Breaking the Two Decade Curse of the Reference Dynamic.
PROS: Exemplary midrange linearity and timbral correctness that accurately captures the natural weight of acoustic instruments and vocals | Disciplined low-end response with linear extension down to the sub-bass octaves, no mid-bass bleed or muddy masking | Pioneering Thin-Ply Carbon Diaphragm (TPCD) implementation delivering planar-like transient recovery and crisp edge definition | Thoughtfully engineered, 100% modular, and consumer-serviceable chassis optimised for long-term professional or personal use | Industry-leading 5-year manufacturer warranty demonstrating superb post-purchase support and confidence | High efficiency and low impedance make it exceptionally accommodating of low-power portable sources and reference dongles.
CONS: A narrow, highly localised treble elevation around 12-13 kHz that can occasionally sound slightly sizzly | The absence of a decoupled suspension strap can concentrate downward pressure on the crown of the head out of the box | The stock cable could’ve been better | Tonal balance in the upper registers remains moderately sensitive to physical placement and clamp force optimisation.
The product was provided free of charge for my honest take on it.
Introduction.
In the landscape of high-performance personal audio, certain engineering compromises have been reluctantly accepted as gospel by enthusiasts and professionals alike. For over two decades, the iconic Sennheiser HD600-series has stood as a towering monument to mid-range correctness. Yet, selecting it always required signing an unwritten contract: you happily embraced its legendary timbral purity while quietly tolerating its rolled-off sub-bass, compressed soundstage geometry, and moderate transient resolution. To step away from that signature usually meant crossing over to bright, analytical planars or heavy, colored dynamics that sacrificed that organic vocal magic.
Unboxing & Accessories.
True to their pro-audio lineage, HEDD bypasses flashy wooden and leather presentation for a very utilitarian pro-audio design. The unboxing experience is direct, secure and professional. The D1 arrives snugly packaged inside a rigid, low-profile EVA zippered hard case contoured specifically to the headphone’s dimensions. The included 2-meter split-Y cable terminates in standard dual 3.5mm plugs at the earcups, ensuring easy integration with aftermarket balanced upgrades. The textile sheathing is remarkably soft and does an excellent job of filtering out mechanical microphonics when draped over clothing. However, I wish they’d included a more premium balanced cable with swappable terminations or pig tail connectors to have the ability to switch between devices with ease. Also from a practical standpoint, because the cable is unbraided, it exhibits noticeable structural memory that causes it to twist and kink during long desktop listening sessions.
Otherwise the most impressive inclusion in the box is the 5-year comprehensive manufacturer warranty. This is an exceptionally rare consumer safety net at this price tier (mostly otherwise only offered by Ollo Audio as fas as I know), showcasing HEDD’s long-term commitment to durability.
Design, Build and First Impressions.
The D1 embodies a philosophy of pragmatic industrial utility. Covered in a completely non-reflective, monochrome matte-black finish with minimal branding accents and an exposed metal mesh outer grille, it looks less like jewelry and more like a high-performance studio tool designed for a mastering console. In the hand, the structural design feels incredibly integrated and solid. There are no creaks or plastic rattles when flexing the assembly. At approximately 350 grams, the D1 is refreshingly nimble, shedding a massive amount of physical mass compared to HEDD’s older, unwieldy AMT models like the HEDDphone Two lineup. The centre of gravity is excellently balanced, avoiding the forward-heavy tilt that plagues oversized planar headphones. What makes this design highly commendable from an engineering standpoint is its fully modular, consumer-serviceable architecture. Rather than using glued assemblies or proprietary sonic seals that mandate factory servicing, the D1 is assembled to be easily maintained. The perforated velour ear pads, headband cushions, structural yokes, and individual driver-baffle units can be disassembled and replaced independently by the user. Coupled with the five-year warranty, it sets a remarkable standard for long-term ownership security.
Comfort & Ergonomics.
The physical interaction with the D1 is comfortable, but it presents a distinct mechanical footprint that requires careful initial fitting. The anatomical oval ear pads are deep and plushly padded with high-quality perforated velour, allowing excellent heat dissipation and ensuring that even larger ear shapes never press against the internal dust mesh.
