Portable Headphone Amp Kit with Korg NuTube
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HiGHFLYiN9

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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.

Korg HA-K1 Nu Tekt Tube Amp Kit Pieces HF.jpg

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 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 :wink: The entire build took about an hour and a half, and I like to take my time.

Korg HA-K1 Nu Tekt Tube Amp Kit Assembly 4 HF.jpg

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.

Korg HA-K1 Nu Tekt Tube Amp Kit Unit HF.jpg

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.

If you decide to build one, please feel free to post in this thread. I'll try and answer any questions that come up.
 
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bootdsc

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Thats a nice update to the kit, i have the original nutube kit and i still haven't put it together lol.
 
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alpha421

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Didn't find it yet on Petes eBay store. Any idea on the kit price?
 
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HiGHFLYiN9

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It should be up within the week if I'm not mistaken. Pete said the price would be $249. While that may be a little more than some other DIY portable kits, remember it includes the $50 NuTube and a $40 op-amp (MUSES 01 made by New Japan Radio), as well as the pre-populated circuit board, so I believe you're getting a good value for the dollar.
 
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lambdastorm

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I really hope there's a desktop version of this amp. Amazed at the idea but not too thrilled I'll need spare batteries all the time.
 
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tomb

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baseonmars

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I picked up this kit a couple of months ago - really simple fun build and it sounds excellent, especially for the money. I find it does a great job of cutting off the “digital edge” from the FiiO Q5s.

Ny only real complaint is how hard it is to change the batteries. You have to unscrew the lid and swap them over. This lead to it sitting on my desk not getting the play time it deserves... so, I figured out a simple mod so I could charge it from USB.

little rough around the edges, but it works and works well. Here’s a few pictures of the insides: https://imgur.com/gallery/iAB5UL0
 
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