Korg HA-K1 Portable Tube Amp with NuTube

Flash back to mid-2017 and you may remember the super cool "NuHybrid" Headphone Amp that Pete Millett designed around the Korg NuTube. If you're not already familiar with the NuTube, the current model is called the 6P1, which is a thoroughly-modern, dual, very low power, directly-heated triode tube. The NuHybrid used the 6P1 as a driver tube for a desktop headphone amp with very good results.

Back to October 2018, Korg has not been resting on its laurels, and has released a couple of cool new DIY designs. Both make a lot of sense given the low power requirements of the NuTube: a portable headphone amp and a guitar pedal. Some of you may say "wait a minute! the NuHybrid was sensitive to vibration, I can't imagine it bumping around in your pocket or getting stomped on during a performance". I was thinking the exact same thing, however I'm pleased to note that Korg has completely eliminated this concern in the new portable headphone amp (the HA-K1) by mounting the Nutube off the main board and using a modicum of foam to dampen things.

Being the ravenous DIYer that I am, Pete was kind enough to send me an HA-K1 to play with. Included in a relatively non-descript box are the instructions in Japanese and English, several baggies with parts, the enclosure, a sticker, and a PCB which is semi-populated with SMD parts. This is similar to Elekit's portables that knock out some of the steps. I personally don't mind as I don't enjoy futzing with teeny tiny SMD resistors.

Pre-populated PCB bottom

Pre-populated PCB top

The case resembles an Altoids mint tin case in both size and material, which is commonly the enclosure choice for a CMoy. The top is a dark piece of translucent acrylic which has the labels for the controls printed on it. In the parts baggies are a variety of Nichicon caps, trim pots, standoffs, switches, a smaller PCB for the NuTube to mount on, and a couple of op-amps.

The HA-K1 kit parts

The tin case and acrylic top

The instructions discuss the order in which the PCB is populated, and refer to a numbered bill of materials. There was an instance that I wasn't quite sure what the BoM was referring to e.g. "connector", but since there are so few parts to worry about it's pretty easy to just visually check the board and confirm. A couple of the parts need to be visually distinguished, like the codes on the side of the different value trim pots, and the solder eyelets are a little on the small size and close together, but there is nothing particularly difficult about the build.

I would recommend having at least decent solder skills or plan to use some extra flux to ensure you don't bridge any eyelets accidentally. I also found myself using a solder sucker a little bit to tidy up the joints, but I'm more OCD than most ;) The entire build took about an hour and a half, and I like to take my time.

I also needed to whip up a short mini to mini cable as surprisingly I did not have a spare.

3.5mm mini to mini cable 

Once the build is complete, there are a host of fun little things to tweak like the internal gain, the load on the anodes, a bass boost switch, and choosing which op-amp you prefer. I found that I liked the load on the anode a little over halfway and the volume a little under halfway turned. I tried both op amps, liking the basic JRC for alternative rock with more electric guitars (overdriven guitar sounds more raw) and the Muses 01 for the indie rock I prefer which is more electronic and vocal based (smoother, more balanced presentation). The bass boost I liked better for poorly recorded rock that lacked body, but was a little aggressive with already bassy music.

Bottom of amp with stick-on footers

Top of completed unit

So bottom line, is this piece of kit worth your time? If you enjoy tube sound and want it on the go, I'd say definitely. Overall the sound is very full and engaging, warm, dynamic and non fatiguing, just the way I like it. It's a little more lush than amps like the Chord Mojo and iFi xDSD. It is also probably the lowest cost portable tube amp on the market that is properly implemented and doesn't suffer from any microphonics what-so-ever.

If Korg releases a future (or alternative) model, I'd like to see a built in USB DAC so we can bypass the phone's internal amp, but for the price and the sound, it's a good value.