Out of the box, the horizontal clamping force is firm and secure. Because the headband design employs a direct, traditional notched metal slider mechanism without an elastic internal suspension strap, the vertical load can concentrate directly on the crown of the head, causing a slight hotspot after an hour of continuous use. From an acoustic engineering perspective, the D1 is structurally clamp-sensitive due to how the ear pad compression impacts the front volume acoustic seal. Users can easily fine-tune this by gently manipulating the internal metal spring structure of the slider arms to dial in their exact head shape.
Slightly optimizing the inward pressure not only re-distributes the downward vertical weight away from the top of the crown to the surrounding temporal bone, but it also alters the high-frequency wave reflections within the ear cup chamber. Achieving a proper, flush seal effectively smooths out the upper-register response, showing just how crucial physical positioning is to unlocking the D1’s full technical potential.
Driver Technology: The Carbon Revolution.
The defining technical feature of the D1 is the implementation of the Thin-Ply Carbon Diaphragm (TPCD). This material tech represents a massive departure from standard vapor-deposited carbon or common PET/Mylar materials. By layering ultra-thin, woven sheets of carbon fiber at alternating structural angles, HEDD has produced a 50mm diaphragm that possesses an incredibly high stiffness-to-weight ratio.
Under heavy dynamic loads, a standard dynamic driver membrane will naturally flex, causing localized cone breakup and harmonic distortion. The extreme rigidity of the TPCD driver ensures uniform pistonic motion across its surface, pushing distortion down to negligible levels while rendering sharp transient edges with remarkable precision.
Operating at a nominal 32-ohm impedance and a sensitivity of 100 dB, the D1 avoids the need for massive voltage swings to sound awake, making it a highly efficient, versatile transducer that handles complex acoustic signals with absolute composure.
Sound Analysis.
The HEDDphone D1 comes across as a highly refined, Sennheiser HD650-inspired uncolored reference headphone that prioritizes acoustic transparency, midrange correctness, and clean transient imaging over romanticized mid-bass warmth or artificial top-end sparkle. Effectively picking up where the HD600/HD650 left off, its presentation behaves like a well-calibrated studio monitor in an acoustically treated space—resolutely refusing to flatter poorly mixed files while effortlessly decoding the underlying arrangements of complex acoustic recordings. The overall tonal profile balances a flat, fairly extended low end with an “almost” transparent midband and an extended upper octave.
Diving into the specifics, it digs noticeably deeper into the sub-bass than its spiritual predecessors, extending remarkably flat down to about 40–45Hz before a gentle, low-slope roll-off takes over. It’s a highly optimised, late-stage roll-off, whereas the classic HD600 and HD650 exhibited a much steeper, far more aggressive drop-off slope. This controlled low-end transitions into an almost flat lower-midrange that completely prevents any muddy masking, leading into a slightly sloped, Diffuse Field-like ear gain of about 12-12.5 dB. This specific pinna rise ensures that vocals and solo instruments retain pristine clarity and a realistic presence, very rarely crossing the line into a honky or shouting presentation—though it can occasionally lean that way depending heavily on its placement- and clamp-force-sensitive character.
Further up, the lower-treble remains remarkably linear and neutral. The D1 completely avoids the artificial lower-treble valleys so often deployed by manufacturers to fake a smooth, safe sound; by keeping this frequency band fully intact, instrument attacks and micro-dynamic edges remain completely resolved, textured, and alive. This linear transition feeds beautifully into a slightly sizzly, airier upper-treble register that effectively unmasks room boundaries and acoustic decay trails—a technical tuning balance that is exceptionally impressive for a moving-coil transducer, even if it does occasionally grab your attention on brighter, hot recordings.
