Elekit TU-HP02


Headphoneus Supremus
Pros: battery life (50 hrs!), build quality, sound (surprisingly clean and detailed), quite low output mimpedance
Cons: little hiss with sensitive BA in-ears, no rubber feet, not the best low-level adjustment range for sensitive headphones/earphones



Before I start with my actual review, let me start by thanking Elekit Japan for providing me with a sample of the TU-HP02 portable headphone amplifier (http://www.elekit.co.jp/en/product/TU-HP02) in exchange for my honest opinion. The conversations I had with them were always very friendly, competent and helpful.

The story of the Japanese company Elekit begins in the 1970s: the owner of a shop for electrotechnical parts saw many children with a technical book in their hands who came in to buy electronical parts for building a radio. Out of these observations, he created various DIY kits that gained a lot of fame among Japanese children, and so it is not much surprising that Elekit’s web shop today also contains many kits that mainly aim at children. But besides that, the Japanese company also offers serious and technologically well-made hi-fi products that are either sold as a kit or already finished, like their portable headphone amplifier TU-HP02.

Technical Specifications:

Price: 29700 Yen (~ $240)
Type: discrete transistor amplifier
Headphone Impedance: 16 – 300 Ohms
Max. Output: 170 mW (with Alkaline batteries, into 16 Ohm headphones)
Residual Noise: 6 µV
Frequency Response: 1 Hz – 230 kHz (-3 dB)
Power Supply: 4x AA Alkaline (1.5 V) or 4x rechargeable AA NiMH batteries (1.2 V)
Power Consumption: no signal: approx. 40 mA; at 10 mW into 16 Ohms: approx. 60 mA
Battery Life: ~ 30 hours of continuous use (10 mW, 16 Ohms output, 20 °C, Alkaline or 1900 mAh NiMH batteries)
Dimensions (WxHxD): 75 x 19 x 132 mm  l  75x19x112 (Chassis only)
Weight: 175 g (without batteries)

Delivery Content:

The TU-HP02 arrives in a compact cardboard box that is designed in the colours red and white and has got a picture of the amplifier on the upper side, along with the technical features in Japanese as well as English. The rear side shows the serial number.
Opening the box, one will find the amplifier which is securely covered in pink bubble wrap, along with a 3.5 mm to 3.5 mm interconnection cable and a Japanese instruction manual (on request, one can get an English version as PDF file as well).
Batteries are not included.

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Looks, Feels, Build Quality:

The amplifier’s enclosure entirely consists of silver-coloured, brushed aluminium as well as red PCB material on the front and rear (as you can see, the colours of the packaging continue on the amp itself and also resemble Japan’s national flag’s colours). The build quality is sublime and the enclosure feels very sturdy.
On the front is a centric volume potentiometer which is machined of aluminium too, and is knurled, which guarantees for a good grip. The turning resistance is not too weak, wherefore it is unlikely that the volume level gets changed unintentionally when you are using the TU-HP02 on the go. It is also unlikely that the gain switch gets activated unintentionally.
All connectors and switches are labelled white and easy to read.

However, what I am missing are rubber feet on the bottom.

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Features, Connectors:

The analogue volume potentiometer that also acts as combined on/off switch is located on the front’s centre. Turning the amplifier on, the “Virtual Triode” sign lights up in green and signalises that the amplifier is ready to use.
On the front’s far left is the 3.5 mm headphone output socket, with a 3.5 mm audio input on the far right. Next to the quarter jacks is enough free space so that big jacks can be connected as well. Between the audio input and the potentiometer is the two-position gain switch located.

In the centre of the rear side is a screw cap. Once it is unscrewed (which can easily be done with your fingers), the aluminium enclosure can be pulled off.
What you will find then is a chassis that is entirely made of red PCBs, with the amplification stage in the upper section and the battery compartment in the lower (the TU-HP02 receives its power entirely from four AA batteries, which is something you won’t find often these days, but it is very handy).

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Battery Life:

Elekit states a battery life of around 30 hours with continuous use. Of course, this value depends on the used batteries but also the headphones, volume level and gain.
For testing, I used the Eneloop Panasonic BK-3MCCE 1.2 V, 1900 mAh rechargeable batteries that were fully charged before testing. The connected load was an inexpensive Superlux HD681, the volume potentiometer was at roughly 1 o’clock and the gain was set to “low”.
With this test environment, I was able to reach astonishing 50 hours and 15 minutes of battery life.
By the way, the “Virtual Triode” sign’s illumination turns from green to red when the battery capacity is low.

