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Headphone amplifier
Bakoon Products HPA-21

Price (in Korea): 2995 USD

Manufacturer: Bacoon Products International

B1 1022-4 Unjung-dong | Bundang-gu, Seongnam-si
Gyeonggi-do | 463-440 | Korea
tel.: +82-70-8677-5513 | fax: +82-31-706-5513


Manufacturer’s website:

Country of origin: Korea

Text: Wojciech Pacuła | Photos: Wojciech Pacuła Translation: Andrzej Dziadowiec

Published: 3. July 2013, No. 110

In April 2013, Srajan Ebaen published a long and extensive review of the HPA-21 headphone amplifier from the Korean manufacturer Bakoon Products International (see HERE). I found out that our Korean friend and the company’s head, Soo In Chae, was preparing this type of product at the end of 2012, about the same time Srajan did. We made an agreement with Soo In that the chief editor of “” will be the first to publish his test, seeing as he was the one who discovered the company to Western Europe and he deserves to go first. Having access to Bakoon’s materials about the new design at a very early prototype stage, he superbly portrayed how the amp was being developed. I won’t repeat that, sending instead all interested to Srajan’s review.

For those who don’t feel like it, don’t have the time, or don’t care about other reviews let me say a few words of introduction. The first component from Bakoon Products International that gained recognition in the West was the AMP-11R integrated amp. It’s a small, wonderfully made modestly powered machine with an outboard power supply and optional, fantastic RCK-11 anti-vibrational platform. Its heart is the SATRI-IC-EX implementation of the proprietary SATRI Circuit. It’s a current-mode integrated circuit with amplification in the current not voltage domain. It therefore works best with current-mode signal and the amp features special BNC SATRI-Link inputs for that. Until now, the only source with suitable outputs has been the EQA-11R phono preamplifier, but there’s a DAC in the making that should be soon released.
SATRI is the fruit of research and development and lifetime’s achievement of Mr. Akira Nagai (featured on the picture from the High End Munich 2013, see HERE), the owner of Bakoon Products Co. Ltd. The company offered SATRI amps and unique DACs, which you can see on the last picture from this year’s Munich show. Korean Bakoon came into existence through the determination and ambition of Mr. Soo In Chae whom I met in Munich, and who turned out to be a fantastic person. He designs the enclosures and carries out additional calculations wherever applicable, adapting Mr. Nagai-san’s ideas. I don’t know what their exact division of responsibilities is. A clue may be found on unit’s back panel, which states that the amp was designed in Japan and made in Korea. I’m not entirely convinced that Soo In Chae only “re-packs” Japanese devices, because their mechanical build quality is a hundred times better than that of their Japanese predecessors, and I know for a fact that Soo In made many of the calculations for the HPA-21 design. One thing is for sure, though – the SATRI circuit, surface mounted on small printed boards that plug into the main board, was made in Japan, which is confirmed by the corresponding marks on the boards. The rest was made in Korea.

The HPA-21 is the outcome of a careful reflection on the incredible popularity of the previous AMP-11R model commonly used as a headphone amp. Although its 6.3 mm port was added just for user comfort and the amp was designed with speakers in mind, many users including myself began to treat it as a headphone amp with an option to drive the speakers. This is nothing new to mention another integrated amp, the Leben CS-300 (and its following versions) that met a similar fate. Its Custom edition custom made for me has long been a reference headphone amp in my audition system.

Due to much lower HPA-21’s power output, Mr. Soo In was able to design a cheaper unit that offers even a better sound than the integrated amp. The new machine could also be equipped with another trademark company product, battery powered power supply. Based on the BPS-02 power supply circuit (see HERE), it has a much higher output voltage. As Mr. Soo In says, the power supply features a dual design and employs four Samsung batteries, working at the highest permitted voltage of 16.8 V. The unit is powered by plugging in a small switched mode PSU. When we power on the HPA-21, the battery-charging section is disconnected and the amp works solely on battery power. The batteries allow for 6.5 hour of continuous playback, despite the amp’s Class A output topology. Charging them fully takes nearly the same amount of time. Bakoon’s proprietary bias system allows for a quick thermal transistors stabilization. The machine is ready for playback and achieves optimal sound after as little as 10 minutes.

