FiiO M33 R2R

FiiO’s First R2R DAP: Proprietary Chip, Two Decoding Modes, One Pocket Device

The M33 R2R represents something genuinely different in FiiO’s DAP lineup: their first attempt at true R2R decoding in a portable player, using a proprietary in-house chip rather than an off-the-shelf delta-sigma solution. R2R DACs have long been the preserve of four-figure desktop hardware, prized for an organic, analogue-like character — whether FiiO could deliver that in a $699 pocket device using their own silicon was worth exploring properly.

I have been living with the M33 R2R for about a month — at my desk driving full-size planars, on the go with IEMs, and on a recent holiday in Japan paired with the FiiO JT7. What I will come back to in the sound impressions is how much resolution this player manages to preserve even in NOS mode, where most listeners would expect detail retrieval to soften. But first, let’s take a look at what’s in the box.

Unboxing

The packaging is confident and purposeful. The outer sleeve communicates the key specifications clearly without overloading the design — FiiO knows their buyers arrive informed. Lifting the lid reveals the player cradled in precisely cut foam, with accessories tucked neatly beneath.

In the box you get: the FiiO M33 R2R, a leather protective case with an aluminium grille at the back, a USB-C to USB-C data cable, a pre-applied screen protector, a quick start guide, a warranty card, and a GPL V2 manual.

It is a practical rather than lavish bundle, which is exactly right for a device at this price. The leather case is a highlight — it ships ready to use and the aluminium grille at the back is a thoughtful touch that allows for ventilation during extended listening sessions. I’ll return to the case in the build section. Before moving on, it is worth noting how much slimmer the M33 R2R’s box is compared to the M23’s — a tangible first signal of the generational shift in form factor philosophy.

Build Quality and Design

FiiO has taken a deliberate step away from the dense, glass-heavy aesthetic of the M23 with the M33 R2R, adopting a slimmer and considerably more pocketable profile.

At 138.2 × 71.5 × 17mm and approximately 258 grams, it is noticeably lighter than its predecessor and more practical for daily carry. The AG matte glass back panel handles fingerprints exceptionally well — it feels closer to the brushed aluminium finish of a recent MacBook than conventional tempered glass, smooth to the touch and resistant to smudging.

The port layout is comprehensive without feeling crowded. Up top sits the 3.5mm output — which doubles as a line-out and coaxial output — alongside the 4.4mm Pentaconn balanced output. The bottom houses the dual USB-C configuration: one USB 3.0 port handling both charging and data transfer, and a second dedicated Power In port for Desktop Mode. A microSD slot, compatible with cards up to 2TB (I used my 512MB without issues), completes the bottom edge.

On the sides, the button layout prioritises tactile reliability. Physical volume buttons replace the capacitive wheel found on the M23, and in practice the change is an improvement for blind navigation — no accidental presses, and the actuation is clean and precise. That said, the low profile of the buttons against the chassis makes them slightly harder to locate by feel on a first attempt, particularly in a deeper pocket.

The included leather case partially addresses this by adding grip and a more defined surface texture, though it does add a touch of bulk around the button area. Access to the ports and buttons remains straightforward.

That aluminium grille on the back of the case earns its place during extended desktop listening sessions, where the player does generate a modest amount of warmth. FiiO’s decision to build ventilation into the bundled case rather than leaving it as an afterthought reflects careful thinking about real-world usage patterns. Overall build quality is tight and consistent — no flex, no gaps, no hollow resonance. A step below the premium glass density of the M23, perhaps, but a deliberate trade for pocketability rather than a corner cut.

Features and Performance

Operating System and Interface

The M33 R2R runs a heavily customised build of Android 13, paired with a Qualcomm Snapdragon 680 and 8GB of RAM — a combination that delivers genuine smartphone-grade fluidity. Navigation is snappy, app switching is seamless, and scrolling through large local libraries feels effortless. The 5.5-inch fully laminated IPS display at 1080×2160 is bright, colour-accurate, and renders album art with real fidelity — a meaningful step up from the M23’s screen.

Five operating modes are available — Android, Pure Music, AirPlay, USB DAC, and Bluetooth Reception — giving the device enough flexibility to serve as a desktop DAC, a network streamer, a portable transport, or a dedicated local player depending on the session.

FiiO Music App

The FiiO Music app has matured considerably over recent years and on the M33 R2R it is genuinely polished — fast, reliable, and well-organised. Local files, NAS libraries, and streaming services are all accessible from within the app, and the interface for managing playback filters and EQ is straightforward. For those who prefer third-party apps — Apple Music, Qobuz, Tidal, Spotify — the full Google Play Store is available and works without friction.

