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. 

The Millett DCPP Engineers Amplifier

Another great project from the man himself, Pete Millett. The Engineer's Amplifier is a distortion canceling push-pull amplifier that makes use of low-cost TV tubes, namely four 6JM6s and 6CB6s. If you prefer the top cap look, you can substitute 6JN6s for the 6JM6s. The amplifier puts out about 18 watts in stock configuration, or well over a hundred with some tweaking (take a look at TubeLab posts especially). As seems to be the recent tradition from Pete, the board is large and bright red, and makes use of a compliment of Edcor transformers. The audio path is pure tube except for the CCS in the LTP tail, a few tube purists may take issue with that but they'd be missing out on a great amp ;)

There they are! Fairchild FQPF8N60C

Some nice concessions to us vintage parts lovers are the lovely green Koa Speers carbon film and traditional carbon comp resistors, the latter being used as grid stoppers. There's also some good old silver mica caps in there. I decided that I wanted this build to be unique, and had some interesting ideas in mind, including a nice Zebrawood wood base with an illuminated slot for the logo and a copper / antique copper powdercoat scheme.

Board top with ceramic tube sockets

Board bottom with resistors populated

I initially build this amp with Russian paper in oil caps:

Russian PIO

But eventually settled on AmpOhm aluminum paper in Oil.

AmpOhm aluminum foil paper in oil capacitors

Here's some final photos of the build:

Orangey Red LCDs

Cardas copper binding posts and Vampire RCAs

The illuminated logo was achieved by using a piece of perforated powdercoated metal as the base, then fastening the "Millett" brass logo I designed on the front. Lighting the logo from behind is a small circuit featuring 16 diffused orange LEDs that runs at 6.3V off the heater. The nice top plate with cool little perforations around the MOSFETs was made by Bob Collins (DIYAudio member PPP6l6gc) for a very reasonable price.

Some other fun little upgrades from the stock build were:

• Cardas Copper Binding Posts
• Vampire RCAs
• Kimber and Oyaide Wiring
• AmpOhm Paper in Oil Aluminum Capacitors

The sound is very detailed and engaging. Quite the opposite from the lush Dynaco ST70. Another recommended build from Pete Millett.


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

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. 

An Elma Stepped Attenuator Populated with Kiwame Resistors

Typically inexpensive kits come with a generic potentiometer to control volume, maybe something like an Alpha or an Alps Blue Velvet if you're lucky. A potentiometer is basically a is a three terminal resistor with a sliding contact. In a typical amp, the input signal runs directly through it and it references the signal to ground at a percentage determined by how much the pot is turned one way or the other. Pots are made using a resistive element formed into a circular arc, and a sliding wiper traveling over the arc. The resistive element is usually made from carbon, cermet or graphite.

If you skimp on the pot, you'll likely end up with something that may not be properly matched between left and right, you may experience static-like noise when turning it, and it likely won't sound all that great either. Really, a $14 Alps is the cheapest potentiometer I will go with on a normal-sized build. The Alps has it's own sound signature, and when compared to more expensive pots, you'll notice a nice difference. Many people swear by the TKD 2CP-2511 ($75), Noble AP25 ($25) and others; you really have to experiment to see where your preferences will lie.

There's another class of products for those overly concerned with taking good care of the input signal attenuation called stepped attenuators. Stepped attenuators can be set up in a series, shunt or ladder configuration through a network of fixed resistors and a rotary switch. There's an excellent comparison between these three configurations on DIYAudio by Arn Roatcap, Inc. A series attenuator (the most common) is the cheapest and has the fewest parts, it uses all resistors up to a given point as the voltage divider. The problem is the resistor noise is added up, the more that are in the way of the signal.

Series Attenuator - Image Copyright Arn Roatcap, Inc.

A ladder attenuator is in theory the best of the bunch, as it only uses two resistors at any one given time, this results in low resistor noise and good channel-to-channel signal level matching, it's the most expensive and largest of the bunch though.

Ladder Attenuator - Image Copyright Arn Roatcap, Inc.

Then there is shunt style, which is a voltage divider that is formed with one single value resistor, and one additional resistor selected by the position of the rotary switch. This style will vary input impedance so it requires one to consider the equipment it will be used with to avoid overloading the input signal source equipment.

Shunt Attenuator - Image Copyright Arn Roatcap, Inc.

Typically, a series stepped attenuator can range anywhere from a $25 cheapie on eBay to several hundred dollars. Ladder steppers require many more resistors and boards so the cost increases higher still. Some companies that make attenuators are Goldpoint, Elma, Shallco, Glassware Audio, DACT and Seiden. Elma, Goldpoint and DACT all seem to make use of similar rotary switch mechanics and are closely priced. Resistors may be full sized through-hole resistors or SMC. You can get them with the resistors presoldered or you can choose your own by figuring out the logarithmic scale you'd like to follow and how many dB of attenuation you want between each step. There are programs to help you decide this on tonnesoftware.com and Glassware among others. You don't really save much money going this route, but you can choose your own flavor of resistor this way and you'll likely be a little more proud of your completed build.



