Short answer
Both are portable players, $649 against $599.
The FiiO M21 reaches 100 dB on 566 of the 567 models here, the Cayin N3 Ultra on 565.
| Specification | Cayin N3 Ultra | FiiO M21 |
|---|---|---|
| Power into 32Ω | 250 mW | 405 mW |
| Power into 150Ω | 53 mW | 88 mW |
| Power into 300Ω | 27 mW | 45 mW |
| Power into 600Ω | 13 mW | 22 mW |
| Output impedance | 0.5 Ω | 1.0 Ω |
| Output noise | not published | 2.0 µV |
| Price | $649 | $599 |
| Headphones it reaches 100 dB on out of 567 | 565 | 566 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Balanced | Cayin N3 Ultra | FiiO M21 |
|---|---|---|
| Socket | 4.4mm | 4.4mm |
| Power into 32Ω | 600 mW | 950 mW |
| Power into 300Ω | 64 mW derived | 185 mW |
| Output impedance | 1 Ω | 1.5 Ω |
| Output noise | not published | 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. Derived figures are worked out from the loads the maker did publish.
| Load | Cayin N3 Ultra | FiiO M21 |
|---|---|---|
| 32 Ω | Published | Published |
| 150 Ω | Derived | Derived |
| 300 Ω | Derived | Published |
| 600 Ω | Derived | Derived |
Both makers publish a power rating. Cayin N3 Ultra publishes at 32 Ω; FiiO M21 at 32 Ω and at 300 Ω. The other rows are worked out from those.
So the two columns are not equally firm row by row: at 300 Ω only the FiiO M21 figure is the maker’s own. On those rows the derived number is the one to doubt first.
1 headphone reaches 100 dB on the FiiO M21 but not on the Cayin N3 Ultra — Moondrop PARA.
14 headphones are left alone by the Cayin N3 Ultra under the one-eighth rule but coloured by the FiiO M21 (64 Audio Nio, 64 Audio Volur, Campfire Audio Andromeda 2023, 11 more).
565 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 Shure SE846: FiiO M21 has +38 dB of headroom there against +32 dB. 6 dB is about 3.8 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for Cayin N3 Ultra because that maker publishes no output noise figure.
| Headphone | Impedance | Needs | Cayin N3 Ultra headroom | FiiO M21 headroom | Cayin N3 Ultra hiss | FiiO M21 hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +32 dB | +38 dB | not published | 20 dB |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +12 dB | +18 dB | not published | 0 dB |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +22 dB | +24 dB | not published | -1 dB |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +17 dB | +19 dB | not published | -6 dB |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +14 dB | +17 dB | not published | -7 dB |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +10 dB | +13 dB | not published | -13 dB |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +6 dB | +8 dB | not published | -17 dB |
| HiFiMAN Susvara | 60 Ω | 50 mW | +4 dB | +6 dB | not published | -19 dB |
The Cayin N3 Ultra costs $50 more than the FiiO M21. Out of the 567 models here, that difference buys 14 more models left alone under the one-eighth rule.
It does not run one way only: the cheaper FiiO M21 drives one model to 100 dB that the Cayin N3 Ultra 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.
Each in full: Cayin N3 Ultra and FiiO M21.