Short answer
Both are desktop amplifiers, $100 against $99.
The JDS Labs Atom+ (2nd gen) reaches 100 dB on 566 of the 567 models here, the Fosi Audio SK02 on 564.
| Specification | Fosi Audio SK02 | JDS Labs Atom+ (2nd gen) |
|---|---|---|
| Power into 32Ω | 190 mW | 1.0 W |
| Power into 150Ω | 41 mW | 545 mW |
| Power into 300Ω | 20 mW | 272 mW |
| Power into 600Ω | 10 mW | 136 mW |
| Output impedance | not published | 0.7 Ω |
| Output noise | 3.7 µV | 1.99 µV |
| Price | $100 | $99 |
| Headphones it reaches 100 dB on out of 567 | 564 | 566 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Balanced | Fosi Audio SK02 | JDS Labs Atom+ (2nd gen) |
|---|---|---|
| Socket | 4.4mm | no separate rating |
| Power into 32Ω | 755 mW | — |
| Power into 300Ω | 80 mW derived | — |
| Output impedance | not published | — |
| Output noise | not published | — |
| Headphones it reaches 100 dB on out of 567 | 566 | — |
Only the Fosi Audio SK02 has a separate balanced rating from its maker; through it, it reaches 100 dB on 566 of the 567 models here. The JDS Labs Atom+ (2nd gen) may still have a balanced socket — its spec sheet simply gives no figure for one, so there is nothing to put beside it. Derived figures are worked out from the loads the maker did publish.
| Load | Fosi Audio SK02 | JDS Labs Atom+ (2nd gen) |
|---|---|---|
| 32 Ω | Published | Published |
| 150 Ω | Derived | Published |
| 300 Ω | Derived | Derived |
| 600 Ω | Derived | Published |
Both makers publish a power rating. Fosi Audio SK02 publishes at 32 Ω; JDS Labs Atom+ (2nd gen) at 32 Ω, at 150 Ω and at 600 Ω. The other rows are worked out from those.
So the two columns are not equally firm row by row: at 150 Ω and at 600 Ω only the JDS Labs Atom+ (2nd gen) figure is the maker’s own. On those rows the derived number is the one to doubt first.
2 headphones reach 100 dB on the JDS Labs Atom+ (2nd gen) but not on the Fosi Audio SK02 — Moondrop PARA, Symphonium Crimson.
26 headphones stay below the audible-hiss threshold on the JDS Labs Atom+ (2nd gen) but cross it on the Fosi Audio SK02 (64 Audio Fourte, AFUL Performer 8, Audio-Technica ATH-CKS1100, 23 more).
564 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 Audio-Technica ATH-R70X: JDS Labs Atom+ (2nd gen) has +20 dB of headroom there against +9 dB. 11 dB is about 13 times 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 | Fosi Audio SK02 headroom | JDS Labs Atom+ (2nd gen) headroom | Fosi Audio SK02 hiss | JDS Labs Atom+ (2nd gen) hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +31 dB | +38 dB | 26 dB | 20 dB |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +11 dB | +18 dB | 6 dB | 0 dB |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +20 dB | +29 dB | 4 dB | -1 dB |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +16 dB | +27 dB | -1 dB | -6 dB |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +13 dB | +20 dB | -2 dB | -7 dB |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +9 dB | +20 dB | -7 dB | -13 dB |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +5 dB | +16 dB | -12 dB | -17 dB |
| HiFiMAN Susvara | 60 Ω | 50 mW | +3 dB | +11 dB | -13 dB | -19 dB |
The Fosi Audio SK02 costs $1 more than the JDS Labs Atom+ (2nd gen), and on these three axes the money buys nothing: among the 567 models here there is not one that the Fosi Audio SK02 handles and the JDS Labs Atom+ (2nd gen) does not — not on power, not on the one-eighth rule, not on hiss. The traffic runs the other way: the cheaper JDS Labs Atom+ (2nd gen) drives 2 models to 100 dB and keeps 26 below the audible-hiss threshold that the Fosi Audio SK02 does not.
Only published figures count here; blank axes are counted separately.
Each in full: Fosi Audio SK02 and JDS Labs Atom+ (2nd gen).