Enleum HPA-23RM headphone amplifier
Brands seldom hit the big time with their first product but Enleum’s AMP-23R integrated amplifier that Kevin Fiske reviewed in 2021 immediately earned the respect of hi-fi and head-fi enthusiasts alike and is no less revered three years on. Such an achievement is rarely the result of happenstance and the apparent overnight success of Enleum’s founder and driving force, Soo In Chae, was actually a journey of steady progress that began all the way back in 2009 when he founded Bakoon International. In the twelve years that followed, Chae was successful in bringing several innovative, high end designs to market, culminating in the AMP-13R integrated and HPA-21 headphone amplifiers – precursors of the AMP-23R and HPA-23RM.
Intended for not only desktop but also mobile usage, the battery-powered HPA-23RM headphone amp inherits technologies from both the AMP-23R and HPA-21 and packages not one but two high-powered headphone amplifiers in a reassuringly solid and highly tactile precision-machined slab of aluminium. It takes its minimalist, ergonomic designed cues from the 23R, measures an almost pocketable six and a half inches at its widest and weighs just over one and a half pounds.
Equipped with not only a stereo RCA input for desktop sources but also a 3.5mm TRS input for portable players, it is impeccably detailed with crisp lines framing each of the five functions on the front of the unit and scalloped radiators along each side to dissipate heat. Care has been taken to ensure that no parts snag when used on the move. The RCA sockets are recessed into the rear of the unit, while the faceted volume control is flush-mounted into the fascia and accessible only by rolling a finger-tip across the small, exposed section on top; creative solutions that no doubt contributed to it winning the Red Dot and iF Design Awards in 2022 and 2023.
Improvements to the battery power supply and thermal efficiency allow the HPA-23RM to be Enleum’s smallest and lightest design to date, taking up just a quarter of the space of the HPA-21. Listeners can expect up to five hours of uninterrupted use from the voltage output or up to three and a half hours from the current output. The unit is fairly quick to recharge from the included 15W 5V USB-C power supply and can be enjoyed whilst either charging or in a fully-charged state, though the manual recommends charging for at least 10 minutes before use if its battery has been completely depleted. Battery and charging status is communicated via the three white LEDs next to the volume control. The battery itself comprises two 18650 cells that are good for 2,000 cycles, widely available and nowhere near as expensive to replace as the bespoke batteries found in many other portable audio devices.
Like its much larger desktop companion, the HPA-23RM is a Class A/B, current-mode, push-pull, zero feedback design that’s single-ended from input to output. Having reviewed my fair share of each, well-designed single-ended topologies sound just as good as their balanced counterparts to my ears, especially when they have enough grunt behind them. While this diminutive Enleum may not have the AMP-23R’s seemingly limitless headroom, the respective 500mW and 1W rms of power available from the voltage and current headphone outputs into a 30Ω load ought to reproduce thrilling dynamics with all but the most challenging earspeakers.
Serviced by a 3.5mm TRS socket to appeal especially to portable headphone and IEM wearers, the HPA-23RM’s constant voltage source amplifier is based on the circuit found in the 23R and uses four MOSFETs (two per channel) with a simplified JET2 biasing system that monitors and corrects each pair in real-time to ensure they’re always performing optimally. It has very low output impedance (less than 1Ω), meaning headphones with variable impedance will not be subject to damping factor-related tonal colourations. Sporting a more traditional 6.3mm TRS socket, the HPA-23RM’s constant current source amplifier is a more technologically-advanced version of the HPA-21’s bipolar transistor circuit and utilises the MOSFETs in the voltage circuit as drivers for four bipolar transistors.
According to Soo In Chae, current drive is superior to voltage drive as it eliminates the deleterious effects of impedances that are in series with the circuit – the resistance and inductance of the wiring and voice coil, capacitance, contact resistance and EMF – and provides “more precise and direct driving of the voice coil for faster response and lower distortion”. There is, however, a major caveat to current drive, which is that voltage gain – which ultimately determines the volume you hear – changes with load impedance. In other words, when a drive unit is driven by a current source its frequency response tracks its impedance curve, similar to the effect of being driven by a voltage source that has extremely high output impedance. The more the impedance varies, the more the frequency response is modulated. Unless tonal colouration is sought, current drive is best reserved for loads with ruler-flat impedance curves – an occurrence that used to be so rare that demand for this type of amplification was extremely limited. Continued growth in the popularity of planar-magnetics is, however, making current source amplifiers relevant to an ever increasing number of headphone users.
It also means that the audibility of circuit noise increases not only with a headphone’s sensitivity but also its impedance. Chae believes this is an acceptable trade-off in the pursuit of a simpler and more direct circuit and it’s difficult to argue with this when you realise that the noise is below the threshold of audibility via the low-sensitivity, low-impedance planar-magnetics the current output is designed for.