Let’s dig in deeper…
Bass – The low end of the D1 demonstrates excellent structural discipline. Unlike many traditional open-back dynamic designs that sacrifice sub-bass extension to protect midrange clarity, the TPCD driver remains cleanly audible down to ~35Hz. It has a low slope roll-off post 40-45Hz, which is remarkably better than competing open-back dynamic headphones under $1000; much better than the HD650, more similar to the HD660S2 but without the mid-bass bump. The more linear extension allows it to reproduce low-frequency information more convincingly stopping just short of delivering the kind of visceral rumble associated with headphones utilising a proper sub-bass shelf to convey the sub-bass rumble more authoritatively.
To some extent, this presentation is simply an inherent characteristic of the acoustic architecture. Closed-back headphones naturally convey low-frequency energy with greater physical slam because the sealed enclosure allows pneumatic pressure to build around the ear, creating an organic sense of weight, compression and impact. Open-backs can reproduce those identical notes with matching extension and transient detail, but they typically present them in a more airy, less tactile manner. This acoustic reality is precisely why I generally prefer a modest sub-bass lift in open-back headphones, as it helps restore the perceived weight and visceral authority that can otherwise be lost to the open air.
Beyond that sub-bass transition, the D1 ensures there is absolutely no artificial mid-bass boost or upper-bass bloom to inject false warmth or mask lower-midrange information. When tracking bass guitars or acoustic kick drums, the driver renders fundamental notes with realistic weight, crisp leading edges and convincing texture. On bass-heavy electronic tracks, it delivers clean, tightly controlled bass without ever bleeding into neighbouring frequencies. The speed of the low end is exemplary; the driver starts and stops promptly enough to prevent rapid, complex percussion passages from smearing or trailing behind the mix.
Midrange – The midrange performance is where the D1 draws its strongest spiritual comparisons to the legendary HD600 lineage, delivering an utterly transparent, lifelike and revealing midband presentation. HEDD has implemented a calculated presence lift centred around 3 kHz to optimise vocal intelligibility and transient bite, which is beautifully anchored by a realistic amount of foundational weight in the 100–200 Hz region. However, that exact 3 kHz peak pushes about 1.5–2.5 dB past what my personal HRTF agrees with, which explains why the D1 can occasionally come across as slightly shouty at louder listening volumes – a typical trait of reference monitors that are tuned without a compensatory sub-bass lift to counterbalance the upper-mids.
When listening at nominal volumes, the tonal delivery is remarkably pure. Male vocals sound deeply rooted, natural, and entirely free from boxy or honky colourations, resolving fine vocal gravel and chest resonance with ease. Female vocals possess brilliant clarity and upper-midband air, projecting forward without crossing into inherent harshness. Similarly, acoustic instrumentation such as grand pianos, cellos and brass overtones – retains their true-to-life acoustic timbre and natural harmonic decay characteristics. The D1 never resorts to filtering or warming up the midband to fake a romantic sense of musicality; it achieves its emotional engagement strictly through pure tonal balance and tracking precision.
Treble – The treble region is fast, extended and highly informative, characterised by highly deliberate tuning. By keeping the 5–8 kHz lower-treble region linear and un-dipped, the D1 preserves the natural bite and harmonic crunch of overdriven guitars and cymbal strikes without ever dulling the sound. The upper-treble leans slightly toward the sizzly and airy side, a trait driven mostly by a narrow emphasis peak situated around the 12–13 kHz mark.
While this specific peak acts as a fantastic diagnostic tool for resolving soundstage boundaries, spatial cues and low-level reverb trails, it can simultaneously impart a slightly wirey/tizzy/sizzly character to high-frequency cymbal overtones and vocal breath elements on brighter, hot recordings. Crucially, because this is a narrow-band peak rather than a wide, elevated shelf, so it can effectively be tamed by optimising your head placement and ear cup clamp force or simply EQ. Otherwise the treble behaves with excellent coherence, capturing fine micro-details without introducing the fatiguing, grainy treble character found in many competing planar designs.
Technical Performance.