The R2R Sound Modes — NOS and OS

The most consequential feature of the M33 R2R is not its connectivity or its processing power — it is the flexibility of its R2R DAC implementation. FiiO has built a proprietary R2R chip into this device rather than adapting an off-the-shelf delta-sigma solution, and they have given users direct access to two fundamentally different decoding approaches. NOS ( non-oversampling) mode lets the R2R architecture express its organic, analogue-like character in full; OS (oversampling) mode brings the presentation closer to a conventional delta-sigma character — detailed, precise, and clean — while retaining the R2R chip’s underlying texture. Layered on top of that is a second axis of customisation: Natural Sound and Warmer Sound profiles, which represent genuinely distinct tuning signatures rather than simple tone adjustments.

In practice this means you have four meaningful listening configurations available, and swapping between them takes seconds. Critics who have found previous FiiO DAPs too analytical will find the NOS mode a direct answer to that concern; those who want more of the conventional delta-sigma precision alongside the R2R hardware can reach for OS mode without compromising on features.

The engineering behind the implementation is worth a closer look. FiiO’s discrete R2R design uses 192 precision thin-film resistors in total — 48 per channel, covering 24 bits of resolution in R and 2R pairs, across all four channels of the fully balanced circuit. Each resistor is specified at 0.1% tolerance, which is a deliberate cost-performance decision: 0.1% precision components cost roughly one tenth of equivalent 0.01% parts, and it is this tradeoff that makes a 192-resistor discrete design achievable at $699 rather than in four-figure desktop hardware exclusively.

FiiO also makes an interesting technical claim about their DSD implementation that deserves a mention, even for listeners like myself who do not use DSD sources. On most R2R DACs, DSD input requires conversion to PCM before the ladder circuit can process it — the exact inverse of the problem delta-sigma DACs have with PCM. FiiO’s approach is different: they configure the R2R ladder resistance circuit to function as a FIR filter when DSD is the input, theoretically enabling genuine DSD native decoding without an intermediate conversion step. I cannot personally verify this from a listening standpoint, but for DSD users already drawn to R2R hardware, it is a meaningful technical differentiator worth investigating.

Equalisation

The M33 R2R includes a 10-band parametric EQ with AutoEQ support for FiiO’s own IEMs and headphones. The parametric implementation is granular enough to address meaningful frequency response adjustments, and the Retro preset is particularly well-matched to the NOS mode character — reinforcing the lower-midrange warmth without muddying the bass or blunting transient attack.

VU Meters and Visualisations

The VU meter and spectrum visualisation screens are a genuinely enjoyable addition that fits the retro-audio identity running through the M33 R2R’s design language. They are small touches, but they make the listening experience feel more intentional — more like sitting down with a piece of equipment than simply pressing play on a smartphone.

Desktop Mode and Power

Plugging a fast charger into the dedicated Power In USB-C port activates Desktop Mode, which bypasses the battery entirely and feeds the amplifier directly from the power source. In this configuration, Super High Gain unlocks, pushing the balanced output to 1100mW + 1100mW into 32Ω — a figure that transforms the M33 from a portable player into a credible desktop source capable of driving demanding planars with authority. This bypass feature also protects the battery’s longevity over time, a patented implementation that is worth considering for anyone planning to use the device intensively at a desk as well as on the go.

Connectivity

The dual-band Wi-Fi (2.4GHz and 5GHz) supports DLNA, AirPlay, and wireless file transfer. Bluetooth 5.0 covers SBC, aptX, aptX HD, LDAC, and LHDC for transmission, with SBC and LDAC for reception. The coaxial output via the 3.5mm port supports up to 384kHz/24bit and DSD128, making the M33 a credible digital transport for a higher-end desktop DAC if needed. FiiO claims coaxial jitter as low as 71.5ps via SPDIF — competitive with dedicated transport hardware at this price.

Sound Impressions

My primary testing chain throughout this review was the Sennheiser HD600 for tracking tonal character and midrange rendering, the FiiO JT7 for IEM-specific evaluation, and the Hedd D1 for planar headphone testing. I also used the Crosszone CX12 extensively during my Japan trip, which gave me a sustained real-world portrait of how the M33 R2R performs as a travel device. Local files were played back via the FiiO Music app; streaming was handled through Apple Music in Hi-Res where available.

The overarching character of the M33 R2R in NOS mode is one of organic warmth combined with, frankly, more resolution than the architecture might lead you to expect. Coming from the M23’s clean, precise delta-sigma presentation, the shift is immediately apparent — notes decay more naturally, the leading edge of transients is slightly softer, and the overall texture of the music feels more cohesive and less partitioned. What struck me most, however, is how much micro-detail the R2R architecture manages to preserve within that warmth. This is not a player that sacrifices transparency for a pleasant tonal coating. It is a player that sounds simultaneously more analogue and more resolved than the delta-sigma competition at this price — a combination that takes real engineering to achieve.

