Extremely open, revealing tuning, Outstanding resolution and control, Top-class build quality, Great separation
Mediocre isolation, Recessed, occasionally dry male vocals, Unapologetically bright
Those wanting an earphone with top-class build quality, excellent resolving power and a hyper-revealing signature will find they probably won’t need to look higher than the HS1503AL.
Though hybrids and BA earphones with 2-digit driver counts have become commonplace as of late, there’s no denying the appeal of a well-done single dynamic driver. Acoustune is a Japanese audio manufacturer that specialises in single dynamics, utilising tough Myrinx drivers and meticulously designed acoustic chambers to provide a resolving yet coherent sound. I first encountered their HS15XX earphones at Portafes, Japan. Their industrial designs allured many but their sound was is undoubtedly the factor that left a lasting impression. Their new HS1503AL is a successor to the HS1501, implementing similar technologies while introducing greater treble extension and a more balanced tuning all at a slightly cheaper ~$420 USD asking price. Further details on the HS1503AL and Acoustune’s innovations can be found on their website.
The unboxing experience of the HS-range of Acoustune IEMs is sensational. All come within a huge metal hard case that keeps the IEMs and accessories well-protected during transit. Inside are the earphones within foam with the cable wrapped around the inlet.
The faux leather magnetic carrying case lies to the right. Underneath is are a plethora of ear tips, each offering different fit and sound. In addition, Acoustune provides a matching faux leather cable organiser and Velcro strap that can be attached to the cable. It’s a terrific experience that leaves an impression on the buyer and a practical accessory set for daily use.
Acoustune’s IEMs are the definition of unique and the embodiment of “made in Japan”. There’s nothing on the market that even remotely resembles their industrial design; with segregated sound chamber and external frame that serves to reduce mechanical interference. Their entirely aluminium build astounds with its density and the level of finish is top-notch, even under one-one pixel peeping scrutiny.
Despite their unorthodox styling, the earphones actually have quite a comfortable fit. This can be attributed to their smooth, rounded inner face that feels similar in the ear to a standard pod-style IEM such as those from Westone and Shure. As such, none of the earphone’s sharp angles contact the ear, mitigating hotspot formation and enabling a comfortable fit over extended listening.
They are fairly shallow fitting, however, they are stable due to their well-shaped design and over-ear fit. Still, though not obviously vented, isolation is just average. With a bassier tuning, they suffice for most public transport but will struggle on the subway and during air travel.
The earphones utilise a removable MMCX cable that’s hugely improved over past Acoustune IEMs. It’s just as thick and sturdy but is now far suppler, with zero memory and minimal microphonic noise. It’s built incredibly well-built, with a beefy right angle plug, low-profile continuous y-spit and textured MMCX connectors that enables easy removal. The cable feels built to last and its compliant nature means it isn’t cumbersome during daily use despite its size.
The single flange tips provided the strongest seal for my ears. Of them, I most preferred the yellow tips that provided a slightly smoother high-end than the blue tips. All testing below was conducted using the yellow tips driven from the iBasso DX200 with AMP5. The earphones underwent over 200hrs of burn-in to ensure optimal performance during final evaluation.
The HS1503AL has a very Japanese-style V with heightened bass set to a bright, revealing midrange and energetic high-end. This stems from fairly large sub and mid-bass emphasis, a forward upper-midrange and heightened lower-treble. As the lower and centre midrange sit behind, its vocals are fairly laid-back, but it also sounds very clear and surprisingly transparent as a result. All of this is set to outstanding control that ensures notes remain focused and well-delineated.
The HS1503AL has a powerful low-end with great impact and bold rumble. Its notes are big and rich, aided by very strong extension and considerable sub and mid-bass emphasis. As its tuning slightly biases mid-bass, the earphone sounds especially full and it carries a warmer tone. Rumble is visceral and very well defined, lows also don’t drone, sounding pleasantly dynamic. Still, simply due to the nature of the HS1503AL’s mid-bass focused low-end, separation isn’t exemplary though bass doesn’t sound congested either. To maintain a transparent midrange and bring its tone more in-line with neutral, upper-bass is slightly de-emphasized, gradually sloping into a clearly attenuated lower-midrange.
This heightens bass/midrange separation and redeems some transparency. The earphone also demonstrates outstanding control; its bold notes maintain composure and lows surprise with their texture and detail. Notes are big and more rounded than most, though as decay is on the faster side for a dynamic driver in-ear and control is very high, every note sounds focused and impact is tight, so bass remains clearly defined. I would not consider the HS1503AL an outright bass-head earphone, but it does possess big bass notes and heaps of slam with strong technical traits keeping detail and definition in check.
Though hardly warm, lush or full-bodied, it’s interesting how much body mids have considering how upper-midrange biased the earphone’s tuning is. This can mostly be attributed to their full low-end that injects body into the entirety of its sound. As lower-mids are quite recessed, bass spill isn’t a prevalent issue and mids actually sound quite transparent and neutrally toned, though it can also sound both warm and cool depending on the track. Still, mids are generally on the thinner side and over-articulation is present due to large lower-treble emphasis. Male vocals are most notably affected by that lower-midrange dip, occasionally sounding a touch dry. Vocals are also fairly laid-back due to relative attenuation of the centre midrange, however, they also avoid becoming over-forward or thinned out as a result.