Soundstage & Imaging – Rather than aiming for an artificially vast or diffuse soundstage, the D1 focuses on generating an accurate hemispherical space. It does leaps and bounds better than intimate soundstage of the HD600 series, offering much better width and depth but it’s still not to the level of very wide and deep sounding headphones like the Sennheiser HD800 series. The width is actually still pretty fine for its price it’s the depth that noticeably falls short. D1 sounds like the sound is surrounding your head but the depth is still in the head and isn’t able to go beyond the nose even with songs that are mixed with very deep sounding reverbs. IMO, if Hedd Audio is able to do a flagship level staging version of the D1, that’d be a crazy hit out of the park.
Imaging on the other hand is fairly precise and pinned down. The D1 constructs a remarkably stable phantom centre image and maps out complex multi-instrument arrangements with pinpoint accuracy, ensuring that sonic cues never blur or overlap, even during highly dense musical passages.
Resolution & Detail Retrieval – The unique properties of the TPCD carbon driver give the D1 an immense resolving capability that avoids relying on artificial upper-mid forwardness to simulate clarity. Micro-details such as subtle reverb tails fading into a room, a guitar’s squeaks with the guitar player’s hand sliding on the strings or the delicate texture of a bow crossing a string are laid bare with effortless transparency. It functions like a proper studio tool, exposing layer after layer of a complex mix without sounding clinical.
Dynamics & Transient Attack – Macrodynamic swings on the D1 are punchy, tactile and highly articulate. Abrupt orchestral explosions and fast electronic transients arrive with clean, snappy impact. Because the rigid carbon diaphragm stops instantly without surface flexing, leading-edge attacks possess a physical ‘thwack’ and texture that make percussive instruments sound realistic and alive
Source Pairing and Scaling.
Operating with a friendly 32-ohm impedance and a highly sensitive 100 dB rating, the D1 is an incredibly versatile load that refuses to demand a nuclear reactor to perform beautifully.
Portable Systems: Running the D1 directly from reference-grade portable USB dongles or discrete DAPs yields fantastic volume headroom, excellent midband control, and a flat, transparent presentation. The core listening experience is never compromised by low-power sources.
Desktop Reference Gear: Connecting the D1 to high-current desktop amplifiers or balanced solid-state gear reveals its scaling potential. Extra current headroom tightens the low-end performance, extends sub-bass rumble, and deepens the layers of the soundstage.
The D1 isn’t power-hungry, but it is highly quality-hungry; it acts as a transparent mirror to whatever source chain you feed it.
Comparisons.
HEDDphone TWO.
The HEDDphone TWO is HEDD’s premium full-range AMT model situated just below the flagship GT, offering a unique, hyper-speed presentation that specialised listeners adore, though the D1 stands as the more practical, ergonomic, and universally appealing headphone. Looking at the low end, the AMT driver on the TWO relies on pleat compression rather than a moving piston, meaning it inherently lacks traditional physical weight and displacement and so the D1 wins decisively in this category, delivering the deep sub-bass punch, physical texture, and authoritative dynamic impact that the AMT design cannot quite do. In the midrange and treble frequencies, the TWO’s full-range AMT driver delivers a hyper-resolved, almost ethereal level of micro-detail and treble air that can sometimes feel slightly detached from the rest of the mix, whereas the D1 utilises its stiff Thin-Ply Carbon Diaphragm to distribute its resolution uniformly across the entire frequency spectrum, maintaining a more natural and cohesive dynamic timbre. Finally, their spatial architectures are vastly different: the TWO projects a massive, vertically tall wall-of-sound with extreme, razor-sharp layer separation, while the D1 provides a more traditional, spherical soundstage where imaging feels conventionally placed, cohesive and structurally grounded – all the while being dramatically lighter and far easier to drive from portable gear.
Sennheiser HD600.