Pros

• Fantastic sound, warm and engaging
• Nice portable size and attractive case
• Good quality internal parts
• Sound is highly configurable to user taste
• Well-designed PCB screen print eliminates confusion
• Quick and easy to build

Cons

• No built-in DAC model yet
• Requires at least decent soldering skills
• Bass boost adjustment requires opening amp
• Tin case prone to dings

I'm of the understanding that the only American distributor at the moment is Pete Millett, and they are available via this eBay page. For full disclosure, I have no business affiliation with Korg or Pete Millett. The review sample was provided to me for evaluation / review purposes.

The Fine Print: 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 and/or your surroundings. 


 

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 Budgie SE 3W Single Ended Amplifier

Our good friend Shannon Parks over at DIYTube has designed another great tube-based amplifier, this one using a pair of EL84 power tubes and a single 12AX7 driver tube.The Budgie has been designed from the ground up to be an affordable build, the power and output transformers (sourced from Edcor) run a measly ~$100 or so dollars for all three! Throw in the parts ($35), tubes ($35), PCB ($39), and build your own chassis ($20), and you should be able to scrape by under $250. Granted, this would be a bare bones build, but it just goes to show you that this is an affordable design. You can go hog wild and add Shannon's top custom printed top plate and a Hammond chassis with walnut sides, and maybe some boutique coupling capacitors, and we're still looking at a reasonable price tag for a single ended tube amplifier.

Budgie SE with custom printed top plate and walnut sides

The DIYTube Budgie SE Schematic

I chose to skip the top plate as I enjoy building my own chassis (it prolongs the whole building process which I find inherently enjoyable anyway). I had a scrap piece of copper which I trimmed to 7" x 12", which is a little larger than the custom printed PCB and will allow a little more air between the components. The base would be an intricate mitered wood frame made from chair rail molding from the hardware store. 

The copper plate was cut, punched, drilled, Dremeled, etc. to hold all the parts in place. The layout used to plan the initial plate size was used to determine where each hole would fall. Small holes were drilled using the drill press, larger holes for tubes and the Neutrik locking plug were punched using GreenLee punches, the extra-large holes for the capacitors were cut using hole saws on the drill press, and square holes, like for the transformer and IEC inlet were cut using a Dremel metal cutting disc. The copper is a little denser than aluminum and requires a bit more effort to cut through.

Once the chassis plate had all the holes necessary to accommodate the parts for the build, it was sanded down, starting with 80 grit sandpaper and ending with 600 grit. Next, a coat of clear satin finish was sprayed on each side to protect the copper, otherwise it tends to pick up fingerprints and oxidize. I've had good luck with the clear satin enamel from Krylon. I tried the Valspar outdoor clear satin and it clouded up and flaked off, so I won't be using that again. Gloves needed to be used after the sanding process so my fingers wouldn’t leave any oil on the copper. Once the clear coat was dry, parts could finally be fastened onto the plate.

Copper Plate with myriad holes

First the various connectors and jacks could be installed. The RCA females are nice quality Philmore gold plated units with a Teflon dielectric. The binding posts are generic heavy duty gold plated brass with clear plastic on top, made to look like WBT's early binding posts I assume. Also installed is the ubiquitous Neutrik 1/4" TRS locking jack, which I elected to install with the release up front for ease of use, the IEC inlet with built in fuse, and a small toggle switch for power.

The Budgie gets some cost effective connectors

Populating the PCB were a number of parts I had lying in the parts bin, some Koa Speers resistors (green) from Mouser, and some spiffy Elna RFS Silmic II electrolytic capacitors from Digikey. Both the Koa Speers and Elna Silmics are considered to be "warm" sounding parts (the opposite of bright or harsh), which is usually what I'm going for. The big blue caps are 150uF Cornell Dubiliers that have good ripple current ratings, which will help smooth the power supply a bit in addition to the Triad choke called for in the design. In addition, I added some surplus Nichicon 0.1uF film caps bypassing the power caps, which would help ripple to some minor degree as well. I also found some oddly shaped Mundorf EVO Aluminum 1uF film caps that I installed on the side of the board. These were relatively low cost and a bit more interesting to look at than a standard film cap.