Bass

The bass is where the R2R architecture makes its most compelling case. There is a weight and physicality to low-end reproduction here that the M23 simply cannot match — kick drums have genuine authority, sub-bass extension reaches deep without bloom, and bass lines have a textural presence that makes them feel three-dimensional rather than just accurate. The decay is natural without being slow, and the mid-bass remains well-controlled even on densely produced electronic music. Jennifer Warnes’ “Way Down Deep” is an excellent reference here — the sub-bass that anchors the track’s opening sits with real mass and layered definition through the M33, each stratum of the low end distinctly rendered rather than compressed into a single block of energy. The 4.4mm balanced output pushes this further still, adding grip and dynamic headroom that makes even the most demanding low-end material feel fully in command.

Midrange

The midrange is the standout quality of this player. Vocals carry a textural richness and body that makes the M23 sound almost clinical by comparison — there is a presence and intimacy to voices that feels closer to what you’d expect from a good analogue source than from a portable digital player. Acoustic guitar and piano have real note weight; strings carry genuine harmonic richness; the upper midrange is smooth without sounding recessed or distant. Joni Mitchell’s “A Case of You” is a reliable test of vocal body and micro-detail, and on the M33 the subtle textural character of Mitchell’s voice is rendered with a naturalness that is genuinely difficult to achieve in the portable category at any price. Switching to the Warmer sound profile adds further lower-midrange fullness, making long sessions with brighter transducers an easy, unhurried listen.

Treble

The treble is smooth, controlled, and — relative to the M23 — gently relaxed. The frequency response measurement confirms this: a 1dB attenuation at 20kHz is audible as a slightly reduced upper-harmonic energy and a somewhat softer sense of air compared to the M23’s cleaner top end. This is the deliberate character of the R2R NOS mode, and for listeners who find brighter sources fatiguing over long sessions it will be exactly right. Switching to OS mode recovers more of that upper-end presence, bringing the treble closer to conventional delta-sigma territory while retaining the organic midrange and bass texture that define the R2R character. The acoustic version of the Eagles’ “Hotel California” is a useful reference for treble evaluation — on the M33 the guitar pick attack has presence and definition without any etch or glassiness, and cymbal decay is natural and unhurried throughout.

Soundstage and Imaging

The soundstage presentation is coherent and well-layered rather than conspicuously wide. Instrument separation is precise, centre-image focus is excellent, and the depth dimension is more convincing than the width — the M33 R2R places instruments with real specificity without artificially widening the stage. This suits the organic character of the R2R architecture, where spatial coherence feels more important than theatrical scale. Miles Davis’ “Kind of Blue” in FLAC demonstrates the player’s spatial strengths well — the intimate recording environment is conveyed convincingly, and the fine separation between instruments in the ensemble is rendered without confusion or congestion.

Comparisons

FiiO M33 R2R vs FiiO M23

The M23 remains an excellent delta-sigma DAP with a clean, detailed, and precise presentation. Against the M33 R2R in NOS mode, it sounds distinctly more clinical — resolving and accurate, but lacking the organic warmth and midrange body that the R2R architecture delivers. The M33 R2R is also notably slimmer and lighter, making it a meaningfully better everyday carry device. In OS mode, the M33 comes close to the M23’s character while retaining the superior form factor, the more capable amplifier section, and the significantly better display. For listeners who valued the M23’s precision above all, the M33 in OS mode is a natural and worthwhile upgrade; for those who want something genuinely different in character, NOS mode is the compelling reason to switch.

FiiO M33 R2R vs FiiO JM21

The JM21 is FiiO’s value proposition in the Android DAP space — a genuinely impressive device at a fraction of the M33 R2R’s price. What the M33 offers over it is a step up in both amplifier power and sonic character: the R2R architecture delivers an organic warmth and midrange richness that the JM21, for all its qualities, simply cannot replicate. For listeners driving planars or full-size over-ear headphones, the additional output power of the M33 is meaningful. For IEM-centric listeners on a tighter budget, the JM21 remains an outstanding choice — but the M33 R2R is a clear step up in both performance and character for those willing to invest accordingly.

FiiO M33 R2R vs Hidizs AP80 Pro Max

The Hidizs AP80 Pro Max occupies a very specific niche: a tiny, genuinely lightweight DAP with decent output power for easy-to-drive IEMs and headphones. If that is precisely what you need — the smallest and lightest capable player you can slip into a jacket pocket without thinking about it — the AP80 Pro Max is worth knowing about. The comparison with the M33 R2R is otherwise limited. Feature-wise, the AP80 Pro Max is less comprehensively equipped than even the JM21, let alone the M33 R2R — it is a stripped-back proposition rather than a full Android platform. Sonically, the two converge only in a narrow set of conditions: when the M33 R2R is running in OS mode with easier-to-drive IEMs, the presentations are broadly comparable in character, both delivering a clean delta-sigma-style result. The moment NOS mode enters the picture, the comparison dissolves entirely — the R2R architecture’s organic warmth, textural midrange, and analogue-like decay are things the AP80 Pro Max’s delta-sigma implementation simply cannot approximate. The Hidizs is the right answer for listeners who need miniature and light above all else; the M33 R2R is the right answer for everyone who has decided the R2R sound is what they are specifically after.