The HD600 remains a timeless gold standard for vocal correctness, but the HEDDphone D1 represents its true modern upgrade path, building directly upon a shared philosophy of flat, uncoloured midrange neutrality. When evaluating the lower frequencies, the contrast is immediate: the HD600 suffers from early sub-bass roll-off and soft-clips when pushed, whereas the D1 delivers better linear extension, providing better authentic physical authority and lightning-fast dynamic slam without masking lower-midrange information. In the midrange, both headphones exhibit an organic lifelike timbre, but the D1 pulls ahead significantly in raw resolving power, unmasking faint vocal breath control and instrumental textures that the Sennheiser naturally softens. Moving into the upper registers, the HD600 presents a smoother, more forgiving, yet ultimately rolled-off top end, while the D1 opts for a highly articulate, extended treble response that shows micro-details effortlessly, despite its narrow 12–13 kHz peak requiring some attention to physical fit. Spatially, the comparison is entirely one-sided; the HD600 is notoriously constrained to an intimate, “in-your-head” presentation with a distinct three-blob imaging pattern, while the D1 completely breaks free from these physical boundaries to establish a more vast, continuous hemispherical stage with precise lateral panning and clear depth layering.
Sennheiser HD650.
The Sennheiser HD650 leans heavily toward a warm, laid-back, and forgiving presentation characterised by a soft mid-bass hump and relaxed highs, making the D1 sound significantly faster, cleaner and more transparent by comparison. In the bass band, the HD650 fakes its low-end punch via a pillowy, slow mid-bass rise that introduced a warm, coloured haze, whereas the D1 counters with a tight, disciplined macro-dynamic slam that strikes hard in the true sub-bass octaves and recovers instantaneously. This tuning choice directly impacts the midrange, where the D1 completely strips away the famous “Sennheiser veil,” replacing the HD650’s thick, romanticised lower-midband wash with studio-grade linear accuracy and pristine vocal clarity. The discrepancy widens in the treble, where the HD650 heavily rolls off upper-frequency air to maintain its dark, relaxing character, contrasting sharply with the D1’s fast, extended, and highly informative top-end presentation. This tonal variation fundamentally shifts their spatial performance; the HD650 shares the intimate staging of the HD600 but introduces a hazier, more diffuse imaging boundary due to its prominent mid-bass bloom, whereas the D1 carries much sharper instrument separation, imaging sonic images down to exact coordinates on a much wider canvas.
Sennheiser HD660S2.
The HD660S2 attempts to modernise the classic Sennheiser series by boosting bass punch and energy, but the D1 pulls ahead comfortably in terms of absolute resolution, spatial width, and technical control. While the HD660S2 successfully yields a punchier, more rhythmic mid-bass experience than its predecessors, its slam remains concentrated in a narrow band that can struggle with congestion during complex arrangements; the D1’s low-end slam, by contrast, extends uniformly into the sub octaves, offering a cleaner, more physically tactile punch that doesn’t bleed into neighbouring frequencies. Across the midrange, the HD660S2 retains a slightly intimate, warmer, and more forgiving character that feels intimate, whereas the D1 operates a tier above in absolute resolving power, pulling out faint, quieter acoustic cues and micro-nuances from the background with much better transparency. In the treble, the 660S2 keeps its top end relatively safe and smooth, missing the sheer transient speed and upper-octave air that the D1’s rigid TPCD driver naturally provides. This structural difference allows the D1 to offer far superior spatial width and separation, creating an expansive soundstage where dense orchestral or electronic tracks sit independently without overlapping or sounding vague.
Focal Elex.
The Focal Elex focuses heavily on explosive, dramatic macrodynamic slam and an energetic, forward top end, whereas the D1 counters with a far more linear, natural midrange presentation and a more coherent tonal balance. In the low frequencies, the Elex behaves like a macro-dynamic monster, offering an aggressive, volatile and highly tactile punch that is tough to beat in the dynamic driver world, though it runs a risk of driver bottoming; the D1 cannot quite match that raw, explosive violence, but it answers with significantly better low-end control, linear sub-bass extension, and zero acoustic smearing. The midrange reveals a clear stylistic divergence, as the Elex can sometimes sound aggressive, uneven, or metallic in its timbre (more noticeable in A/B testing), while the D1 delivers an uncoloured, textbook reference midband that sounds exceptionally lifelike and cohesive. In the treble, the Elex forces an artificial sense of clarity by emphasising a slightly brighter, forward profile whereas the D1 achieves its high level of micro-detail retrieval naturally through raw driver speed and rigidity, keeping the overall timbre pristine and organic despite its narrow upper-treble peak. Physically and spatially, the Elex throws a tall, vertically stretched soundstage that can sometimes feel detached in depth, while the D1 builds a more proportional and coherent stage with superior left-to-right localisation and panning accuracy.
