Both are portable players, $199 against $730. This page puts them side by side on the only three things a maker can be held to — power, output impedance and noise — and then asks the question that actually matters: which headphones do they treat differently?
On raw power they are level: each reaches 100 dB on 566 of the 567 non-electrostatic models here.
| Specification | FiiO JM21 | FiiO M33 R2R |
|---|---|---|
| Power into 32Ω | 245 mW | 470 mW |
| Power into 150Ω | 56 mW | 109 mW |
| Power into 300Ω | 30 mW | 60 mW |
| Power into 600Ω | 15 mW | 30 mW |
| Output impedance | 1 Ω | 0.8 Ω |
| Output noise | 1.6 µV | 5.5 µV |
| Price | $199 | $730 |
| Headphones it reaches 100 dB on out of 567 | 566 | 566 |
A figure marked not published is missing from the maker’s own material, which is not the same as being good or bad. Where one of these two publishes a number and the other does not, the comparison on that axis cannot be made at all — and saying so is more useful than guessing.
| Load | FiiO JM21 | FiiO M33 R2R |
|---|---|---|
| 32 Ω | Published | Published |
| 150 Ω | Derived | Derived |
| 300 Ω | Published | Published |
| 600 Ω | Derived | Derived |
Both makers publish a power rating, which is less common than the table above makes it look: of the 280 sources here, 209 publish a figure at at least one load and only 2 publish one at all four. FiiO JM21 publishes at 32 Ω and at 300 Ω; FiiO M33 R2R at 32 Ω and at 300 Ω. Every other row is worked out from those — between two published loads, or beyond them holding voltage constant, which understates rather than flatters.
The two publish at the same loads, which is the one case where the rows of this table can be read straight across.
What each of these labels means, and why a derived figure is still worth printing, is set out in how this data was built.
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.
3 headphones are left alone by the FiiO M33 R2R under the one-eighth rule but coloured by the FiiO JM21 (Shure AONIC 4, Simgot SuperMix 4, Symphonium Giant).
74 headphones stay below the audible-hiss threshold on the FiiO JM21 but cross it on the FiiO M33 R2R (64 Audio Fourte, 64 Audio Nio, 64 Audio U12t, 71 more).
566 of the 567 models here reach 100 dB on both. That is a power count and nothing more; whether either source suits a given headphone also depends on the two lines above.
The counts above say how many models the two treat differently. This says by how much. The same 8 headphones appear on every page of this kind, ordered from the easiest to drive to the hardest, so a row can be read across pages as well as across columns. Headroom is at a 100 dB SPL peak.
The widest gap on the list is the Sennheiser HD 600: FiiO M33 R2R has +10 dB of headroom there against +7 dB. 3 dB is twice the power, which matters for peaks rather than for how loud the music sits.
Hiss is this source’s own noise, in decibels SPL, in that headphone — not a rating. Below about 20 dB SPL it is under the threshold of hearing in a quiet room; a negative figure means it is far below it.
| Headphone | Impedance | Needs | FiiO JM21 headroom | FiiO M33 R2R headroom | FiiO JM21 hiss | FiiO M33 R2R hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +37 dB | +39 dB | 19 dB | 29 dB |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +17 dB | +19 dB | -1 dB | 9 dB |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +22 dB | +24 dB | -3 dB | 7 dB |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +17 dB | +20 dB | -8 dB | 3 dB |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +15 dB | +18 dB | -9 dB | 2 dB |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +11 dB | +14 dB | -15 dB | -4 dB |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +7 dB | +10 dB | -19 dB | -8 dB |
| HiFiMAN Susvara | 60 Ω | 50 mW | +4 dB | +7 dB | -21 dB | -10 dB |
The FiiO M33 R2R costs $531 more than the FiiO JM21. Out of the 567 models here, that difference buys 3 more models left alone under the one-eighth rule.
It does not run one way only: the cheaper FiiO JM21 keeps 74 models below the audible-hiss threshold that the FiiO M33 R2R does not, which is the part worth checking against your own headphones before paying the difference.
Everything in this paragraph counts only figures the two makers published. An advantage on an axis one of them leaves blank cannot be counted, and is not counted — how often that happens is its own page.
Each has a page of its own with every power figure traced to what the maker published and a full headphone list: FiiO JM21 and FiiO M33 R2R. Or put either against your own headphones in the source comparison tool.