The Bottlehead Mainline

If you are not already familiar with Bottlehead, they make some fantastic audio kits, mostly tube-based, to power sensitive speakers and headphones. They have some other cool stuff too like a guitar amp, so check them out if you haven't already.

The good people at Bottlehead have a slightly unusual naming convention for their products... sex, drugs, and I'd imagine the rock and roll is when you fire up a build for the first time. The Mainline continues this tradition, and if you are ignorant to these references as I was, mean intravenous injection, which I surmise alludes to addictive sound quality... or something like that ;)

Stepping away from the marketing side of things, the Mainline is a headphone amp... and yes, I am building ANOTHER headphone amp. I'm beginning to think I might need to grow a few more ears. The Mainline should be good enough, refined enough, that I won't have to build another for a while... we will see.

The Bottlehead Mainline

The Mainline offers a balanced output for those of you lucky enough to have balanced headphones. If your headphones aren't balanced, you can contact Zynsonix Audio for the conversion, or just take the boring route and use the single ended output on the amp.

The Mainline has 3 tubes in total. A 12AU7 is used for power duty and two 6C45-Pi tubes take care of the signal. The signal tubes each get their own C4S board (often an upgrade on other Bottlehead kits) which provides an active load for the tubes and improves sound quality.

The output is a healthy 600mW with adjustable impedance. Doc, the head honcho at Bottlehead, has stated that the amp works well with the more power-hungry Audeze LCD series. It features the same volume attenuator as the BeeQuiet upgrade that Bottlehead offers on their top-of-the-line BeePre pre-amp. Using a pair of knobs labeled "coarse" and "fine", you adjust the volume. The coarse attenuation has 9dB steps (from 0dB to -45dB) and the fine adjustment has 1.5dB steps (from 0dB to -7.5dB).

The Bottlehead gear makes use of a gravity mount, which uses a wood frame with a rabbeted edge to hold up an aluminum plate, which all the parts are mounted to. It's a nice easy way to make a chassis if you don't have a metal bender and it looks nice too. I like to build my own with some decorative molding and then copper leaf it, which is a process where you take extremely thin sheets of metal and adhere them to the wood surface.

Copper leaf being applied

If you've seen my other builds, I tend to switch out some of the parts for my own personal favorites I've found over the years, namely Teflon tube sockets, Cardas wire, Kiwame resistors, and various boutique capacitors. That being said, the quality of the parts included with the Bottlehead kits is perfectly good, so don't feel like you have to swap things out.

Below is the initial mounting of the parts with a little bit of wiring done. The teflon tube sockets have blue PCBs mounted that give you some extra room for soldering. The blue wires are Cardas 23.5 x 2. The design calls for shielded wire, so the mesh shields are grounded to the chassis.

Additional progress pics below:

You'll note the large Jupiter beeswax flatstack 10uF capacitor mounted on the right. I initially was going to mount a piece of FRP material then mount the caps to that, however I ended up with a standoff and a capacitor mount instead, it offered a slightly cleaner look. The cap narrowly avoids touching the transformer and the clip on heatsinks, and the leads are just long enough to reach. The cool gold colored zip ties are from Michaels craft store.

Very precise mounting of Jupiter capacitor

I re-oriented the capacitor for the final build when I was satisfied with the fitment (outer foil faces inward on the right cap). These are big caps (big and beautiful!) so they needed a second standoff with capacitor mount to keep them nice and secure.

Final interior photo

After some listening tests with the resistor and the choke, I couldn't really tell a difference, but since I had the choke already, I went ahead and installed it. It fits so nicely in the corner. 

This is the final photo of Bottlehead's most sophisticated headphone amplifier to date (at least publicly available, maybe they have something spectacular going on behind the scenes).

The knobs, if anyone is wondering, are matte aluminum made by KILO. You can purchase them from Digikey or Allied Electronics. The copper colored tube shields I believe were from Angela Instruments.

Hope you enjoyed this post. Be sure to visit Zynsonix Audio if you need a great sounding headphone cable or some new interconnects to connect your Bottlehead gear.

The Fine Print:

The owner of this blog and Zynsonix Audio LLC do not have any affiliation with Bottlehead. Please remember that building/modifying circuits 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. 

DIYTube Get Set Go 300B Headphone Amp

It's been a little while since the last post, and there's a good reason for that; I've been gathering parts for a very special DIYTube Get Set Go made specifically for the Audeze LCD-2 headphones. The Get Set Go is a single-ended triode design made first-and-foremost for high-sensitivity speakers that only need a few watts to fill a room with sound, such as the Planet 10 Audio Frugel Horn. I did have the fortune to build the Get Set Go for speakers previously and was impressed with the overall tonal character and quality of the sound.