Audeze MM-500.
The Audeze MM-500 is a planar magnetic studio monitor tool designed for flat, quick reference tracking, setting up a fascinating technical showdown with the HEDDphone D1. In the bass band, the MM-500 offers classic planar performance – taut, perfectly flat, and quick, but ultimately missing raw physical air displacement; the D1, being a high-performance moving-coil transducer, moves air viscerally, giving kick drums and bass drops a weightier, more satisfyingly physical slam. The midrange of the MM-500 aggressively spotlights leading-edge transients and forward vocal monitoring to assist with critical studio tracking, whereas the D1 delivers a more balanced, lifelike reference midrange that seamlessly integrates both the initial transient attack and the organic, trailing harmonic decays of acoustic instruments. The top end has a similar contrast, where the MM-500 presents a dry, hyper-focused treble aimed at dissecting mix flaws, while the D1 provides an extended, airy profile that handles complex signals with absolute composure. Spatially, since the MM-500’s soundstage can feel a bit flat and aggressively upfront. The D1 counters with a much more holographic listening field, serving up deeper spatial layering and superior retrieval of low-level ambient room reflections. It makes for a far more immersive experience overall, which honestly gives one a much better sense of environmental depth, whether you’re critically placing elements in a mix or just kicking back as an audiophile.
Conclusion.
The HEDDphone D1 stands out as a very deliberate, hyper-focused reference headphone. It completely scorns artificial tuning tricks, steering clear of bloated bass shelves or over-emphasized treble regions designed to fake a “reference-y,” high-fidelity sound. Instead, it delivers a deeply competent, remarkably transparent presentation in an open-back design that effortlessly bridges the gap between surgical studio-monitor accuracy and pure musical engagement. By capturing the organic vocal realism and textbook tonal correctness that cemented the legendary status of the Sennheiser HD600 and HD650, and weaponizing those traits with modern sub-bass extension, impressive transient speed, and highly capable micro-detail retrieval, HEDD has engineered something genuinely special. Backed by a fully consumer-serviceable modular chassis and an industry-leading five-year warranty, the value proposition they’ve established here is incredibly tough to beat.
That said, the D1 isn’t entirely free of first-generation quirks – which, to be perfectly honest, aren’t a major issue, but they definitely leave room for optimization in a future iteration. Acoustically, that mild 12–13kHz emphasis can occasionally introduce a sizzly edge, which is highly temperamental when it comes to fit; both clamp force and exact ear cup positioning exert a noticeable influence on its prominence. From a personal preference standpoint, I would have loved for it to exhibit absolutely zero sub-bass roll-off. A subtle sub-bass shelf would have been a welcome touch here, serving to compensate for an open-back’s structural limitations when pitted against a closed-back’s innate ability to trap pressure and render deeper sub-bass rumble. Ergonomically, while shedding enough mass to hit the 350-gram milestone is a monumental engineering triumph, opting to forgo an internal elastic suspension strap is a bit of a miss. Without it, that downward pressure concentrates squarely on the crown of your head, carving out physical hotspots during marathon mixing or listening sessions. If HEDD wants to elevate this platform from an exceptional debut into an unassailable next-gen masterpiece, the blueprint is straightforward: integrate a decoupled suspension strap mechanism, iron out that double-digit treble spike to make the presentation less finicky about exact head placement, and pack a truly premium braided balanced cable with swappable terminations in the box.
Ultimately, these minor caveats do very little to overshadow the sheer brilliance of HEDD’s achievement. By successfully pulling off a near-ideal reference tuning, they have delivered an incredibly versatile tool that speaks fluently to analytical professionals and demanding audiophiles alike – setting a fierce new benchmark for performance and value in its class. The whole package has been profoundly impressive from day one, and it for sure earns my highest recommendation!