This is especially notable as the earphones quickly pick up into the upper-midrange where they explode with clarity and energy. This creates a revealing over organic presentation and one that biases female over male vocals. However, as Acoustune employ a typical 4KHz dip to redeem some vocal density, the HS1503AL’s midrange reconstruction remains pleasantly smooth despite considerable treble elevation. So though not linear, the HS1503AL is very revealing and simultaneously quite clean. Sibilance can be enhanced on tracks that are already prone, though not to the extent of sharpness or fatigue. Male vocals and instrument timbre also aren’t especially realistic, however, the HS1503AL’s clarity, openness and the ability to uncover every ounce of micro-detail are jaw-dropping.
Highs are very crisp yet clean, clearly lifted but not peaky. In particular, lower treble emphasis creates a very aggressively detailed presentation, but gradual climb to emphasis results in more accurately bodied treble instruments than most brighter earphones. Interestingly, the earphones aren’t actually that sharp sounding or sibilant, at least to a lesser degree than models like the Dunu Falcon-C, despite having similar emphasis. This can be partially attributed to the HS1503AL’s forward upper-midrange that provides a more organic treble instrument reconstruction. However, as opposed to earphones like the Campfire Comet, the HS1503AL also has a lot of crispness, derived from 6KHz emphasis.
Where the HS1503AL deviates a little more from most is its middle-treble. Most would employ some form of attenuation to maintain a dark background and provide contrast, however, the HS1503AL is rather lightly elevated, delivering enhanced air. It’s background remains mostly clean and avoids excessive brightness, thereby avoiding stridence. Still, the earphone are more clear-cut bright as a result. The HS1503AL follows up with linear extension into the upper-treble, producing very high resolution and imbuing additional sparkle. These factors contribute to an energetic, open presentation with very high resolving power on top.
With strong extension and a tuning that maximises separation, the HS1503AL delivers a very spacious stage with especially pleasing width. Imaging is fairly good if not pinpoint precise like more linear earphones though layering is excellent, well-defined and well-resolved. Separation the HS1503AL’s prime asset. As bass is quite full, it isn’t hyper-defined, but still well-separated due to high control and both midrange and high-end are hyper-separated, open and easily deconstructed. Directional cues are sharp and centre image is strong. The HS1503AL has a very grand, immersive presentation even if it isn’t perfectly coherent and precise in its placement.
The HS1503AL is quite efficient with a fairly average 32ohm impedance matched with a very high 110dB sensitivity. As such, it reaches very high listening volumes from portable sources but it also picks up hiss fairly easily, something to note for low volume listeners. On the contrary, it isn’t overly source sensitive as it is a single dynamic driver with a flat impedance curve, output impedance has little effect on its signature as demonstrated below with the Hiby R6 that has a 10-ohm output impedance. Still, the HS1503AL benefits from a dedicated source and scales very well due to its technical ability, especially with regards to soundstage expansion and bass definition. Select pairings below:
HTC U11: Bass is slightly less extended and controlled, smooth texture lacking fine detail and slam. Midrange is slightly more veiled, warmer tone and less transparent, vocals sound slightly strained, low resolution. Treble is present and aggressively detailed, retreival is quite good but the background is bright and treble is less controlled. Mediocre extension and soundstage expansion, imaging is just average and layers aren’t well delineated.
Shanling M0 ($90): Nicely controlled low-end, a touch longer decay and a very slightly warmer tone, more impact. Mids are slightly more full-bodied, upper-midrange sounds denser and more intimate. Nicely detailed, but missing some micro-detail compared to larger sources. Good top-end extension and resolution. Medium soundstage expansion, still layered. Imaging is precise and the source has zero hiss.
Fiio M7 ($200): Controlled low-end, well defined and impactful. Delicate vocals, slightly brighter timbre but clear and slightly more intimate. Slightly denser vocal reconstruction. Highs are nicely detailed, slightly more presence in the middle treble produces a brighter background. Nicely detailed, slightly smoother detail presentation. Medium soundstage expansion but still well separated and nicely layered. Barely audible hiss.
Hiby R6 ($600): Bass is nicely controlled with appropriate decay and tight impact. Slightly smoother bass texture. Mids are well extended but female vocals are more laid-back and smoother. Slightly enhanced detail presence, slightly thinner instrument body, some micro-detail smoothed off. Good resolution and a darker background. Very good soundstage expansion, well separated, layers are slightly less delineated. Zero hiss.
iBasso DX200 w/AMP5 ($900): Very controlled low-end, great definition and separation. Mids are very well extended and open, neutrally toned, perhaps upper-mids are even a touch forward. Highs are more aggressive, very detailed and micro-detail retrieval is outstanding. Regardless, the background remains clean. Great extension and resolution, slightly enhanced sparkle. Great soundstage expansion with more precise imaging. Great layering, zero hiss.