Here's an Elma stepped attenuator I have without resistors mounted:

Elma Stepped Attenuator

Here's one I populated with some Kiwame Carbon Film resistors:

Elma with Kiwames

It takes about two hours give or take to populate all these resistors and solder them in (in this case it was 48 resistors for 24 steps). You'll find that a stepped attenuator gives you a clearer and more detailed presentation than a low cost potentiometer. Is it worth the work or extra dollars involved? You'll have to be the judge of that ;)

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 modification posted below 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. 

A Simple Recipe for a High Quality Digital Coax Cable

The audiophile world has really changed in the past decade. Most of us have moved from a stand-alone CD player to a computer-based music server. Whether you use iTunes, Foobar2000, WinAmp, MediaMonkey or some other program, you really need a quality DAC to extract your music from your PC to your audio rig. The industry has caught on and there have been myriad high-end DACs being released in recent years.

Carrying your precious 1s and 0s to your DAC should be a quality cable you can trust. A good digital cable should have an impedance of 75ohms. Cable impedance is important in digital audio because the signal runs at high frequencies. Because of this, impedance needs be matched from the source to the cable to the load. If it's not, then your end up with reflections. The concept seems to be that some of the signal is reflected back to the source along the cable. If the source isn't 75 ohms, it will return the reflection to the load, creating a time delayed reflection. I've also read some essays on the matter stating that the coax cable should be 1.5 meters (appx. 5 feet) or longer to further minimize the possibility of this effect. 

It seems that there are plenty of opinions on the above that spark long debates running thousands of pages through hundreds of threads, so that's why I'm only going to touch on it and move on ;)  The important thing for the DIYer to get out of the above is that one should use 75ohm connectors and 5 feet of 75ohm coax cable when possible (unless it's a balanced AES/EBU connector, which is a different story). 


There aren't a lot of audiophile quality DIY Coax connectors out there, so you have to spend a little bit to get the good ones. Furutech makes the FP-3-117(R) Audio grade BNC connector which is attractive and rhodium plated. There really isn't much else out at the moment that falls between this and the pro-audio quality Canare connectors. I believe Vampire used to make one but the usual suspects (PCX, Percy) don't carry it. A pair of the Furutechs will set you back approximately $40 at the time of this writing.

Furutech FP-3-117(R)

To connect these two lovely coax connectors together will be some nice quality wire. DHLabs, a company that has been around since 1992, is known for providing audiophile quality parts at reasonable prices. I'll be using the DHlabs D-75 wire, which has a solid silver-coated center conductor to maintain uniform impedance. From DHLab's website: The use of a solid center conductor minimizes signal reflections, which are very important in digital data cables. This center conductor is encased in a very uniform closed cell foam Teflon dielectric. The result of these efforts is a cable that can provide excellent performance to beyond 2 Gigahertz ... In continuing with our policy of providing maximum value, the D-75 costs much less than its performance level would suggest. I would tend to agree. I'd imagine if other companies were selling the same wire, it would be at least twice the price.

D-75 Cutaway

Using these parts together is actually fairly easy. One strips the jacket, folds back the braided shield, trims away a small amount of the dielectric, then inserts the cable into the connector. The solid core center conductor is soldered to the center pin of the Furutech, and the connector clamps down on the shield for a solderless connection. 

Example of a solderless connection

So you're thinking to yourself, "those BNC connectors look a little strange, I don't think my DAC / CD Player / etc. has those". It is actually a little less common to have nice BNC females on your gear than a generic RCA female to accept or send a digital signal, but more and more new gear seems to be including BNCs now. If one isn't already mounted on your gear but has room to mount it, one of the Vampire BNC female connectors below typically works:

Vampire BNC Female

One can either mount it free-hanging and solder wire to it or it's a drop in replacement depending on certain PCB layouts. PartsConnexion carries these, and I'm sure other retailers do as well. If one would rather not futz with it, DHLabs still makes a nice RCA plug that matches the D-75.

DH Labs RCA-3

It's certainly possible to have an RCA-3 on one side and a Furutech BNC on the other depending on the application. Here's what my completed Furutech / DHlabs coax came out like:

Furutech / DHlabs Digital Coax Cable

And here's an example of an RCA to BNC using the aforementioned DH Labs RCA-3. 

RCA to BNC Digital Coax Cable

If you'd like to have your own custom Digital Coax Cable made, please contact Zynsonix. 


I hope you enjoyed this detailed discussion of a Digital Coax cable. Please note that this post is for entertainment purposes only and not intended as a walkthrough. The owner of this blog, Blogger, and all associated parties will not be held responsible if you attempt to recreate the above cable and damage yourself or your surroundings.