Short answer
Both are portable players, $499 against $599.
On raw power they are level: each reaches 100 dB on 566 of the 567 non-electrostatic models here.
| Specification | FiiO M11S | FiiO M21 |
|---|---|---|
| Power into 32Ω | 200 mW | 405 mW |
| Power into 150Ω | 44 mW | 88 mW |
| Power into 300Ω | 23 mW | 45 mW |
| Power into 600Ω | 12 mW | 22 mW |
| Output impedance | 1.5 Ω | 1.0 Ω |
| Output noise | 1.8 µV | 2.0 µV |
| Price | $499 | $599 |
| Headphones it reaches 100 dB on out of 567 | 566 | 566 |
| Balanced | FiiO M11S | FiiO M21 |
|---|---|---|
| Socket | 4.4mm | 4.4mm |
| Power into 32Ω | 670 mW | 950 mW |
| Power into 300Ω | 90 mW | 185 mW |
| Output impedance | 2.6 Ω | 1.5 Ω |
| Output noise | 3.1 µV | 2.7 µV |
| Headphones it reaches 100 dB on out of 567 | 566 | 566 |
Through the balanced sockets the two are level on the same test: each reaches 100 dB on 566 of the 567 models here.
| As published | FiiO M11S | FiiO M21 |
|---|---|---|
| DAC | Dual ES9038Q2M | Quad CS43198 (CS43198 x4) |
| Processor | Qualcomm Snapdragon 660 | Qualcomm Snapdragon 680 |
| Operating system | Android 10 (global SRC bypass) | Android 13 |
| Storage | 32 GB storage (about 16 GB available to user), 3 GB RAM, microSD slot up to 2 TB | 64 GB storage (about 52 GB available), 4 GB RAM, single microSD up to 2 TB (in theory) |
| Outputs | 3.5 mm single-ended headphone out, 2.5 mm + 4.4 mm balanced, 3.5 mm line out (shared PO), 3.5 mm coaxial out (shared PO) | 3.5 mm headphone out, 4.4 mm balanced headphone out, 3.5 mm line out (shared), 4.4 mm balanced line out (shared), 3.5 mm coaxial (shared) |
| Bluetooth | Transmit SBC/AAC/aptX/aptX HD/LDAC/LHDC (version not stated); receive Bluetooth 5.0: SBC/AAC/LDAC | Bluetooth 5.0; transmit AAC/SBC/aptX/aptX HD/LDAC/LHDC; receive SBC/AAC/LDAC |
| Wi-Fi | 2.4 GHz / 5 GHz | 2.4 GHz / 5 GHz, DLNA, AirPlay, Wi-Fi song transfer |
| Formats | FLAC up to 384kHz/32bit; DSD64/128/256; USB audio 768kHz-32bit/DSD512 | Local decoding up to 384kHz/32bit, DSD256 (Native); USB audio up to 768kHz/32bit, DSD512 |
| Screen | 5.0-inch IPS incell, 1280 x 720 | 4.7-inch, 750 x 1334, bezel-less |
| Battery | not published | 4000 mAh |
| Battery life | >14 h (3.5 mm headphone output, volume 48) / >14 h (2.5 mm + 4.4 mm balanced output, volume 37) | 15 h (3.5 mm headphone out) / 12.5 h (4.4 mm headphone out) |
| Weight | 271 g | 193 g |
| Dimensions | Approx. 125.2 x 74 x 18.5 mm | About 68 x 121 x 17 mm |
Copied from the makers’ own pages: fiio.com/m11s, fiio.com/M11S_parameters, fiio.com/m21_parameters, fiio.com/m21. Battery life is the maker’s own test figure.
| Load | FiiO M11S | FiiO M21 |
|---|---|---|
| 32 Ω | Published | Published |
| 150 Ω | Derived | Derived |
| 300 Ω | Published | Published |
| 600 Ω | Derived | Derived |
Both makers publish a power rating. FiiO M11S publishes at 32 Ω and at 300 Ω; FiiO M21 at 32 Ω and at 300 Ω. The other rows are worked out from those.
The two publish at the same loads, which is the one case where the rows of this table can be read straight across.
On power they are interchangeable across this whole database: there is no headphone here that one reaches 100 dB on and the other does not.
36 headphones are left alone by the FiiO M21 under the one-eighth rule but coloured by the FiiO M11S (64 Audio Aspire 1, 64 Audio Fourte, 64 Audio Ocean, 33 more).
4 headphones stay below the audible-hiss threshold on the FiiO M11S but cross it on the FiiO M21 (64 Audio Fourte, Campfire Audio Clara, Shure SE846, 1 more).
566 of the 567 models here reach 100 dB on both.
Headroom is at a 100 dB SPL peak.
The widest gap on the list is the HiFiMAN Susvara: FiiO M21 has +6 dB of headroom there against +3 dB. 3 dB is twice the power, which matters for peaks rather than for how loud the music sits.
Hiss below about 20 dB SPL is inaudible in a quiet room.
| Headphone | Impedance | Needs | FiiO M11S headroom | FiiO M21 headroom | FiiO M11S hiss | FiiO M21 hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +36 dB damping | +38 dB | 20 dB | 20 dB |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +16 dB damping | +18 dB | -0 dB | 0 dB |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +21 dB | +24 dB | -2 dB | -1 dB |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +16 dB | +19 dB | -7 dB | -6 dB |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +15 dB | +17 dB | -8 dB | -7 dB |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +10 dB | +13 dB | -14 dB | -13 dB |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +5 dB | +8 dB | -18 dB | -17 dB |
| HiFiMAN Susvara | 60 Ω | 50 mW | +3 dB | +6 dB | -20 dB | -19 dB |
The FiiO M21 costs $100 more than the FiiO M11S. Out of the 567 models here, that difference buys 36 more models left alone under the one-eighth rule.
It does not run one way only: the cheaper FiiO M11S keeps 4 models below the audible-hiss threshold that the FiiO M21 does not, which is the part worth checking against your own headphones before paying the difference.
Only published figures count here; blank axes are counted separately.