Bacoon Products International featured in “High Fidelity”
  • REVIEW: Bakoon Products International AMP-11R + RCK-11 – integrated amp + antivibrational platform, see HERE (RED FINGERPRINT award)
  • AWARD OF THE YEAR 2012: Bakoon Products International AMP-11R + RCK-11 – integrated amp + anti-vibration platform, see HERE
  • REVIEW: Bakoon Products International BPS-02 – battery AC, see HERE (in a system)

    Albums used during this review

    • Estampies & Danses Royales, Hesperion XXI, Jordi Savall, Alia Vox AV 9857, CD (2007).
    • Filia Praeclara, Ensemble Peregrina, Divox CDX-70603, CD (2008).
    • L’Amor de Lonh, Ensemble Gilles Binchois, Glossa GCP P32304, CD (2010).
    • Ludi Musici, Hesperion XXI, Jordi Savall, Alia Vox AV 9853, CD (2007).
    • Audiofeels, Uncovered, Penguin Records 5865033, CD (2009).
    • Bach, Violin Concertos, Yehudi Menuhin, EMI/Hi-Q Records HIQXRCD9, XRCD24, CD (1960/2013).
    • Bogdan Hołownia, Chwile, Sony Music Polska 5052882, kopia z taśmy master, CD-R (2001).
    • Frank Sinatra, Where Are You?, Capitol Records/Mobile Fidelity UDSACD 2109, “Special Limited Edition No. 261”, SACD/CD (1957/2013).
    • Guillaume Du Fay, Diabolus In Musica, Antoine Guerber, Alpha 908, CD (2007).
    • Hildegarda von Bingen, Canticles of Extasy, Deutsche Harmonia Mundi 77320 2, CD (1994).
    • Jean Michel Jarre, Essentials & Rarities, Disques Dreyfus/Sony Music 62872, 2 x CD (2011).
    • John Coltrane, A Love Supreme, Impulse! 589 945-2, “Deluxe Edition”, 2 x CD (1965/2002).
    • Michael Jackson, Thriller. 25th Anniversary Edition, Epic/Sony Music Japan EICP-963-4, CD+DVD (1982/2008).
    • Miles Davis, In A Silent Way, Columbia/Mobile Fidelity UDSACD 2088, "Special Limited Edition No. 1311", SACD/CD (1969/2012).
    • OMD, English Electric, 100%/Sony Music Japan SICP-3810, CD (2013).
    • Perfect, Perfect, Polskie Nagrania Muza/Polskie Radio PRCD 1596, CD (1981/2013).
    • Xymox, Metamorphosis, X-ULT Records/Alfa Records (Japan) ALCB-653, CD (1992).
    Japanese editions of CDs and SACDs are available from

    I’ve been listening to this amp for the past four weeks, nearly non-stop, with obvious breaks for sleep, auditions and family life. However, all my remaining time, even while writing this review right now, I’ve spent with headphones on my ears. During this time a few things changed in my system and every single one of these changes only reinforced my belief that the HPA-21 is one of the best, if not the best, headphone amp I’ve known, regardless of price, technology, country of origin or my preferences. This Korean amp clearly showed all the changes in a bullet-point list, both in terms of its sound and design.

    Klutz Design                          
    CanCans Leather Ed Black

    In addition to the new RCA-RCA interconnect between the player and preamp (or headphone amp) – now it’s the Siltech Royal Signature Series Double Crown Empress (see HERE) – new headphone cables – previously the Entreq Konstantin 2010 (see HERE), now the new Challenger 2010, I’ve finally received the newest Klutz Design CanCans stand (see HERE), finished in black leather with a white thread. I’ll write about the cable some other time, this time I’ll be talking about the stand.
    Michael Hollesen, the owner of Klutz Design (pictured below), showed me the orange-thread edition in Munich. It’s made specifically for use with Bakoon products, which Michael is a big fan of. I decided to get the white thread version, mostly because it’s more universal. It’s the fourth stand from this company I own.
    When you see the CanCans for the first time, you’ll be surprised by how large they are – they’re 310 mm tall, the width of the base is 140 mm, and they weigh 2.1 kg. They’ll be visible in any system and in any room. Their finish precision is perfect. You can order them in different colours, in different finishes with different “pins” which you spool the cables around when you’re not using your headphones; you can set them up to ideally cater to your expectations.
    The class of Bakoon and Klutz designs are from the same top shelf. It’s no wonder that the companies work together very closely – people with a similar mindset quickly find a common language. The newest version of stand is finished in black leather. I think it was black matte first, with black pins and the unofficial name Black Knight, prepared as a tribute to the latest Batman movie. The leather is a natural extension of that project.