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My listening tests were mainly made with the iBasso DX80 and DX90 with a wide variety of my in-ears and full-sized headphones. Music material was mostly stored as FLAC files; the audio levels were metrologically adjusted for my listening tests.

Technical Background/Implementation:

Mainly the harmonic distortion (provided the amp’s output frequency response is neutral) is responsible for the typical, gentle tube sound. Besides the warm-up time, one disadvantage of tube amplifiers is their typically high output impedance which alters the frequency response of in-ears and headphones that don’t have a flat impedance response (i.e. all multi-driver in-ears and many open-back dynamic headphones).
The TU-HP02 is a fully discrete transistor amplifier with semi-conductor elements and no OP-Amps, and therefore does not run on tubes. Instead, its input circuit is “trioderized” (as Elekit calls it), which means that harmonic distortions are added on purpose to achieve a more gentle, tube-like sound.

Channel Balance/Potentiometer Balance:

After turning on the amplifier, its adjustment range starts at pretty exactly 9 o’clock with a very minor channel imbalance of only 1 dB (with analogue stereo potentiometers, a channel imbalance at the low adjustment range is normal and usually greater than the small 1 dB of the Elekit). At ca. 10 o’clock, the channel mismatch is only 0.5 dB and entirely gone at 11 o’clock, with a perfect channel balance.


Frequency Response:

The (unloaded) frequency response is perfectly flat, wherefore all sonic characteristics can’t be attributed to the frequency response:


Output Impedance:

Especially multi-driver in-ears that have a varying impedance over their frequency range require a low-impedance source, else the source device’s output frequency response gets altered. As usual, I used the Ultimate Ears Triple.Fi 10 for recording the frequency response (which is a complex load that has a varying impedance response) and calculating the output impedance:


From the (peak to peak) deviation, an output impedance of about 1.5 Ohms can be calculated (if I haven’t miscalculated). This value is still good and ideal for most multi-driver IEMs and literally all full-sized headphones.


As expected, the “trioderized” input circuit adds harmonic distortions to the sound which are however still much lower than with a typical, real tube circuit:



The amplifier’s gain (with inserted 1.5 V batteries) was also something I measured.
In Low Gain mode, the amplification is -1 dB (which means that it is 1 dB lower than the input signal) as well as 8.5 dB in High Gain mode. Upon request, Elekit also confirmed these measured values.
In a chart, the gain looks like that:

(The minor bass roll-off here is produced from my soundcard and not the TU-HP02, as I used a different audio interface for measuring the Gain. The Elekit amplifier itself is perfectly flat, what my frequency response measurements with other rig also prove further above.)


For testing the ground-hiss, I used two of my most hiss-sensitive IEMs, namely the Shure SE846 and Ostry KC06A.
With those “hiss-microscopes”, I (who considers himself as very sensitive to hiss as well) was able to spot some higher than quiet hiss (louder than the 1st gen FiiO X3) that lays between the LH Labs Geek Out IEM 100 and iBasso DX80, but is closer to the iBasso’s.
With less sensitive in-ears like the Sennheiser IE 800 and all full-sized headphones, there was no hiss at all, just as expected (and I also see the TU-HP02 as being mainly made for portable full-sized headphones).
With very sensitive, yet not extremely sensitive IEMs (Ultimate Ears Reference Monitors, Ultimate Ears Triple.Fi 10, …), there was some hiss at moderate listening levels when music was being played, but it was not distracting.

Resolution, Precision, Soundstage:

Now to the more subjective part of my review. My opinion and experience regarding the sound signature of source devices and amplifiers goes like this: there is an existing audible difference between various devices, but it shouldn’t be overrated – as the basic character of a headphone won’t be completely changed (if the circuit follows a clean design philosophy), but sometimes rather “shaped” a bit and is rather subtle in many cases.
Let’s go on with my subjective impressions:

Starting with the volume: when the amplifier is directly connected to a Line-Out, the low adjustment range could be too rough with very sensitive in-ears and if you are someone who is rather a low-level listener (like me). Therefore, as also mentioned above, I see the amp to be better suited with portable full-sized and on-ear headphones (unless you are a high-volume listener with IEMs).
With headphones like the Sennheiser Amperior, HD 800 or the Beyerdynamic DT880 Edition (600 Ohms), the amplifier had more than enough power (at least for me, so your mileage may vary).