Top of the PCB with Tube Sockets, Film Caps

Bottom of the PCB

The PCB could now be fastened to the copper plate with short aluminum standoffs.

Directly beside it would sit the 1H Triad choke which I pulled from an ST70 when I upgraded that choke to Classic Valve's heavier duty model.

Sitting directly beneath the output transformers are a pair of matched vintage Russian silver mica capacitors. If a manufacturer was making these today, I'm sure they'd be pricey, but given that they are old military surplus, they're downright affordable. Some audiophiles feel these sound comparable to the Russian Teflons, so I figured I'd give them a shot. They are certainly interesting looking little metal brown boxes.

Check out those Russian silver mica caps!

With all the small parts in place, I could secure the Edcor power and output transformers. I purchased some unpainted bells from Edcor (I'd imagine these are popular products on their page for anyone who doesn't want blue) and had them powdercoated with an antique copper color, then replaced the nylon washers and the nickel hardware with brass for a more uniform look. These were secured to the copper chassis with rubber washers to dampen a bit of the vibration they produce.

You can always hide a depth mounted screw under an xformer

Since a copper plate cannot sit on its own, a wooden base was fashioned out of some decorative molding from the hardware store, the edges rabbited with a router so the plate could sit on top via "gravity mount", and then mitered to be a perfect fit for the plate. Once the four pieces of molding were secured together with woodglue and allowed to dry, I went through the process of copper leafing and antiquing the wood. If you'd like to see more details on this process, please check out my Bottlehead Crack Coppermine project.

Completely wired up

And the photos of the finished unit:

Front

Rear

I had a listen with a pair of Sennheisers and really enjoyed the quality of the headphone output. I'll be testing the unit with some speakers shortly. Looks like Shannon has another winner on his hands!

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. 

A Customized Dynaco ST35 Kit

The ST35, also known as the Stereo 35, was released by Dynaco in 1963. It's the baby brother of the Dynaco ST70 and tends to be overlooked by many vintage amplifier aficionados. I think the main reason this is the case is that there isn't a large price difference between the two units, and with a tube rectified power supply, power choke, bias system and twice the wattage, it's hard not to want to step up to the ST70. For those of us with relatively sensitive speakers, the ST35 is a nice little unit with a small footprint and more than ample power (17.5 watts x 2). I've also read from more than one source that the ST35 is the best sounding of the Dynaco units stock, for whatever that's worth ;)

Because there are a lot less ST35s floating around, there aren't really any boards with modern circuits that fit into the original chassis. Shannon Parks of DIYTube has created a brand new circuit board that is similar to the ST35 circuit with some modern sensibilities if one would like to go that route. I prefer the looks of the original ST35 chassis myself, with the exposed PCBs on the ends. For this, I turned to Dynakit Parts. Dynakit Parts is a company based in Clifton, New Jersey and has an impressive selection of old school reproductions of Dynaco products. The kits are about as close to the originals as you can get, from the can capacitor to the screw-down terminal strips.

Dynakit Parts ST35 Kit

The original Dynaco ST35 Schematic

Rather than purchasing the whole kit, I purchased a number of the parts a la carte, as I was interested in adding a little bit of customization to the kit with a power choke, power switch, triode / ultra-linear switches, a C8 power inlet for a custom C7 removable power cord and a number of boutique parts. I also added the ST35-BCU bias control upgrade kit which fits nicely above the chassis. As stated on the Dynakit site:  The ST35-BCU employs an adjustable fixed bias circuit which allows for user adjustment of the bias setting of each (EL84) output tube via (4) separate on board precision bias pots. This allows you to not have to purchase a matched set of power tubes. And even if you did purchase a matched quad, power tubes tend to age differently, so they can be adjusted after this occurs. 