    If you remember my review of the AMP-11R integrated (which can also work as a headphone amp), you’ll know that its incredible cleanness combined with outstanding resolution opened before the listener’s a wide open window to what’s before the unit. It’s an outstanding machine. I’ve heard a very similar effect when plugging in headphones to the HPA-21’s output labelled “Voltage”. I think that lowering the output section requirements and powering the whole unit with an ultra-stable battery voltage has pushed the whole deal forward. Although the improvement isn’t massive, one can hear even more information that the signal carries and the top-range is even more open and precise. The amp used in this way would probably be the cleanest-sounding machine I have ever come across to drive headphones, with the least amount of own coloration. Everything else, including my beloved Leben, seems saturated, as if their sound was more “set up”, and often even manipulated. I’m not talking about flawed designs but about the best ones, like the Leben CS300XS [Custom Version], SPL Phonitor 2730 (see HERE) Funk Tonstudiotechnik LAP-2.V3 (see HERE), Ear Stream Sonic Pearl (see HERE), or even the Phasemation EPA-007 (see HERE). Only the last one was equally resolute and selective. The Leben, on the other hand, showed a better saturation across the whole range and was fuller. It was also much more natural at presenting low frequencies. None of them, however, I repeat: none of them sounds like the Bakoon does via its “Current” output.

    I had a great time with all the aforementioned amps, listening to one album after another, changing headphones, sources, and cables. At the end, when I finally had to describe what I heard, I would return to my Leben to find again something new and more absorbing in its sound. After a few days I was right back under its spell, embarrassed that I’d ever doubted in it.
    Enter the HPA-21 headphone amplifier from Bakoon Products International, coming from Korea but having its roots in Japan. The first machine I’d listened to without feeling the need to return to my Leben. My custom CS-300 XS is still a wonderful work of audio engineering, my beloved baby I’m not going to give away. The thing with the Bakoon is that I’ve got no need to analyse what I’m hearing. It’s probably a transition state, and surely someday I’ll hear something even better and I’ll change my mind (that’s exactly what progress and “learning the sound” is about) and find this amp’s weak points. As of now, I can’t see any. They are definitely there as there is no such thing as “ideal sound”. Remembering how shocking the Siltech interconnects were after the Acrolinks that seemed perfect back in the day, I am absolutely, painfully aware that the same thing will one day happen with the HPA-21, and I’ll be standing there surprised at how I missed such obvious things.

    But now is not the time. At the moment I don’t feel the need to analyse anything. The amp’s sound in combination with the Sennheiser HD800 headphones is unbelievably natural. It’s obviously resolute and selective, differentiating and detailed, except that none of these traits “jumps out”, asking for attention. The most important thing for me is the excellent balance between resolution and selectiveness. The first is infinite in its character – the more, the better. There is no such thing as too high resolution (or differentiation, at that). On its own, without proper tonal support it is irrelevant. Yet it is the driving force, the power behind a natural, normal presentation which is close to what we hear live. The resolution shows HOW the individual sounds differ from one another. Selectiveness on the other hand, or the ability to separate really close, similar-sounding sounds, instruments, and events – often mistaken for detail – is a quantity which should be kept in balance. It can be exaggerated and when the selectiveness is too high, it will shatter the illusion of live sound, leaving nothing but pitiful hi-fi in its place. The amp we’re talking about does a wonderful job at setting it up – it’s very good, but not exaggerated, and the notes aren’t forcefully separated. The resolution, on the other hand, is so remarkable that it gives the feeling of smooth, silky sound, despite lots of information and detail. Together they give a lot more than in other machines, allowing to break through the analytical layer.

    Despite the high selectiveness, and probably thanks to what I’ve described above, the amp gives lots of pleasure regardless of what album you’re listening to, no matter how it was mastered and produced – as long as it’s musically ‘digestible’. We notice many elements we didn’t hear before, and that’s normal. But its key to success is something different – a perfect harmony of color, soundstage, imaging, body and tangibility. At first, and for a very long time, I listened to recordings from before the year 1600, mostly vocals recorded in large churches. It’s easy to mess something up and to dig out vocalists or reverbs. And although the amp clearly pointed out better and worse recordings, I had no problems listening to any of them and nothing really pushed me away. Only after an audition and giving it some thought I started deciding which CDs I wouldn’t listen to and should put them back in a box. They were decisions based on musical taste, however, not their sound.
    The same was true with electronica and rock, for example the new Daft Punk and OMD albums. I compared the Japanese CD release of Random Access Memories to its hi-res 24/96 FLAC version played back on the Naim NDS, and I chose the CD every time. Although the Naim had previously nearly killed me with the quality of 24/192 and 24/96 classical music. The “box” version had a deeper tone, saturation and palpability. Hi-res showed more detail and flavor, but I still preferred listening to the CD. The Bakoon showed these differences with ease, as though in passing and without making a fuss. As if it were saying “this is how it is” and carrying on with its life. It’s a new quality, because we’re used to emphasizing differences and in the audio world.