But the more interesting question is, how does the TU-HP02 sound?
Definitely different than what I expected. Based on Elekit’s description, I expected a warm, probably even slightly woolly sound, but the amplifier is far from being that – the small Japanese sounds very clean, detailed and the “trioderized” stage is less obviously implemented than expected.
If I had to describe the sound, I would say the Elekit sounds “defused analytical”. Due to the small harmonic distortions, the midrange and especially treble sounds less sharp, but doesn’t lose the slightest amount of details: subjectively, the treble doesn’t sound darker at all, but “splashy” and ringing treble attacks as well as cymbals sound a bit spread out, wherefore their attack is not as hard anymore and the ringing gets reduced: the sound gets smoother, but not audibly coloured. The bass is clean and quick, however not as fast as the Geek Out IEM 100’s, but also not slow by any means.

Regarding spatial presentation, the TU-HP02 takes a similar approach as the LH Labs Geek Out IEM 100, with a soundstage that is rather on the narrow side (like the Geek Out’s), but has a very good and deep layering – it could even educe some spatiality from the Triple.Fi 10.

Summarised, I can say that the TU-HP02’s sound is very well-made and the “trioderized” stage gentle, but audible, however rather unobtrusively implemented.


The Elekit TU-HP02 is a superb portable headphone amplifier with a good build quality, user-replaceable AA batteries, an impressive battery life as well as a well-layered sound signature that tends to being analytical, but also smooth and defused due to the “trioderized” input stage.
On the negative side are the lack of rubber feet and the fact that it is probably not the best match for listening to extremely sensitive IEMs at low levels, although the output impedance is still quite low.

Well done, Elekit!
Thanks for the review. 
Nice, informative review of an amp I've never heard of. Thanks.


IEM Reviewer Extraordinaire
Pros: Excellent Build Guality, Good Sound Quality.
Cons: None really.
Elekit Japan is not your typical audio company. Their core business is in fact not audio, but the design and manufacture of educational electronic kits. Perhaps it is because of this unique background, they are more willing to exploring the different paths to music enjoyment. The new TU-HP02, the company’s second portable amp, best reflects that idea by purposefully introduces distortion that mimics the sound of a tube amp. The idea is however not as outlandish as it sounds since it has been tried by others in the past, though this may be the first portable amp to do so, or at least to my knowledge.


Recommended Headphone Impedance: 16-300ohm
Maximum Output: 170mW @ 16 ohm
Frequency Response: 1Hz-230kHz (-3dB)
Power Supply: 4 x AA battery or 4 x rechargeable NiHM AA battery
Power Consumption: Approx. 40mA (no signal) or 60mA (10mW @ 16 ohm)
Battery Life: Approx. 30 hours of continuous use (10mW @ 16 ohm)
Dimensions: W75 x H19 x D132 mm
Weight: 175g [excl. batteries]
MSRP: US$245

Build Quality
Like the TU-HP01 I have reviewed before, the TU-HP02 has an excellent build quality. The one I received for this review is a pre-production unit so some of the marking are wrong, but otherwise it should be identical to the retail version. The amp takes either 4 AA size batteries and they can be either alkaline or NiHM rechargeable. The front and back panels are basically the same material as the PCB, which is the same style as they have done it on the TU-HP01 and to be frank, quite to my liking. Unscrew it on the back and the whole PCB and front panel will easily slide out of the housing for battery exchange. Battery life is estimated to be around 30 hours depends on volume and load, which is pretty good consider they are running on AA batteries.  I am actually still on my first set of batteries as it seems to go on forever and I just give up trying to drain them empty after the first 10 hours.
Gain, Hiss and EMI
Based on my measurement, the two gain settings are rather far apart, with low gain at about -8dB and high gain at around +9dB. Those these number seems a bit more extreme, it actually works out rather well in practise. The low gain setting work best if you have a high output source (i.e. a FiiO’s DAP with well over 1Vrms of line-out) with very sensitive headphone to prevent clipping, while the high gain setting works best with low output source (i.e. Sony’s DAP with under 0.5Vrms of line-out) that needed the voltage boost. They are far more versatile than that of TU-HP01’s gain settings to say the least.
Another area that shows good improvement is hiss. The TU-HP01, being an actual tube amp, suffers from microphonics and the resulting hiss. TU-HP02 on the other hand has no such problem. Hiss is extremely fainted (and arguably inaudible) even with maxed out volume in high gain on my most hiss-prone IEM.
EMI is also excellent with no detectable interference from a cellphone making/receiving call in close proximity. Click and pop during startup is fairly well behaved too.