The Get Set Go Board from DIYTube

You may be wondering why someone would be using a speaker amp for a pair of headphones. A typical headphone amplifier will output relatively a small amount of power compared to the speaker amplifiers (in small fractions of a watt measured in milliwatts). The Audeze, as well as certain other planar magnetic headphones like the HiFiMan, really benefit from some added wattage to sound their best as they aren't the most sensitive drivers. Hense, using a low-powered speaker amplifier makes sense, but there are some changes that are necessary.

The biggest change is the output impedance. While speakers are typically 4, 8 or 16 ohm, headphones often range between 32 and 600 ohm. The Audeze headphones that the amplifier is being made for are 50 ohm (although I have also seen them listed as 60 ohm). Since 50 ohm single-ended output transformers don't really exist on the market, you can either have one custom wound, or try and find something that would have an equivalent turn ratio. There's a handy calculator that Shannon of DIYTube recommended that will give the equivalent turn ratio for this purpose. A 3.5K / 50 ohm output transformer would have a turns ratio of 0.12, or 8.367:1. Gery at Transcendar offers a 1.6K / 16 ohm single-ended transformer, the TT-023-OT, which is a 10:1 ratio and close enough for our purposes. Per Shannon, The reflected load would be a touch more linear and lost max output would be minimal.

Secondly, learned from previous experiences like with the Millet Jonokuchi, and to a much lesser extent, the ECP Audio Torpedo, headphone amplifiers do not like their output transformers to be close to the power transformer. When they are too close in proximity, there is something called flux interaction when the magnetic waves interfere with one-another and cause an audible hum with certain headphones. So not only will the output transformers be moved to the opposite side of the chassis, but rather than using an EI transformer for power, a toroid will be used instead as they create much less of a magnetic field. Antec is one of the few manufacturers out there that builds certain toroids specifically for tube amplifiers. The Antec AS-2T300 would be a good substitute for the PA774 with a bit higher ratings.

So other than the transformer choices and their position, this build will be fairly similar to the previous Get Set Go. To make things a little different, 300B tubes will be used rather than 6B4Gs. Also Mundorf Silver Oil capacitors will be taking the place of the Jupiter wax paper caps. The Mundorfs should be a little more detailed, which the Audeze can benefit from with their warmer/darker presentation.

The first action-item on the list was the chassis. I always like to create a virtual mock-up of the chassis layout to see different set-ups and settle on what I feel will work the best. Note that in this layout, the output transformers are distanced from the power toroid and chokes.

Virtual Chassis Layout

Once this is settled, I forwarded the dimensions over to Keith (ebay seller po1019) to fabricate the chassis. The virtual layout is then used as a guide to lay out the parts, take measurements and make markings, and ultimately start drilling / cutting away. After prepping a number of aluminum and steel chassis, I have a small collection of very handy Greenlee punches, which are ideal for making clean holes for tubes and sockets. The metal area (which will be behind the little red tab on the Neutrik locking plug) to release the headphone plug was recessed a bit by sanding using a Dremel and by hand for adequate clearance.

The prepped aluminum chassis

I decided that this chassis would have a more unique and artistic finish. Something more worn and rustic looking. I was actually inspired when I was scrapping off the dark grease on an old baking sheet. This finish starts with some black engine enamel that is then sanded and scrapped by hand minutes after spraying. It produces and interesting texture in my humble opinion.

Textured Bottom Plate

Texture Detail

The texture is achieved by hand using worn out sandpaper at different times of paint dryness, along with the sharp point of a scissor to get the longer lines. Once the texture was where I wanted it, I gave the chassis two coats of clear satin enamel. The parts can now be dropped in place. I began with the smaller pieces, including the headphone sockets, which were flush mounted and secured with black oxide screws. The switch, pilot light, RCAs, binding posts, IEC, Teflon 300B sockets and hole grommets were all installed next.

Part installation on the chassis

Next comes the fun part, the population of the PCB. Varying from the BoM, we have Elma Silmic II caps, Nichicon FineGold caps, Kiwame and Mills resistors and Mundorf Silver/Oil Caps. Slight changes to the circuit includes the Wima 0.1uF snubber cap and a pair of Solen 630V 5.6uF bypass caps in the power section.

The populated Get Set Go Board

On the bottom of the board, at Shannon's suggestion, we have a few of the Mills power resistors to help distribute the heat a little better. 

Bottom of the board

Now comes the long process of preparing the wood side panels. I decided on an antique silver leaf finish for these pieces. Since there is no off-the-shelf option for antique silver leaf that was readily available, I'm using some Speedball brand silver leaf pieces and some Old World Art brand leafing and antiquing materials.

Leafing Materials

Below are the wood side panels for the chassis before any prep.