Specifications and Measurements

The frequency response measurement tells a coherent story: a response that is essentially flat through the bass and midrange, with a gentle 1dB attenuation at 20kHz. This confirms what the listening impressions suggest — the M33 R2R does not impose a dramatic EQ curve, but the NOS filter presents the upper frequencies with less leading energy than a conventional delta-sigma design. The effect in practice is a presentation that avoids the occasional analytical edge that can make extended sessions with brighter headphones tiring without sacrificing the midrange and bass precision.

The distortion measurement is equally revealing. THD+N runs at a consistent ~0.02% through the bass and midrange frequencies — notably higher than what a well-implemented delta-sigma DAC achieves at this price, and a straightforward consequence of the R2R architecture’s inherent non-linearity. What is interesting is that distortion decreases meaningfully above 6kHz, which aligns neatly with the relaxed treble character: the region where R2R designs produce the highest measured distortion coincides precisely with the region where the NOS filter softens the presentation, resulting in a combination that is easy on the ear without the harmonic artefacts becoming obtrusive in the audible band.

Rating Explanation

The M33 R2R earns its pragmatic rating of five for one fundamental reason: it delivers what it promises, and delivers it without compromise. The R2R architecture is implemented with enough sophistication that the organic warmth and midrange richness it provides are genuinely meaningful distinctions — not marketing language, but audible reality. The dual OS/NOS mode selection, combined with the Natural/Warm sound profiles, gives the M33 remarkable flexibility for a single device, allowing it to serve as an analytical tool in its delta-sigma-adjacent OS character or as a warmly engaging listen in NOS mode. Add the powerful amplifier section, the excellent Android 13 implementation, and the meaningful improvement in portability over the M23, and you have a device that earns full marks for what it sets out to do.

The price rating of four reflects the realities of the competitive DAP market rather than any dissatisfaction with the M33’s value. At $699.99 — repriced from its launch figure of $649.99 due to global memory price pressures and tariff impacts that have affected the wider DAP market — it asks considerably more than FiiO’s own JM21, which now retails at $ 259.99. The JM21’s own price increase from its original launch price is worth noting: this is an industry-wide shift rather than a FiiO-specific margin play. Part of what you are paying for with the M33 R2R specifically is the discrete R2R circuit itself: 192 precision thin-film resistors, fully balanced across four channels, represent a level of component investment that has no equivalent in any of FiiO’s delta-sigma Android DAPs regardless of their price.

Listeners who are primarily IEM-centric and do not need the R2R character or the additional amplifier headroom may still find the JM21 a more pragmatic choice. For those driving full-size headphones or actively seeking the organic R2R sound signature, the M33’s pricing is entirely reasonable — but it is worth being clear-eyed about whether the R2R experience is what you are specifically seeking before committing. The M33 R2R is, in short, a player for listeners who want the best of both the portable and the analogue worlds: the flexibility and feature set of a modern Android DAP, with the option to step into genuinely warmer, more organic territory whenever the music calls for it.

Conclusion

When I picked up the M33 R2R a month ago, my expectation was a warmer, softer version of the M23 — pleasant, but perhaps blunt compared to the M23’s delta-sigma precision. What I found instead was a player with a genuine sonic personality that coexists with real resolving power, in a form factor I found myself reaching for consistently over the M23 for both portable and desk use. The Japan trip crystallised that impression. Paired with the FiiO JT7 in a hotel room at the end of a long day, playing Hi-Res files from the internal storage in NOS mode, the M33 R2R delivered the kind of musical satisfaction that is easy to take for granted in a portable device but is actually quite difficult to achieve. The R2R architecture’s midrange richness and bass texture made familiar recordings feel genuinely fresh — and the power on tap meant the Crosszone CX12 I brought along was fully in its element without reaching for Desktop Mode.

If you have been looking for a sign that FiiO’s DAPs have moved beyond the analytical-but-safe territory that some critics have levelled at the brand, the M33 R2R is exactly that sign. It does not ask you to choose between features and soul — it offers both, with enough flexibility to let you dial in the listening experience you want from session to session. For $699.99, that is a genuinely impressive achievement, and one of the most compelling portable players I have spent time with in the first half of 2025.