It was nine years ago that that I made an impulse buy of the Bakoon International HPA-01 from what is now Drop.com. At the time, the Korean offshoot of what is almost a local company to me here in Kyushu Japan, had produced a couple of new models based upon the Japanese tech.
I wasn’t particularly impressed with the results, the amp lacking in resolution and the voltage output having clearly higher distortion. The design needed a lot of work, so I reviewed it, sold it on, and forgot about the company.
Fast forward to late last year, and Soo In from ENLEUM, the Korean company that was once Bakoon International (the companies split some time ago) contacted me and asked me if I’d like to try the new HPA-23RM, a successor to the amp I originally owned. Having read good things, I agreed.
Design
The box that arrived was relatively small, but then so was the amp, not much bigger than the Hugo 2 that graces my desk as the gold standard of portable audio. The size is belied by its weight, sporting a hefty, but replaceable battery internally. That provides about 5 hours of output from the voltage output, and 3 hours from the current output (2.5 hours in the H version).
Inputs are RCA or 3.5mm, use of the latter of which disables the former. Charging is via USB-C, and the unit came with a Raspberry Pi power supply, though I used one of the various 100W multi-chargers that hide behind my desk.
While the front of the unit looks conventional, with gain switch, 3.5mm and 6.3mm outputs, indicator lights and a volume knob, simple this amp is not. The volume, to begin with, is digitally controlled with 32 steps. Power the unit on by turning the volume knob to the 9 o’clock position with a “click”. The lights on the front will flash then stay on steadily, indicating an approximate battery level, with one pulsating as appropriate when charging.
Then, turn the knob a little further to begin output, further to which you’ll hear a very slight clicking through your IEMs or headphones with each volume step. Switch gain modes, and the amp doesn’t just jump, but the volume is ramped up, or down to that gain level as appropriate so that you don’t get any sudden surprises.
The sophistication continues where, if the amp detects anything untoward with the output, it’ll stop the output and flash the front lights. Leaving a headphone adaptor in the socket without headphones plugged in can trigger this, requiring powering off and on again.
Technology
Of course the curious reader at this point may be wondering why I talked about the outputs as I did, describing them as voltage and current outputs respectively. That’s where the uniqueness of the Bakoon-derived design gets interesting.
Regular headphone amplifiers are voltage amplifiers. An analog signal matching the sound waves of the music expresses it as changes in voltage, with a fixed current. However, various designers have considered, and used current as a transmission medium, rather than voltage, as there is a potential for lower distortion. As current-to-voltage conversion can be performed with nothing more than a resistor (some amplifiers even use the volume pot, which is a variable resistor), a pair of headphones can adequately do this as well, but with some caveats.
Conventional voltage amplifiers have an output impedance near zero Ohms, so as not to have the headphones or speakers “drive” the amp back, since they are electromagnets in essence, and themselves generate voltage as the driver bounces back as the voltage drops.
Current amplifiers turn this upside-down, and have an output impedance in the mega-Ohms, which means they require headphones with a flat frequency-to-impedance. Conventional headphones don’t have a flat impedance curve, unfortunately, but more recent, and quite popular planar magnetic (orthodynamic) models almost always do, making them the perfect candidates for this form of amplification.
What this means is, a headphone such as Sennheiser’s HD800S, which has an impedance that goes as high as 600 Ohms in the mid-bass region, will have its frequency response change with its impedance (the gain of a current amp is relative to the impedance of the headphones). The result is a crazy mid-bass boost, which is why there is also a second, voltage-based amplifier behind the 3.5mm socket.
IEMs, which often have wildly-varying impedance curves, are not suited to current amps either, so this second, conventional voltage amp covers them as well.
Planar, or more correctly orthodynamic headphones such as you might get from Audeze, Meze, Final, HiFiMan and others (Dan Clark Audio are a slight exception, more on that later) all have flat impedances, so make for being great candidates for this kind of amplifier.
The downside to this design is that while the 3.5mm voltage output amplifier has up to 500mW of power, it is limited to about 5 hours of battery life. The current output only has, respectively, 3.5 hours (HPA-23RM) or 2.5 hours (HPA-23RMH) of use in exchange for the 1W or 1.35W of power available.
The amp, however, can be used plugged into a USB-C charger as a desktop amp without any compromise to the sound.
Listening Impressions
Initially, the HPA-23RM sounded a bit closed-in, albeit with that familiar quality of “black” background and serious depth I associated with this type of amplifier. I left the amp running, plugged into power, for a few days, whereupon the soundstage opened up and came quite close to that of my main amplifiers, the Schiit Audio Mjolnir 3 and ALO Audio Studio Six.
Tonally, the amp was neutral, rivalling the output of Chord’s TT2, which I use with an MScaler for most listening. That had it working excellently with whatever headphones I used with it. Where a pair of headphones wasn’t suited to the current output, such as the Focal Utopias (2022 version) or HD-800s, unlike my old experience with the HPA-01, the voltage amplifier was excellent.
The only pair of headphones that I found more suitable for my Studio Six, or their own brand of DAC amp, were the HiFiMan Arya Organics, which I found synergised best with gear that was slightly warm or intentionally harmonic, whether that be R2R DACs, or a tube amp of some kind.