Both outputs in the HPA-23RM are downstream of Enleum’s Ensence technology that uses discrete transistors with zero negative feedback to preserve both the transient and frequency response of the incoming signal. Handing off to a solid-state relay attenuator circuit comprised of precision switching resistors, instead of attenuating an already amplified signal the desired output level is achieved by varying the gain and, as a result, fidelity and channel matching is preserved at all output levels.
As well as controlling volume, the knurled attenuator knob also powers the amp on and off. At its minimum position the unit is off. Between 7 and 9 o’clock the internals are powered but the outputs are muted by a relay to protect against the momentary shorts that occur when plugging and unplugging headphones. Above 9 o’clock the relay disengages and the knob acts like a conventional stepped attenuator, albeit with only 210-degrees of rotation available instead of the usual 270. Chae has nevertheless managed to squeeze in 32 steps of adjustment that provide a total of 62dB attenuation. Each step in the upper half of its turning range effects a 1dB change in level, this widens to several dBs per step as you continue to turn the dial counterclockwise to reduce the output level.
High gain mode is engaged by flicking the toggle switch on the front of the unit to the up position, boosting output from the Ensence module by a fixed 15dB at any given volume setting. I found this was only really necessary for particularly insensitive headphones or when listening to sources that output less than the industry-standard 2Vrms. I kept the amplifier in low gain for the majority of my audition and only needed to switch to the higher setting when using the 600Ω version of the Beyerdynamic DT880 (95dB/1V sensitivity) and the Dan Clark Audio Stealth (98dB/1V sensitivity), and this was mostly when listening to DSD recordings through HoloAudio’s Cyan 2 DAC that outputs DSD at half the PCM voltage.
Sound quality
I rarely prioritise convenience over sound quality but commuting is one exception and, while I have nothing but admiration for the diehard audiophiles who wouldn’t think twice about carrying a high-end device of the HPA-23RM’s size and weight on their person, I’m more comfortable with an all-in-one DAP that slips easily into in my trouser pocket. That said, I do find Chord’s Hugo 2 very convenient for enjoying high-end playback in spaces away from the listening room, whether that’s in different areas of the house, the garden, the office or a hotel room. Being just over 40% bigger in footprint the Enleum is still portable for this kind of use though, unlike Hugo 2, you do need factor in a separate DAC if you aren’t using your DAP’s onboard converter. The Chord DAC/headphone amp also let’s you go cable-free on the input side, either by streaming directly from your DAP over Bluetooth or by adding its bolt-on streaming companion, 2go.
While the Chord edges it on convenience in these environments, the stronger connection to the music I felt by using Hugo 2 as a DAC and plugging my headphones into the HPA-23RM’s 3.5mm voltage output made the clutter of an extra device a price that was worth paying. Hugo 2’s headphone amp isn’t short of output power, especially into low impedance loads, but the Enleum takes things to another level by resolving timbres that are more organic and a less congested, more holographic field of sound without giving up much, if indeed any, of the Chord’s impeccable timing. This was evident on all of the headphones I partnered it with regardless of impedance but was particularly welcomed on Audio-Technica’s ATH-MSR7, Focal’s Utopia and Sennheiser’s HD600, all of which have a tendency to sound somewhat forward and intimate when driven by solid-state topologies that use negative feedback.
Enleum’s voltage output opens up the soundstage in all directions in You And Your Friend from MoFi’s 2024 DSD remastering of Dire Straits’ immaculately-produced On Every Street. Paul Franklin’s rattly, ‘wild west’-inspired lap steel and Knopfler’s seductive lead guitar and cryptic vocals all emanate from tangible points in three-dimensional space with greater distance between them than on the Chord. The bottom end also cleans up and is more fluid, leaving more space between notes and allowing the background strings, that are key to nurturing the ambience of the track as it progresses, to sit clearer in the mix.
The HPA-23RM spent the remainder of its time in my main system where it faced very stiff competition from Holoaudio’s Bliss – a full-sized, fully-balanced, fully-discrete, Class-A design capable of delivering a prodigious 12W rms from its balanced headphone output into a 32Ω load. While Bliss benefits from a chassis that’s twenty times as big as the Enleum and can output twelve times as much power, the fundamental design philosophies of these two headphone amps aren’t a million miles apart. Partnered with not only my reference Schiit Yggdrasil OG multibit DAC but also Holoaudio’s new Cyan 2 NOS resistor-ladder DAC with external oversampling applied, I was struck by how similar the presentations were and it was only after a period of extended listening that the differences began to crystallise.
Bliss became my reference headphone amp last year – dethroning the superb Sparkos Labs Aries – due to its ability to resolve detail in a more effortless manner and render a soundstage with more square footage than any other solid-state amplifier I’ve auditioned. The Holo and Enleum are equally revealing of what’s upstream and both are exceptionally musical and engaging to listen to, reproducing gorgeous, harmonically-rich tones that are never synthetic. Both have excellent extension and control at both frequency extremes, but the HPA-23RM has a touch less body and bloom in the lower fundamentals and is a little more sculpted and fleet-footed. Their soundstages are comparably huge; while Bliss’s is more panoramic the Enleum’s is slightly narrower but it anchors the centre image a little deeper in the stage in front of you.
