb driver, but can't give much output, similar to a lowther but without the last edge in transparency and dynamics OR the nasty "lowther shout") with a ribbon supertweeter. This arrangement was excellent, very quick, clean, and smooth, but naturally limited in bass extension and maximum output. I always use a sub or two anyway. With my current 2 ways, the SEX amp is serving fine duty, with its limits having been a non-issue thus far.

Building It
The kit comes with a thorough illustrated instruction manual on CD, labeled parts, and is extremely easy to build. An experienced builder will get through the soldering and assembly in a few hours, but however much attention to finishing you wish to take will largely define how long the project takes. I'll not belabor the details, it is a beginner-appropriate project and should not give anyone much difficulty. The layout is simple and easy to work with.

There are also upgrades available. Mine of course utilizes the C4s (constant current sources) upgrade along with better capacitors and upgraded transformers and inductors, available from I assembled, used a simple wipe-on oil finish on the base, and had ‘er singing over the course of a week or so (finish had to cure). I used a hammered finish for the top plate and power transformer.

Sonics
Within its limitations this is a superb little amplifier. I compared it to a Transcendent OTL-SE, and a Fi Super X running 45s. It's, not surprisingly, more like the Fi than the OTL. The Bottlehead didn't quite reach the qualities of the Fi with 45s, which is a much pricier amp, particularly with the Emission Labs 45s I preferred. The amps were very close, and the Bottlehead had the slight edge on resolution, but the 45s sounded fuller and a fair bit more dynamic. This shouldn't be surprising- a good 45 amp is very difficult to beat for dynamics, so long as the tiny power is sufficient. The Bottlehead does have a headphone output that the Fi lacks, and the headphone output is superb. It's also extremely quiet, beating the Fi in that regard as well. To my ears the Transcendent beat the SEX in speed and resolution and seemed to be the better overall amplifier to my ears, but my good friend preferred the SEX for it is more full-bodied presentation.

Sounds negative eh? Not really. The S.E.X. is a SUPERB amplifier, extremely quiet, has great tonal definition and imaging, and a listen all day quality that's very beguiling. The other amplifiers are also excellent, and anyone with an extreme high efficiency system would be well served trying all three- they represent 3 different takes on the flea power amplifier and any of them is a truly high-end amplifier and a bargain due to the reasonable prices made possible by extremely modest power requirements. Using 2A3s in the Fi changed the package a little bit, giving the SEX a little more resolution edge, but having better loudspeaker compatibility due to 3dB more output available. If 1.25-1.5W got you there, I'd take the Bottlehead over a 2a3 loaded Fi X (both used the excellent Magnequest Iron). The ratings below reflect this, as used in an appropriate system. A fleapower amp will never truly do Sub-bass properly, hence the low rating. The rest are 4/4.5, and deservedly so. Value is a 5. There are, of course, better amplifiers than this and the Transcendent or Fi. But they are typically hugely more expensive (these three range from a $699 or so Bottlehead kit (with upgraded iron, caps and constant current sources at the time of building) to a $1350 to $1500 range for either of the others pre-built (the Transcendent is available as a kit, with tubes, for $1000), and the three listed above all are very close in quality, and I'd expect three different rankings of the amps from three different people... or even on three different speaker sets. But I'm the reviewer and this is where the chips fall. Highly recommended!

Specifications
Type: Stereo single-ended vacuum tube kit
Tube Compliment: Two
Price: Kit $469, fully assembled version adds $295.
Stock kit is designed for 120VAC mains, those needing 240VAC add $25 

Manufacturer
Bottlehead
Voice: (206) 451- 4275
E-mail: queen@bottlehead.com
Website: www.Bottlehead.com