Dunu Falcon-C ($200): The Falcon-C is more balanced, especially through its midrange and bass. It has similarly strong sub-bass extension combined with similar emphasis, delivering great slam. However, its mid-bass is more neutral and its upper-bass is a hair attenuated, delivering a neutral tone and greater separation. The Acoustune is slightly more controlled, so it is similarly detailed despite being much fuller. The Falcon-C has a more transparent and accurately bodied midrange. It also has some over-articulation and a hair of upper-midrange emphasis.
It also lacks the dip for density, sounding slightly more extended if not quite as clear or revealing overall. The Falcon-C also shares the HS1503AL’s 6KHz peak, however, it is more isolated on the Dunu, and instruments sound a lot thinner as a result. The Acoustune is clearly more detailed and it isn’t as sharp or sibilant. Both have similar middle-treble quantity though the Acoustune extends a bit further up top and has greater upper-treble energy. As such, its resolution is higher. The Acoustune delivers a similarly sized stage with similarly strong separation.
Cardas A8 30th Anniversary ($350): The A8 is more L-shaped, thicker and warmer. It has similarly strong sub-bass extension combined with a similarly big mid-bass. However, the A8 has more upper-bass and lower-midrange so it is fuller and warmer overall. As such, it also isn’t quite as defined or separated, especially between its bass and midrange which is also quite full. This is especially so as it lacks the same upper-midrange emphasis, sounding denser and thicker. Otherwise, vocals are slightly more present and they are smoother as the A8 lacks the same treble presence.
In isolation, the A8 actually has an emphasized lower-treble response, but by comparison to the HS1503AL, it sounds quite smooth. As its peak is centred around 5KHz before steep attenuation, its background is a lot darker and it lacks the same crispness, instead favouring a warmer treble timbre. The A8 doesn’t extend at the very top like the Acoustune and it lacks the same level of resolution and detail retrieval. Its soundstage isn’t as big overall, but it has greater depth. It isn’t as separated but is slightly more coherent in its placing.
Fidue A85 Virgo ($400): The A85 is another brighter earphone though its markedly more mid-focused than the HS1503. It bass doesn’t extend quite as well and it is a lot leaner throughout. As such, bass is also more neutrally toned and it’s slightly more defined but not by a large degree. The A85 has a similarly recessed lower-midrange but also a more forward centre midrange, prioritising vocals over instruments. This contrasts to the more V-shaped HS1503AL. As the A85’s bass is less present, its timbre is skewed bright and it doesn’t sound as natural with upper-mids sounding a bit over-forward and lacking density.
However, since its treble is less present, it’s also smoother and lacking any over-articulation. The HS1503AL is a lot more aggressive in its detail presentation and also extends futher. Treble is less detailed on the A85 but it is also significantly smoother than the HS1503AL, delivering greater instrument body at the cost of less air and crispness. Resolution is higher on the A85 and it has considerably more sparkle and energy. The A85 has a larger soundstage with better separation, however, it is less coherent than the HS1503AL.
Hyla CE-5 ($940): The CE-5 is similarly tuned but with a darker background and greater overall balance. Its sub-bass extension is slightly stronger and it focuses on sub-bass more heavily than mid-bass, favouring a more neutral bass tone. The HS1503AL is fuller down low, but it also isn’t as controlled and hyper-detailed as the CE-5. The CE-5 similarly has a recessed lower-midrange and its male vocals can occasionally suffer from the same dryness as a result. However, it has more centre midrange so its vocals aren’t as laid-back and its upper-midrange is enhanced.
As emphasis isn’t to the same degree, this grants the CE-5 great clarity without the bright timbre and it is more transparent. The CE-5 also has quite a lot of lower-treble aggression, not quite as much as the HS1503AL and it is immediately more detailed. However, it differs in its middle treble where it employs considerable attenuation to achieve a darker, cleaner background. The CE-5 extends further at the very top, delivering higher resolution. Its stage is bigger and it has a lot more separation. Imaging is slightly better on the Hyla.
So what we have here isn’t realistic in timbre, balanced or orthodox. Instead, it’s a wonderful composition of high-quality Japanese engineering and a vivid tuning that does all in its power to take advantage of that. To turn this impression into more of a statement, I think we’ve become desensitized to pricing as reviewers. Gone are the days where one would expect perfection at $400. Instead, we almost expect flaws in the shadow of higher-end flagships pushing the 4-digit boundary. The HS1503 is refreshing because it treats itself like a flagship, not in pricing but in its no-compromise philosophy.
Its housings feel better than most $1K+ IEMs and its sound demonstrates both scrupulous engineering and responsiveness to user feedback. It’s not a $400 flagship killer, but it doesn’t come across as explicitly compromised in any regard either, and that’s become a rarity in the modern audiophile scene. If you’re looking for something with superb noise isolation or perfect balance, look away. However, those wanting an earphone with top-class build quality, excellent resolving power and a hyper-revealing signature will find they probably won’t need to look much further than the HS1503AL.