Wood Chassis Panels

The first step in the antique leafing process is a red basecoat. This is typically used with a gold foil, so we'll see how it performs with silver. Below is the wood with the basecoat applied.

Red antique basecoat

 After two basecoats and a layer of adhesive size, the wood is covered with the silver foil sheets. This is a very unusual process, the foil is incredibly thin and will tear apart if it catches your fingers, so you have to be delicate. each foil sheet is laid down with a little bit of overlap.

Silver foil sheets laid in place

The foil is then burnished with a cheesecloth to have it form the curves of the wood. This is a messy process and the foil flakes will end up everywhere. They are so thin that you'll find them floating in the air. Once this is complete, a layer of sealant is used to protect the foil. 

Silver foil after burnishing

The antiquing kit included an antique glaze that is made for gold so it's a brownish hue. It didn't look right on the silver. Instead I chose to rub it down with powdered graphite before sealing it again. Then two layers of satin clearcoat were added for additional protection of the finish.

More to come

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 AMB Mini³ (Mini Cubed) Portable Headphone Amplifier

The AMB Mini³ is a cute little portable headphone amplifier designed by Ti Kan of AMB Laboratories in 2007. The size of the amplifier when completed is approximately the size of a pack of cigarettes, or perhaps a stack of credit cards ;) 

The AMB Mini3 Schematic

Available from the AMB Laboratories website is the PCB board, a few selected parts, and pre-drilled panels that fit the Hammond 1455C801 or1455C802 enclosure if you'd rather not make them yourself. I'm not a big fan of the unanodized rim around the FPE (Front Panel Express) panels so I'll be drilling them out myself, but if one doesn't have access to a drill press, the pre-made panels would save a lot of effort. 

Because of the size of the board, one has to mount some very miniscule parts, including 1/8 watt resistors and some SMD opamps. SMD stands for Surface Mount Device and means that the device or component is not through-hole, so it is soldered on the side of the PCB that it is mounted on. Most modern components are made by machines using SMDs, but when soldered by hand, care is required. As explained on the Curious Inventor website, solder flux is first placed on the component legs, then solder is added to the tip of the iron, then the solder is carefully added, please view the video here. 

AMB Mini³ PCB, the size of a credit card

Because of sizing constraints, one really can't drop in boutique resistors, capacitors or other such items, but one can manage a little bit of customization. I chose to replace the 1% metal film resistors with 5% carbon film as carbon films can tend to sound a little more natural. I purchased about 10x the amount of carbon films that I needed and measured them with the multimeter, keeping only the ones that fell within 1% and matching them on both the left and right sides. This is much more obsessive than one needs to be, but I like drawing out projects a little bit longer to get more enjoyment out of them ;) 

My populated AMB Mini³ Board

The Mini³ has a built in battery recharge circuit that runs off a wall adapter. The battery is a very tough fit in the Hammond case. I had to file down the batter contact pins, remove the battery wrapper, sand down the plastic around the battery AND file the top of the inside of the case and it all just barely fits. I chose the 9V CTA 325mAH NiMH rechargeable battery that was recommended, perhaps the latest models are a little larger than before. As you can see below, the fit is quite tight. I also re-snipped some of the soldered leads at the bottom of the PCB to ensure they wouldn't short to the case. 


I wanted to make my Mini³ unique, as there are many out there that have already been built by headphone enthusiasts on HeadWize and HeadFi, so I chose to dremel out a vent area (purely for aesthetics) and have the case powdercoated in a textured antique copper. AMB has a nice set of printouts that you can use to drill the holes in the front and back panels, I simply lined them up, taped them on and used the drill press to make the holes. The vent area was quite a bit more work, requiring four drill holes to be placed in the corners, a cutting disc to cut the area between them, and a metal file to de-burr and even up the cutout. 

Chassis Prep

Once this was complete, I trimmed a small piece of perforated metal and used a permanent adhesive to attach it in place. The grill area is colored with a bright copper powdercoat to provide an accent to the antique copper chassis.

Unit Front without panel on, input, output, LED and potentiometer

Unit back without panel on. LED, power inlet and battery

Completed Mini3 unit with brass hardware and aluminum knob

Antique copper textured powdercoat

One might wonder why I chose to build a solid state amplifier since nearly everything on the Zynsonix blog is tube-based. There are a couple of tube-based portable amplifiers out there. Todd the Vinyl Junkie developed one a few years ago and there's the DIY Oatley Electronics K272A for a mere $27 that people seem to like. I haven't tried either of the two units, but as tubes tend to be fragile and microphonic, I'd be a little concerned about them banging around in my pocket for extended periods of time, even with damper rings. The AMB Mini³ offers a nice warm sound without the tubes, so it is highly recommended in my book. 

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.