TU-HP02 (left) and TU-HP-01 (right)
Tech Prelude
So what exactly is this tube’s distortion Elekit has designed into the amp? Well, it is basically referring to the harmonics distortion that comes with a typical triode tube gain stage, and most importantly, the second harmonics that is associated with added body, depth and richness to the fundamental (i.e. with a 1kHz fundamental, the second harmonics will be the double at 2kHz, so on and so forth). Inside the TU-HP02, Elekit has designed a full discrete amp stage that recreates these harmonics distortion and they refer it as the ‘trioderized’ sound.
Sound Quality
As usual, the sound quality evaluation starts with some basic measurements. As far as RMAA’s result goes, TU-HP02’s noise, dynamic range and stereo crosstalk number are pretty good and close to the reference O2. As expected, intermodulation distortion and total harmonic distortion are both noticeably higher due to the purposefully implemented tube-like distortion circuit, though the number are still below the 1% threshold. Frequency response is flat from 20Hz to 20kHz. Output impedance is calculated at 1.4 ohm, which is fairly low. Plenty of current output which means it should have no problem driving even very low impedance headphone. All and all, the numbers look quite solid.

Subjective listening on the other hand is quite a surprise. Given the emphasis from Elekit for having a ‘tube-ish’ sound, I had somewhat expecting TU-HP02 to have a rather warm, if not thick and mellowed sound signature. However, the first impression that comes to my mind during the first listen is that it is actually quite a clean sounding amp, if not approaching what I will usually refer as being ‘transparent’ sounding – a term typically reversed for my reference O2 amp (and that brings back my memory of the TU-HP01’s review, but we will leave it for a later discussion). Further A/Bing between the TU-HP02 and O2 confirms this. TU-HP02 is very much like a slightly richer sounding O2 – a beautified version without any obvious coloration, if you will. The major (or actually the minor) difference is that TU-HP02 isn’t as clearly defined or sharp in each note because there is more ‘room filling’ detail in between each note. Similarly the soundstage seems smaller on the TU-HP02 because of the increase in detail that fills in more space, though a very close back and forth comparison will reveal that TU-HP02’s soundstage by itself is by no mean small in anyway. I guess an analogue will be a room that is totally empty feels ‘more open’ when compared to the same room with one chair in it. Of course what I am describing here are rather subtle in nature and I have a hunch that most people might not able to tell which is which in a brief listening.
So how does it compared to its elder brother, the portable hybrid tube amp TU-HP01 (with MUSES8820) I reviewed awhile back? Well, they are eerily similar in presentation. I said it in the TU-HP01 review that it is hard to really say whether it has a really tube-ish or solid state sound and the same applies to TU-HP02 as well. Looking the other way around, I guess you can say they both have the best of two worlds – but what makes TU-HP02 better is that it doesn’t have any of the downsides of TU-HP01.

FiiO X5II (left) and TU-HP02 (right)
I like the TU-HP01 and TU-HP02 just does it better in every way (except unless you are counting the higher price tag as a negative). While it might not have the opamp rolling capability of its elder brother for the user to customize, it is more refined in design and more practical in use. This makes TU-HP02 an excellent portable amp that I can recommend without reservation.
A thank to both Elekit Japan and VKMusic.ca for the review sample.
@earfonia At high gain with a 2Vrms input, it will run out of voltage swing at around 2.5Vrms. With a 1Vrms input, the output will be around 2.3Vrms. This kind of number might be okay for a high impedance headphone if the sensitivity is high enough, but I guess it will really be pushing it.
@rmullins08 As I have mentioned in the review, this is a pre-release prototype, so some of the marking are wrong. The circuit however is the same as the retail version.
4 removable batteries?
@mayi They are AA batteries, so yes, removable and replaceable.