Enleum HPA-23RM

Small, but perfectly formed” are the words that come to mind on unboxing Enleum’s HPA-23RM. The new headphone amplifier is just 22mm thick, and in plan view no bigger than the palm of a large hand.

Like all of Soo In Chae’s audio products of recent years, first from his previous brand Bakoon International, and now from his current company Enleum, the HPA-23RM has elements of the Korea-born, California-resident audio wizard’s distinctive design language all over it; classic gunmetal finish, sculpted sides and deep chassis score lines. Pick it up, though, and expectations of a featherweight are instantly dispelled. At 762gms, the HPA-23RM is surprisingly dense for its size. Take to the streets after rubber-banding it back-to-back with a contemporary DAP of commensurate quality, and you’ll be lugging around over a kilo of tech.

In fairness, though, that’s undoubtedly not the kind of portability Enleum primarily has in mind. More likely, one of the brand’s target users is the music-lover who travels and who, upon arriving at their overnight hotel, might place the HPA-23RM and DAP on the bedside table and spend an evening in their room blissing out to hi-rez recordings.

The other target group for the HPA-23RM is what we might term the conventional user whose audio consumption is home-based. In this role, I evaluated the HPA-23RM, using it on the kit table for four weeks as my go-to headphone amplifier. By way of an up-front takeaway, I can report that the Enleum HPA-23RM delivers a sound quality with many – but not all – dynamic and planar headphones that give little to nothing away to top-of-the-line full-sized, mains-powered headphone amps. Moreover, it does it for up to five hours before needing a re-charge via its rear-mounted USB-C port.

Shine a light

Lid off, the HPA-23RM is a marvel of miniaturisation and build quality. There’s no chunky power supply; a pair of 18650 lithium-ion batteries juice Enleum’s mighty mite, the same type used by many domestic torches or flashlights. This is charged by a wall wart. Two amplifier circuits, with zero negative feedback and featuring surface-mounted discrete components, are sandwiched between the batteries. One is optimised for devices such as IEMs that want voltage drive. The other is for designs like planar-magnetic headphones that need current to perform.

We find the USB-C charging socket, a stereo mini-jack input, and a single pair of RCA inputs on the rear panel. Around the front, to prevent the volume knob being turned up to maximum by accident when the user is on the move, the control is sunk into the front panel. This can only be rotated by stroking a finger or thumb over a wide slot in the top surface of the chassis. Three LEDs indicate battery state, and 3.5mm and 6.3mm TRS sockets allow IEMs and headphones to be connected to the Voltage and current outputs, respectively. A miniature toggle switch allows gain to be set at 2dB or 17dB.

Bakoon re-emergence

Audiophiles who lamented the passing of Soo In Chae’s Bakoon brand had reason to cheer when he re-emerged with Enleum. The new brand’s first product was the AMP-23R, a compact integrated amplifier-cum-headphone amp in which combined MOSFETs by British fab-house Exicon with an innovative biasing circuit that enables the amplifier to deliver the kind of approachable warmth, some might say naturalism, we might associate with a Class A design. However, it also has exceptional precision and grip. Mains powered, and with a stiff internal linear power supply that allows it to output 25 Watts into eight Ohms, the AMP-23R was never intended to drive big speakers in big rooms. However, paired sensibly, it achieves such a sonic performance that some reviewers regard it as a benchmark against which other designs can be evaluated. 

When I reviewed the AMP-23R in 2021, I, too, was captivated by its performance. While many other integrated amplifiers treat headphones as an afterthought, using an add-on integrated circuit op-amp to drive them, the AMP-23R brings its complete speaker-driving circuit to bear but with the gain rolled back. The result, even and especially on tricky-to-drive loads, is properly top-drawer. Only my Puritan streak stopped me from buying the review sample exclusively as a headphone amplifier to fill a vacancy on the kit table.

Wait, Watt?

Soo In Chae says that the AMP-23R has heavily influenced the circuit design of his new dedicated headphone amplifier. However, his new creation is not immune to the laws of physics, so we should not be surprised that rather than its larger stablemate’s output of 4 Watts into a 60-ohm load, the HPA-23RM delivers just 0.5 Watts into the same impedance.

I’ve picked on 60 Ohms to get the Susvara question out of the way. As fans will know, that’s the load HiFiMAN’s flagship headphone presents to amplifiers, along with an efficiency that is distinctly on the low side at around 83dB. The mains powered AMP-23R breezes Susvara, but while the HPA-23RM generates perfectly acceptable sound with the headphone – very loud sound too on its high gain setting – it doesn’t have quite the weight, dynamic expression and tonal detail that the Susvara can deliver when more appropriately driven. But, really, what should we realistically expect, given the half-watt output? 

A particular aspect of design contributes significantly to this surprisingly strong level of performance. Enleum’s baby is single-ended, not dual differential, but that’s not unusual. What is unusual is that it operates as a current gain, not a voltage gain device. Rare in headphone amplifiers, this is never seen in commercial speaker amplifiers. Loudspeaker impedance curves vary as the multiple drivers track up through the octaves, pulling the gain of a current source amplifier all over the place. The tail wags the dog, making current gain the wrong tool for that particular job.

Current drive

However, planar-magnetic headphones have a generally flat impedance curve. Enleum’s HPA-23RM takes advantage of this, squaring up to the resultant stable load and using current to drive the headphone voice coils directly with greater precision for lower distortion and a faster, more natural response. If you wonder why the HPA-23RM can cope well with demanding loads using just half a Watt while some other amplifiers with more on-paper grunt can struggle… current drive is part of the answer.