    During the review I mostly focused on the HD800 headphones from Sennheiser. I had a very good reason for that – it’s an excellent combination. I’m full aware of the weaknesses of the top designs from this German brand, but their advantages are much more important to me. The HPA-21 allowed me to discover them all anew, and showed me some for the very first time. The Korean amp sounded very well with all of my other headphones, although the higher the impendency, the better it was. I least liked the combination with the Beyerdynamik DT-770 Pro {Limited Edition} with a 32-ohm impendence.
    The HE-6 HiFiMAN planar headphones are a wholly different ballgame. I know a large percentage of music lovers uses the Bakoon with other headphones of this type (by Audeze) and they’re in heaven. I’ve got to say that the HPA-21 was the second amp, after the Phasemation EPA-007 (in balanced mode), which made Mr. Chang’s headphones finally sound like they’re meant to. I’ll return to the Bakoon and HE-6 someday, but for now I’ll just say that the sound was more precise and better-defined than with my Sennheisers. The perspective was different, though – the vocals and instruments were closer, and the acoustic background seemed less important. The bass didn’t extend as low as it could, but had an excellent, almost model definition.
    What really won me over in the Korean amp was its compatibility with the HD800. I don’t know another equally successful combo, except perhaps the HD800 and Leben CS-300 XS. In this comparison the Japanese amp is clearly oversaturated sounds and sometimes you have to use the “Bass Boost” button to compensate for a recording’s flaws. The Bakoon doesn’t make you do that, and I never felt an absence of anything. I’m immensely impressed, because this headphone system is like nothing I’ve ever heard before.

    The amp receives the RED FINGERPRINT award


    Mr. Soo In Chae, who in my opinion is responsible for the mechanical part of this design, has truly gifted hands. His products are small and extremely solidly and precisely made. The HPA is no different. Its enclosure is made of thick aluminum panels. No joining elements can be seen as the top panel is mounted with long screws from the bottom.
    The front panel sports a large, flat volume knob, three small toggle switches and two headphone jacks. Two ports but not providing the same signal as the Phasemation EPA-007 does or offering balanced signal as the circuit is unbalanced. Here we have a completely different solution. One 6.3 mm port outputs a classic voltage signal as all other headphone amplifiers. The second port is a current output with 2.5 MΩ impedance. The switches are used to select circuit gain, active input and to power on the device. The amp’s active state is indicated by an orange LED visible through a small cutout in the bottom panel. Next to it is a red LED indicating low battery state.
    On the back we have two pairs of inputs – a classic voltage input on Cardas RCA connectors and a current input for SATRI-Link hookup to Bakoon sources on 75 Ω BNC Amphenol connectors. Next to it is a small port for the outboard power supply. The unit sits on tiny felt pads. But there seems to be another possibility while sticking to the company's "code": the bottom panel features symmetrically spaced recesses and threaded holes. Apparently, it’s possible to integrate the HPA-21 and RCK-11 anti-vibration platform, previously detaching the top plate from the latter. This way we end up with an amplifier integrating mechanical vibration control. Since I didn’t have the RCK-11, during the review I used Finite Elemente CeraBall that are base on exactly the same idea – decoupling of two metal surfaces with a sintered ceramic ball.

    Electronic circuits are divided between two printed boards mounted above each other. The boards, the assembly and the components used are all of the highest quality. After removing the top panel the upper board that houses the batteries charging circuit. Next to it are Omron relays to disconnect the outboard power supply after powering on the amplifier.
    The gain circuit is mounted on the lower board. SMD mounted boards housing the SATRI circuit are located near the input and plugged into the main PCB. The signal is then attenuated in the Tokyo Ko-on Denpa 2CP Series potentiometer, located on the front panel. The potentiometer is connected via short, shielded cables. All amplifier stages are built on transistors, with Class A output stage on two pairs of output transistors per channel. The latter are bolted to the bottom panel that doubles as a heat sink. I repeat: beautiful, clean assembly work using excellent passive components (Dale resistors, Wima capacitors and others, tantalum). Congratulations!