The weird outliers were the Dan Clark Audio (DCA) headphones, which, due to some rather unusual aspects of their design, have a slightly higher, if flat impedance from 20 Hz to somewhere in the mid-range. Given the Stealth tends, for my preferences, to sound a bit thin, the resulting bass-shelf-type lift from the current output was welcome.
Likewise the Expanse, which is similar to the Stealth, but already has more bass. I found the lift to be entertaining, if anything, whereas in discussion with Soo In, he stated he preferred the voltage output for those headphones for this reason.
What brought the most joy to my listening was the level of detail that came through the HPA-23RM (and H). The amp responded positively to better source gear very clearly. As a potential candidate to be used with Chord’s Hugo 2 as a (trans)portable rig, as bulky as that sounds, I observed that using a 3.5mm to 3.5mm adaptor, rather than RCA cables between the two, resulted in a slightly narrower soundstage. This corresponds to my understanding that TRS connectors have a slightly higher crosstalk than do RCA ones.
The main question then was, can the HPA-23RM really bring benefit to the Hugo 2, given that Chord’s portable flagship already has an excellent headphone amplifier built in.
Switching to in-ear monitors, particularly the very sensitive Campfire Audio Solaris a combined balanced-armature and dynamic driver model, in low-gain mode the HPA-23RM had sufficiently low hiss from the voltage output for at least half the volume range. The question is always whether or not with an amplifier to just max the volume control and use the Hugo 2’s internal digital volume. In this case, as turned the HPA-23RM’s volume towards the maximum point, hiss became quite clearly audible, so I settled on somewhere around the 12 o’clock mark for IEMs.
The surprise then was that, after I matched the volume levels as closely as I reasonably could, the bass quality from the HPA-23RM was noticeably superior. From the Hugo 2, it seemed boomier and more out-of-control. It wasn’t a trivial difference either.
A similar story was to be had with the Final A8000 IEMs, themselves using a single, high-quality dynamic driver per side, with which the bass was considerably improved by switching to the HPA-23RM from the Hugo 2.
With the Modhouse Tungsten DS
One of the most unexpected combinations that has appeared recently has been of the HPA-23RMH (the higher-power version of the HPA-23RM) with the Modhouse Tungsten (available here). These 3D printed headphones, for which I’ll write a separate review, require crazy amounts of power, with 155 Ohm impedance yet a sensitivity of only 78 dB/mW means that they need amplification with a strong voltage swing at the very least to drive. Basically, it’s speaker-amp-or-go-home territory.
The HPA-23RMH, given it’s current output making the gain proportional to the impedance allows it to drive the Tungsten surprisingly adequately, despite the maximum 1.3W of power output.
Sure, you wont be running movie soundtracks with 20 Hz bass at high volumes (I tried, it clipped) but the combination works surprisingly well.
Using the Chord TT2 and MScaler combination as my source, I hooked up the HPA-23RMH with a pair of Van Den Hul The Hill interconnects and put on Too Young by John Lee Hooker (Chill Out (2007 Remaster). Using the brighter-sounding leather pads, listening to this simple track with the combo was very compelling, the vocals coming through wonderfully, though I did notice a slight bloom in the lower registers.
Since the Tungstens are in for review, I tried Arethra Franking singing Elenor Rigby, a recent unexpectedly interesting discovery. For that tracks, I preferred the slightly more open sound from the leather pads, but, most impressively was the clarity of the soundstage, with each part of the music (unfortunately mixed very left/right) coming through clearly,
Fleetwood Mac’s Sara is presented in a bit more of a “monitor” way in that everything is precise, including imaging, no doubt thanks to the Chord stack, but also the amp and its resolution. I remember feeling a bit more moved by the song the previous night when I was listening with the Susvara Unveiled and Luxman P-750u which I had in on loan from a friend, which more a reflection of the slightly coloured sound the Luxman has.
An old favourite track of mine from The New Basement Tapes is When I Get My Hands On You which has a strong bass line. The bass punches through very well, despite the technical limits of the HPA-23RMH, however I didn’t feel like the bass was out-of-control at all as I had with the Tungsten on most other amplifiers I have here. To do better I had to connect the Tungsten to the Audio-gd Master 10 speaker amplifier, which transforms the Tungsten into what sounds like a totally different headphone.
Yuja Wang playing Rachmaninoff with the Los Angeles Philharmonic (Rachmaninoff: The Piano Concertos & Paganini Rhapsody) didn’t quite have the grandiose presentation I might have liked for an orchestral work, and the presentation with the Tungsten was a touch analytical for taste.
Conclusion
The HPA-23RM and HPA-23RMH are impressive amplifiers. Their ability to take near total control of both IEMs and most headphones while being battery powered is an impressive feat and validation of the current-drive technology inside them.
While the idea of buying a $3000 (or $3300 in the case of the HPA-23RMH) amplifier to go with an already very capable $2000+ DAC such as the Hugo 2 for portable use seems crazy, this kind of thing is not at all unusual in markets such as Hong Kong and Singapore. When Chord’s original Hugo came out, after I got a ~$2400 Astell&Kern AK240 in for review, people used to ask me if they should buy it to use with their Hugo as a transport!
Overall, the amplifiers do their best not to colour the music, which for synergy purposes doesn’t have them matching all headphones and tastes, but for a very “clean” and highly resolving amplifier, both versions of the HPA-23RM I do very much feel meet their asking prices, if the battery life is not an issue.