Kevin of Dynakit Parts was working on a brand new brown PCB board for the ST35 when I contacted him. I really appreciate it when a company isn't resting on their laurels but investing in new products and innovations. The new PC-13 boards from Dynakit is made from aerospace grade 240 degree Centigrade brown Polymide mil spec material and features silk screened component designations. The brown will go very nicely with the vintage patina powdercoat scheme that I had planned. I wish the bias board was made of the same nice brown material, but we can't have it all, can we? ;)

Brown Polymide boards with a few parts in place

I elected to populate the small driver / power boards with Kiwame carbon film resistors, Obbligato and Mundorf Supreme film capacitors and Teflon and ceramic tube sockets (the Teflon sockets even have beryllium copper contacts). The caps fit pretty much perfectly, although the Mundorfs will have to be mounted once the boards are mounted due to sizing constraints. The Dynakit build calls for nicer silver mica capacitors in place of the ceramic disc capacitors that were in the older units. These can be found just about anywhere (Mouser, Digikey, Partsconnexion, Handmade, etc.). 


I chose to go with the cadmium metal chassis as opposed to the newer stainless steel version as the chassis is going to be powder-coated; no need to pay extra for that chromed-out finish when no one's going to see it ;) Below you will see the chassis, I've enlarged the holes to fit the Cardas binding posts and RCAs, and the center hole has been enlarged to hold a piece of FR4 material that will then hold the C8 inlet in place. I then gave the chassis a nice sanding and labelled it for the powder-coater. 

A few chassis mods and a bit of sanding prep

One thing I would be painting myself was the power choke. A choke is a nice cheap upgrade for the ST35 and helps reduce ripple in the power supply. A number of people have used the C354 to replace the 50 ohm resistor, I chose a Hammond 156R with 56ohms DCR, which is pretty much the equivalent. Chokes tend to be on the ugly side, so if they aren't hiding in the chassis (there's absolutely no room in this case), they need to be dressed up a bit. I was prepping two at once, one for the ST35 and one for the Millett Jonokuchi. 


The first step in this process is to sand down the metal frame on the choke. Then painters tape is cut and applied to mask off the inside. I've seen some builders just paint the whole thing, but I don't really care for that look. 

Chokes masked off with painters tape

Next I apply two layers of primer, one per hour, then multiple coats of Hammertone paint. It can be difficult to get the Hammertone paint to lay correctly without making the coat too thick. Any over-spray is corrected using black permanent ink. 

Chokes painted with copper Hammertone paint

Once dry, I measured and cut some strips of black leather and used some epoxy to keep them in place, wrapping around the spool of the choke. I use a couple layers of leather when possible to give the spool a fuller look, it's more aesthetically pleasing.

Completed chokes with leather wrapping the spool. 

UPDATE (6/19/11): The parts arrived for the ST35 power / driver boards, so I went ahead and populated everything except for C4 and C5, which will be 0.1uF Mundorf Supremes that will hang off the board just a bit and will need to be installed once the boards are fastened to the chassis. C1 was not used per Bob Latino's suggestion as its purpose is DC input blocking and nearly all modern-day preamps already have a DC blocking output cap, so I used a pair of thick leads to short C1. As you can see below, the Teflon tube sockets have blue PCB boards installed to ease the soldering process. The sockets had a rather long ground pin that I trimmed down and sanded smooth as there would have been a clearance issue in the height challenged chassis. 

ST35 boards mostly populated and ready for installation

UPDATE (7/8/11): The chassis is pack from the powder-coater so I was able to fit the boards in place. I opted to install them rotated 180 degrees as I wanted the RCAs on the rear of the chassis and there wasn't sufficient clearance with the power tube sockets blocking the way. I took some generic gold-plated RCAs from my parts bin and trimmed off the back with a cutting disc, rendering them nonfunctional but small enough to squeeze between the tube socket and the chassis front, simply for the sake of aesthetics. I was also able to position the painted Hammond filter choke, attach the Cardas copper binding posts and install the Cardas rhodium plated RCA jacks. Also installed was the large silver multicap and the bias control board from Dynakitparts. 

Getting started assembling the parts on the chassis

In the interest of making things convenient, I installed a small 6A SPST toggle switch on the front panel of the chassis so the amp can be powered on and off conveniently.

