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March 17, 2011

The Bottlehead Single Ended Experimenters (S.E.X.) Amplifier

If you've read my other posts, you'd know that I enjoy building Bottlehead gear. I have both a Bottlehead Crack high impedance headphone amp and Seduction phono preamp, both with the obligatory C4S upgrades. The Crack is excellent for high impedance cans (~300 ohms), but can't really handle the low impedance ones. The solution was to build a S.E.X for those more demanding headphones. ;) 

The S.E.X. kit
is a stereo integrated single-ended-triode amplifier with 2 watts per channel output and has been available since 2004. From the Bottlehead site: The two stage zero feedback RC coupled circuit uses parallel feed (a.k.a. parafeed) output, a topology championed for use in modern single ended amplifiers by Bottlehead and MagneQuest. Parafeed output assures articulate, musical bass, great high frequency extension and minimizes the influence of the power supply on the sound. 

The Bottlehead S.E.X. Kit
There are a few special upgrades available for the S.E.X., including the C4S Constant Current upgrade which consists of a pair of small PCBs with mounted MOSFETs, LEDs and resistors (currently $45), and some higher quality MagneQuest transformers and chokes ($300-450 depending on nickel content). And of course you can throw in the nicer caps and such as well.

Building the amp is the typical Bottlehead affair; there's a wooden base to assemble and stain, a top plate made of aluminum you can paint or leave bare, and a nice big instruction manual that makes things easy for beginners. Because my build included the aforementioned upgrades, I had three manuals to look at.

My first concern was the MagneQuest transformers. They may be well made, but they aren't much to look at in my humble opinion. The chokes hide beneath the plate but the outputs would be visible. I sanded down the metal frame, taped off the center with painters tape, and spritzed it with metal primer, copper hammerite paint, a layer of clear coat, then more hammerite paint. It gives it almost a leather looking texture when the clear coat reacts with the hammerite. I then wrapped the center with some thin black leather (Doc's advice). You can see the difference below between the original look (left) and the modded look (right).

Magnequest choke, transformer and stock bell
I got the plate back from the powdercoater with my favorite antique copper finish, so the next step was to begin populating the plate. I had made a few enlarged holes for the Neutrik locking jack, Cardas RCAs and binding posts, Goldpoint attenuator and vintage lamp.

Teflon 8 pin sockets and boards, Goldpoint attenuator

The Goldpoint attenuator required a little bit of work to fit. In the stock hole position, it butted up against the wood frame and the plate couldn't be seated. The hole had to be enlarged vertically to allow clearance. A couple of the little solder strips required mounting to #8 screws to be enlarged. I recommend one takes their time if they use a drill for this, as I broke two of them :P

Further population included a few little tweaks to the stock kit, including rubber washers for the transformers and chokes, shielded Cardas output wire, bypass film caps for the power supply caps, a few nicer Nichicon caps, thicker shielded wire for the power switch, another solder strip for the Neutrik locking plug to keep the installation clean, and some big hulking paper in oil AmpOhm caps, which required the addition of a set of 2" standoffs. Below are a few progress pics of the amp being built. 

Getting Started

Additional solder post used for headphone output

Solen caps placed temporarily for resistance checks
One thing about the build that was a little difficult was soldering the bottom of two of the power supply caps. There were three or so things going into the top hole of terminal 4 with that little solder strip which made it quite difficult to ascertain whether you had a good solder joint or not. With a flashlight a couple minutes of investigation, it could be seen. 

C4S boards fitted

The little C4S boards got in the way of the standoffs for the large caps, so I flipped them 180 degrees and dremmelled into the unused part of the board for clearance. It's almost comical how large the AmpOhm caps are at a mere 2.2uF.

Finished inside - Front

There's a lot of parts shoehorned in here

Finished inside - Back

There's a lot packed into this little chassis, every inch counts ;) I originally had a set of EAR feet on the bottom of the wood frame, but the caps protrude far enough that I had to get larger feet. I initially entertained the idea of some little brass spikes, but the company selling them wanted $80 for a set of four 1" x 1" threaded brass spikes. I picked up a set of 1" black steel spikes and 1" rubber feet (both under a dollar each) from PartsExpress. I ended up liking the black spikes better, so I drilled out the bottom of the wood frame and threaded it for them.

This amp lets it's business hang out ;)

Finished Back

Finished Front
The emblem on the front I had custom made (I had two made in total). I created the logo and sent it off to a promotional company that makes the little metal emblems. 

UPDATE (3/22/11) I got to have a listen to this yesterday, Eileen got me a replacement set of RCA tubes, and I had a pair of Sylvania coin-bases arrive from another supplier. Sound is really excellent, even before burn-in. No lack in the bass department, a nice powerful sound with just enough detail.

I'm currently using a very special custom 10-3 right angle power cord with this kit (my own recipe). Take a look at the details here.

Wondering where I got all my parts for this build? Check out a list of my favorite online vendors here.

UPDATE (3/29/11) Replaced the stock sandcast 0.1 ohm 5W resistor with a Vishay Dale wirewound in the power supply. Can't tell if it made a sonic difference, but would likely be a little more reliable over the years.

The Fine Print:
Please remember that building circuits and performing circuit modifications can be dangerous to you and/or your surroundings and should only be performed by a certified technician. The owner of this blog and all associated parties can not / will not be held responsible if you attempt a build or modification posted above and cause physical harm to yourself or your surroundings. Many electronics contain high voltages that can kill, and mods, if performed improperly, can be a fire hazard. Please keep this in mind. 


  1. The vintage emblem is so nice. Could you tell us who made that for you?

  2. Thanks Coop. I designed the vector art for the emblem and sent it to a friend of mine in the promotional products business. He's since changed jobs, so I'd imagine I'd have to start from square one to find someone who would make more of them. That being said, the cost was much higher than expected, and I doubt I would ever do it again ;)