With alternative, more amplifier-friendly headphones, the results were even more satisfying. Into a load of 16 Ohms the HPA-23RM’s current output punches out 1.8 Watts, and at 300 Ohms it delivers 100 mW (current output) 50 mW (voltage output). As I indicated in my earlier teaser, these metrics enabled the review sample to trade blows with much larger and more powerful mains-powered alternatives, including its bigger brand sibling.

That’s not to say that the HPA-23RM can only be used with headphones with a flat impedance. Depending on where an impedance dip or peak sits, the result might be offensive or not. Soo In Chae points to the Sennheiser HD800, which has a peak of 650 Ohms at 100 Hz, standing proud of a nominal impedance of 300 Ohms. The effect, in this instance, is a bass boost that some users might like. That’s not an admission of sloppy design on his part but a caution that we need to understand the design parameters of the HPA-23RM clearly and pair it accordingly.

Most of my listening was done via an Audeze LCD5 headphone, which exhibited its characteristic mid-band forwardness and slightly rolled off top end, just as it does with other headphone amplifiers. To that extent, what I heard was unexceptional.

Liquidity

What sets the Enleum HPA-23RM slightly apart from the alternatives is a sonic aesthetic that I found both winningly engaging and strongly reminiscent of the sound produced by the mains-powered AMP-23R. This might best be described as a relaxed warmth and liquidity that initially deceives us into thinking we’re not given the complete detailed sonic picture. Only upon more extended listening followed by back-to-back comparison with some alternative headphone amplifiers do we realise the detail is present after all. The HPA-23RM doesn’t mask it but offers a slightly different perspective. Listening to music becomes not an exercise in forensic examination of the source material but rather a cosseting bath in sonic balm. 

Some readers will link that observation to the fact that the HPA-23RM uses no negative feedback. I think they are right to do so; the little Enleum calls to mind the kind of relaxed yet particularly transparent presentation that we might associate with top-of-the-line zero-feedback single-ended triode amplification. 

What’s remarkable about Enleum’s achievement is that the HPA-23RM doesn’t combine this with the soft and loose bass and truncated top-end that experience tells us we might also expect. The HPA-23RM digs basement deep and reaches top floor high, adding texture and control to a rich palette of tonal colour and a generosity of energy transfer that is truly competitive and, indeed quite shocking given its diminutive size. It slams hard and fast when required by plucked, bowed and keyed bass, with just as much weight and transient snap as any number of mains-powered alternatives can muster. Human voices are rendered with believable diaphragmic air compression and timbre, while high-frequency material plays out with sweetness and satisfying texture.

Intimacy

Driving the LCD5, the HPA-23RM produced a more intimate sound stage than some other amplifiers. Musical events tend to be more in-head than external. Specificity and layering proved to be to a high standard.

Whether a slight perceived recess in the HPA-23RM’s midband shows on the testbench or not, its partnership with the LCD5 proved highly satisfactory. The amplifier tamed to a degree the headphone’s forwardness in that region, compensating for its rolled-off top end and beefing up the low end, all without the application of Roon’s excellent EQ. I loved the result.

Brief experiments were made with IEMs. I don’t own this type of transducer because I find it either uncomfortable or impossible to wear. However, I can report that the loaned IEMs exhibited zero intrusive hiss when connected to the HPA-23RM in low gain mode via its voltage output. A household member with more obliging morphology and therefore able to listen to multiple recordings on my behalf reported that the HPA-23RM’s IEM performance is of an extremely high standard.

While Enleum’s claim of up to five hours of operation on the internal batteries – three in current mode – is achievable, it depends on load and how loud we want to listen. I suspect many users with no interest in portability might just plug the HPA-23RM into the wall wart and leave it at that. Pursuing thorough enquiry, I ran the review sample that way for a day. Either the Raspberry Pi wall-wart supplied with the HPA-23RM is ultra-quiet, or the HPA-23RM incorporates effective EMI filtering, or, highly likely, both are true. The upshot was that I struggled to hear any zero sonic difference when the wall wart was connected.

Parity

Some may look at the battery-powered HPA-23RM and wonder if it is Enleum’s last word on the matter or whether a dedicated mains-powered version is in the works. He says that’s it for now, and he encourages potential purchasers to think of the HPA-23RM as his statement personal listening device.

Readers are free to distrust my audio memory since it is two years since I heard the mains-powered AMP-23R. However, after some four weeks of living with the Enleum HPA-23RM I was convinced that it may perform at the same level as its larger integrated amplifier sibling.

That it does this yet costs £3,000 less than the AMP-23R makes Soo In Chae’s dinky creation a shoo-in for any headphone amplifier shortlist. To potential buyers: suspend disbelief until you’ve tried one. 

Technical specifications

• Power: 1 Watts (Current Output) / 500 mWatts (Voltage Output) @ 30ohms
• Inputs: RCA and 1/8” Mini Jack (Analogue)
• Outputs: 1/4” (Current) and 1/8” (Voltage) Headphone Jacks
• Operation time: Continuous or depending on the external battery capacity up to 5 hours (Voltage Output) / up to 3 hours (Current Output)
• Dimensions: 116mm x 164.5mm x 22mm (w x d x h)
• Weight: 730g
• Price: £3,300, $3,000, €3,000

Manufacturer

Enleum

www.enleum.com

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