A 3 Amp power switch installed on the front of the unit

The ST35 Bias Control Unit from DynakitParts makes use of an adjustable fixed-bias circuit, allowing for adjustment of the bias setting of each EL84 power tube via four separate precision bias potentiometers. 

Wires run to the Bias Control Unit from DynakitParts

The initial wiring was completed using Kimber TCSS 19 gauge stranded copper in teflon dielectric. I opted to drill out the area originally designated for the fuse holder and the cable in the interest of fitting a C8 power inlet. Because the re-purposed real estate on the chassis back was a little too large for the C8 inlet, I mounted it to a piece of FR-4 material and then mounted the combo to the chassis. This adds a degree of convenience, allowing a removable C7 power cord to be used to power the amp. The fuse was then moved inside the unit and mounted to one of the chassis braces, really a perfect place for it in my humble opinion. Grounding lugs were added to both chassis braces to connect to the RCA grounds (the Cardas RCAs are insulated from the chassis unlike the stock Dynaco RCAs) and the negative connector of the speaker binding posts. 

Initial wiring of the ST35 complete

UPDATE (7/28/11): The transformer bell-ends arrived back from the powdercoater and were reattached to the iron with brass hardware. Rather than using the cut transformer wire, I prefer the Kimber TCSS Teflon wire for all extraneous point-to-point wiring. The dielectric doesn't melt, it's a little thinner so it's easier to route, and I'd imagine the copper's of a higher purity. I opted to add a set of three 1uF 630V Solen film capacitors as bypass caps for the can capacitor. The Solen film caps are only a couple bucks a pop and have the potential to add refinement to the power supply. It's a narrow fit in the ST35 case, no room for zip tie mounts for the caps. 

Transformers wired in place, Solen bypass caps added

The wiring was finalized by adding a pair of DPDT switches to manage the triode and ultralinear switching. This was wired based on the following advise from Bob Latino and Tazsmonn on the DynacoTubeAudio Forum: Remove green or green white wire from pin #9 of each tube and solder to separate poles of DPDT switch on the same end, Solder wire from pin #9 to center pole of DPDT switch, Solder 100 ohm [resistor] from pin #7 to DPDT switch opposite end from green or green white wire, All wiring to and from switch side must be to same tube.

Triode mode switches wired up

Now that the wiring is complete, a final checkout of the circuit is performed, tubes and installed, the unit is powered on and the tubes biased using the ST35-BCU. The tubes installed were Mullard EL84s and Phillips EGC 12DW7. The sound is quite excellent, very resolving and holographic imaging. It actually sounds quite "hifi" for a circuit from the early 1960s. I could use a tiny bit more bass, but I was listening on a small set of Tektons with 4" Fostex full range drivers. Here's some additional progress pics:

Dynaco ST35 front view

Final Dynaco ST35 back view

Dynaco ST35 side view

UPDATE 8/8/2011: I was able to have a pair of custom engravings made for the unit. A 1" gold circle with the word "Dynaco" engraved in black and a small rectangle with a Dynaco Stereo 35 logo that I made in Adobe Illustrator. These are the final touch.

Final Dynaco ST-35 front view

Final Dynaco ST-35 top view

Engraving detail on quad-cap

Final Dynaco ST-35 rear view

To recap, the customizations to the kit were as follows:

• Teflon tube sockets with Beryllium copper contacts
• Ceramic gold plated driver tube sockets
• Mundorf and Obbligato driver board capacitors
• Solen power bypass caps
• Kimber TCSS internal wiring
• Kiwame / Koa Speers carbon film resistors
• Hammond 156R Choke - 56ohms DCR
• Dynakit Parts ST35 BCU (Bias Control Unit)
• Front power switch
• Triode / Ultralinear Switches
• Cardas RCAs and Binding Posts
• C8 power inlet jack with custom C7 power cord
• Custom powdercoat color-scheme
• Alternate fuse holder and placement
• Custom Brass Engravings

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.