I found its presentation equally appealing with all genres from classical to rock ’n’ roll. The National Symphony Orchestra is generously distanced around your head but arranged in more of a spherical formation than an elliptical one in Chasing The Dragon’s stunningly realistic binaural recording of España. Even though AC/DC’s legendary stadium rock anthem Thunderstruck is delivered with thrilling speed and attack, the tipping point at which the Young brothers’ guitar riffs breach your aural defences is suppressed just enough to let you crank the volume up one more notch to be pummelled by the full force of Chris Slade’s demonic drumming.
With special thanks to Dan Clark Audio’s UK distributor Electromod I was able to borrow their flagship closed-back planar-magnetic, Stealth, to test the credentials of Enleum’s current output. I had great fun in 2020 reviewing the Aeon 2 closed-back DCAs and I was excited to hear the potential of their v-Planar technology when it isn’t constrained by cost. Progressing from the previous tuning method of layering combinations of foam and felt in front of the driver to a 3D-printed AMTS lattice insert has been a game-changer when it comes to controlling unwanted reflections without harming transparency and transient response. Stealth is not only the smoothest, clearest and most resolving closed-back from DCA I’ve heard, it’s the most accomplished closed-back I’ve heard period. Plugging this headphone into the 6.3mm current output leaves you wanting absolutely nothing. Timbral trueness, openness, separation, and full-range extension with speed and slam – it’s all there in equal measure and sums to deliver a truly transcendent listening experience.
I was a highly impressionable 7-year-old when the Bee Gees’ Size Isn’t Everything was released and my affection for this album has grown stronger over the years as system upgrades have revealed more of its incredible production. Between my various headphone and loudspeaker systems I wondered if, three decades on, I’d heard everything there is to hear in these twelve tracks. Experiencing them through the Enleum/Stealth combo showed I hadn’t. The bottom end was always a highlight of this album but I’ve never heard it delivered with such speed and precision. Feeling the visceral impact from the percussion and bass in Anything For You without harmonic distortion and overhang was a revelation; textures that were previously masked were now finally apparent. I’m not normally a fan of headphones with a rising low end (Harman curve) but Stealth’s distortion is so inherently low that it’s able to deliver thunderous bass lines that are exceptionally crisp and clean with an amplifier that’s up to the job. The HPA-23RM’s current output controls Stealth’s planar drivers with authority and ensures those powerful sub-bass notes that extend all the way down to the limits of audibility never modulate and obscure the crystal clarity of the middle and high frequencies.
I experienced a somewhat reserved and watered-down version of this headphone when I switched to the voltage output but it was still mightily impressive, just not quite as hard-hitting or enveloping. Hearing a similar effect with the inexpensive and easy-to-drive Audio-Technica ATH-MSR7 suggests it isn’t only high-end, power-hungry, reference-level transducers that benefit from Enleum’s current output. The closed-back, linear-impedance MSR7 is far from being the most resolving or involving earspeaker in my collection but the extent to which it grew bigger and bolder – becoming not only more impactful but also more layered and immersive – convinced me that this output’s perceived superiority isn’t solely the result of it having more power on tap. I’m inclined to agree with Soo In Chae that a simplified circuit that provides more direct driving of the voice coil is good for fidelity.
Euphonic indulgence
I know roughly how much tonal deviation to expect when my variable impedance headphones are driven by voltage sources with specified levels of output impedance but I was in uncharted waters with the HPA-23RM’s current output. I knew there’d be increased output around the driver’s resonant frequency as this is where the impedance is highest in open-backs, but how pronounced would the effect be? The +1dB bass bump the bipolar transistors imparted into the 600Ω version of the Beyerdynamic DT880 was definitely a shift in the right direction, albeit a very subtle one, as I still found it difficult to melt into my music with this model’s razor-sharp top end. I also welcomed the more noticeable +3dB hump that appeared in the Sennheiser HD600, it added judicious measures of warmth and gravitas to this well-tuned classic. The +6dB hills that formed in both the Sennheiser HD800S and Audio-Technica ATH-ADX5000 skewed their tonality and obscured textural information too much for my tastes, and the +14dB mountain that erupted in the Focal Utopia would have beaten even the most ardent bassheads into submission! Reverting to voltage drive for these three flagship open-backs soon restored order and served up enough detail and slam to dispel the notion that being confined to the HPA-23RM’s voltage output for this type of headphone is in any way a hardship.
Verdict
With dedicated voltage and current amplifiers that are optimised for different loads, Enleum’s transportable, battery-powered HPA-23RM is arguably in a league of its own when it comes to driving both dynamic and planar headphones with authority. Yes it is a high-ticket item but its ability to compete with much larger desktop designs means this punchy and exceptionally musical little headphone amp could well be the only one you need. And that’s before pricing in the fun to be had from driving variable-impedance headphones with its current output, a euphonic indulgence I expect many will find irresistible.