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. 


 

Millett NuHybrid Headphone Amp Using Korg NuTube

In Mid-March, the prolific DIYer Pete Millett introduced a new hybrid headphone amp called the NuHybrid using the Korg Nutube 6P1. The design is similar to the original Millett Hybrid, which spawned both the exceedingly popular Millett MAX and stripped-down Starving Student builds. The Starving Student was super-popular as it could be built for as little as $35, which is unheard of for an amp with two tubes.

To shake things up (Millett designs are rarely status quo), the old car radio tubes were eschewed for the new Korg Nutube. The Nutube is a low-power, directly-heated dual triode tube built using a process originally used for Vacuum Fluorescent Displays (VFDs) and emits a bluish-white glow when the tube is powered up. The Nutube has been popular with DIYers in Japan since it was released at the tail-end of 2016, but hasn't really caught on with DIYers elsewhere, at least yet. The cost of a Nutube is around $50, which may seem a little steep, but two noval signal tubes will set you back at least $30-35 these days, along with another $5 for sockets and the potential to have to replace them in a few years, which makes $50 not seem all that bad.

The Korg NuTube

One of the nice things about the design is you don't have to feed it plenty of amps. The whole thing runs off of a 24V power supply, so it is much safer than a typical tube-based build. A pair of OPA551 op-amps to drive the headphones, but others can be rolled in for a different sound signature. Pete has made the design so it can be built cheaply (~$120) and placed in a Serpac plastic case. Much like Pete I don't like status quo, so I'll be building a case from scratch for my NuHybrid amp. I wanted to make the amp look like an old tabletop radio, so a wood frame would be needed.

A quick trip to Home Depot yielded a 6ft long by 3.5" wide piece of maple.

A little bit of routing and a few 45 degree cuts on a miter saw and we have a frame. Note the inside of the frame is rabbeted so the front and rear plate can sit flush. The 5/8" pieces in the middle are for screws to hold the plates in place. This is a photo after an initial coat of Minwax Pecan colored stain and seal combo. Yes I know it's sacrilege for anyone who works with wood regularly, but it's quite and easy.

Frame to hold PCB inside

As wood doesn't protect from errant EMI/RFI, I've added some copper shielding tape which will be electrically connected to the front and rear aluminum plates to get 100% coverage.

Copper EMI/RFI Tape for shielding

The board is quick and easy to populate and should take even the most OCD solder slinger less than 3 hours. Below is an image of the PCB. The checkered areas indicate where double-sided tape is applied to the board to hold the NuTube in place.

NuHybrid unpopulated PCB

Below you'll see my PCB about 90% populated. I changed a couple of the parts in the BOM. The BOM calls for Wima polyester, but you can switch them out with Polypropylene for about $2.50 more and they fit without issue. I also went with Elna Silmics in the power supply. The Silmics sport a 7.5mm lead spacing versus 5 on the Nichicons, so they have to be pressed into place otherwise they'd stand crooked. They also encroach a bit on other areas of the PCB, so the Wimas needed to be mounted on the bottom. I also went with Takman carbon film resistors in most areas. The 1/4 watt variety are only 36 or so cents a pop at Sonic Craft, so it won't break the bank to use them.

Note the large-and-in-charge Silmics in the power supply section  

I picked up two aluminum plates 1/8" thick for the front and back. I usually get them pre-cut from eBay but will also use a miter saw with a metal blade if the size I need isn't available. The back of the unit needs holes for four RCAs and 24v DC. Although I doubt the unit gets very hot due to the low power requirements, I drilled a hole pattern at the top for heat to escape, as with wood it will trap it in otherwise. The aluminum is spritzed with Hammertone paint in a copper finish.

1/8" thick aluminum plate with holes for RCA, DC

Attaching the unit to the front panel are the 1/4" inlet nut and the TKD potentiometer nut. I went with the nicer TKD potentiometer as suggested by Pete. $40 is a bit painful but I'm tired of using Alps Blue Velvets for every build. The front LED, switch and coupling capacitors (Russian Silver Mica) are connected with fly leads (22ga. silver clad teflon solid core wire I had left over from a Bottlehead build). The wiring to the RCAs is Cardas 24ga. stranded in teflon. Note the space between the two mica caps in order to reach the adjustment pots. The RCAs are Switchcraft units I had on hand. The switch is much higher rated than it needs to be, but I have tons of surplus switches and like the big old school ones. If you're looking for low cost and high quality, check out Apex Jr.

Attaching the panels gives the nearly finished product...

Vent holes probably not needed, but look nice

The last touches were an engraved front plate and changing the pilot light to a funkier looking one.

All done and sounds great!

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. 

The Millett MiniMax - My First Tube Amp

The Millett MiniMAX is an excellent starter kit for budding tube enthusiasts. The design began with Pete Millett who has designed an enormous amount of fantastic DIY projects, and has been further refined by Colin Toole (CeToole) as the MiniMax board. As stated from DIYForums: The Millett MiniMAX was designed by Colin Toole (cetoole) to incorporate many new features and advances previously unavailable, while still adhering to the famous Millett heritage. Two new outstanding features distinguish the MiniMAX from its predecessors:

1) A superior low-noise power supply. The standard configuration has been optimized for 0.045mVAC in the power supply, measured at V+ and Gnd, with the tubes burning and the buffers biased at 50ma each. This is outstandingly low noise performance for a linear regulated power supply. Moreover, this is all imbedded in the PCB design - only requiring a 24VAC walwart for complete power.

2) A custom-machined case design from Lansing Enclosures. Based on their "DT1" case design, the MiniMAX is optimized for the smallest volume, width, and length practically possible with through-hole components and full-size extruded heat sinks. With the custom-machined case, commercial-grade finish quality is possible for every MiniMAX builder. Optionally, DIY-versions of the Lansing DT1 and CT1 case are easily constructed.

TomB of HeadFi offers this excellent kit for a mere $220 + shipping (as of Feb 2011) on his website Beezar.com. Below is a photo of all you get with the kit, including the above mentioned chassis:

My experience ordering the kit from Tom was excellent, everything was well packed and labeled for ease of installation. The documentation on this amp is above and beyond anything else I have seen. 

In the interest of adding a few personal touches, I swapped out the high output LEDs for diffused orange ones from Digikey (I'm not a big fan of bright LEDs), I also swapped four of the Nichicon caps in favor of the newer release Elna Cerafines from Partsconnexion. You'll also notice some Kimber TCSS wiring and Cardas RCAs. The knob is a milled-aluminum type from Digikey.

Here's some photos to enjoy:

Millet MiniMAX Internals - Click to Enlarge

Millet MiniMAX Internals - Click to Enlarge

Millet MiniMAX Front - Click to Enlarge

Millet MiniMAX Back - Click to Enlarge

For the price, the sound is very good. There is better out there, but for more money of course ;) A very recommendable first build.

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.