    Specifications (according to the manufacturer)

    Input Impedance: 3.68 Ω (SATRI-LINK), 100 kΩ (Voltage, RCA)
    Gain: High/Low – switchable (current output only)
    Maximum Power Output: 1 W (32 Ω, 1 kHz)
    Frequency Response: 10 Hz ~ 1 MHz
    Battery: Li-ion, Samsung
    Battery Operation Time: up to 6.5 h
    Battery Recharge Time: 4 hours (80% capacity) | 6 hours (full capacity)
    Dimensions: 195 mm (W) x 237.5 mm (D) x 55 mm (H)
    Shipping Weight: 4.5 kg

    Since my opinion and final verdict on the HPA-21 largely coincides with Srajan Ebaen thoughts and findings, we both felt that the new Korean amplifier deserves a special award. Srajan suggested that our joined award might be called blue fingerprint which I thought to be an excellent idea. The Bakoon HPA-21 thus becomes the first ever recipient of this special Blue Fingerprint award. It is a mark of our appreciation to two great minds behind this design. Congratulations Nagai-San and Soo-In Chae!

    The amplifier receives Blue Fingerprint award


    - Turntable: AVID HIFI Acutus SP [Custom Version]
    - Cartridges: Miyajima Laboratory KANSUI, review HERE | Miyajima Laboratory SHILABE, review HERE | Miyajima Laboratory ZERO (mono) | Denon DL-103SA, review HERE
    - Phono stage: RCM Audio Sensor Prelude IC, review HERE
    - Compact Disc Player: Ancient Audio AIR V-edition, review HERE
    - Multiformat Player: Cambridge Audio Azur 752BD
    - Line Preamplifier: Polaris III [Custom Version] + AC Regenerator, regular version review (in Polish) HERE
    - Power amplifier: Soulution 710
    - Integrated Amplifier: Leben CS300XS Custom Version, review HERE
    - Stand mount Loudspeakers: Harbeth M40.1 Domestic, review HERE
    - Stands for Harbeths: Acoustic Revive Custom Series Loudspeaker Stands
    - Real-Sound Processor: SPEC RSP-101/GL
    - Integrated Amplifier/Headphone amplifier: Leben CS300XS Custom Version, review HERE
    - Headphones: HIFIMAN HE-6, review HERE | HIFIMAN HE-500, review HERE | HIFIMAN HE-300, review HERE | Sennheiser HD800 | AKG K701, review (in Polish) HERE | Ultrasone PROLine 2500, Beyerdynamic DT-990 Pro, version 600 - reviews (in Polish): HERE, HERE, HERE
    - Headphone Stands: Klutz Design CanCans (x 3), review (in Polish) HERE
    - Headphone Cables: Entreq Konstantin 2010/Sennheiser HD800/HIFIMAN HE-500, review HERE
    System I
    - Interconnects: Acrolink Mexcel 7N-DA6300, review HERE | preamplifier-power amplifier: Acrolink 8N-A2080III Evo, review HERE
    - Loudspeaker Cables: Tara Labs Omega Onyx, review (in Polish) HERE
    System II
    - Interconnects: Acoustic Revive RCA-1.0PA | XLR-1.0PA II
    - Loudspeaker Cables: Acoustic Revive SPC-PA
    System I
    - Power Cables: Acrolink Mexcel 7N-PC9300, all system, review HERE
    - Power Distributor: Acoustic Revive RTP-4eu Ultimate, review HERE
    - Power Line: fuse – power cable Oyaide Tunami Nigo (6m) – wall sockets 3 x Furutech FT-SWS (R)
    System II
    - Power Cables: Harmonix X-DC350M2R Improved-Version, review (in Polish) HERE | Oyaide GPX-R (x 4 ), review HERE
    - Power Distributor: Oyaide MTS-4e, review HERE
    - Portable Player: HIFIMAN HM-801
    - USB Cables: Acoustic Revive USB-1.0SP (1 m) | Acoustic Revive USB-5.0PL (5 m), review HERE
    - LAN Cables: Acoustic Revive LAN-1.0 PA (kable ) | RLI-1 (filtry), review HERE
    - Router: Liksys WAG320N
    - NAS: Synology DS410j/8 TB
    - Stolik: SolidBase IV Custom, read HERE/all system
    - Anti-vibration Platforms: Acoustic Revive RAF-48H, review HERE/digital sources | Pro Audio Bono [Custom Version]/headphone amplifier/integrated amplifier, review HERE | Acoustic Revive RST-38H/loudspeakers under review/stands for loudspeakers under review
    - Anti-vibration Feets: Franc Audio Accessories Ceramic Disc/ CD Player/Ayon Polaris II Power Supply /products under review, review HERE | Finite Elemente CeraPuc/ products under review, review HERE | Audio Replas OPT-30HG-SC/PL HR Quartz, review HERE
    - Anti-vibration accsories: Audio Replas CNS-7000SZ/power cable, review HERE
    - Quartz Isolators: Acoustic Revive RIQ-5010/CP-4
    - FM Radio: Tivoli Audio Model One