Short answer
Both are dongles, $46 against $50.
The ddHiFi TC35Air reaches 100 dB on 564 of the 567 models here, the Hidizs SD2 on 556.
| Specification | ddHiFi TC35Air | Hidizs SD2 |
|---|---|---|
| Power into 32Ω | 210 mW | 70 mW |
| Power into 150Ω | 45 mW | 15 mW |
| Power into 300Ω | 22 mW | 7.5 mW |
| Power into 600Ω | 11 mW | 3.7 mW |
| Output impedance | not published | not published |
| Output noise | not published | not published |
| Price | $46 | $50 |
| Headphones it reaches 100 dB on out of 567 | 564 | 556 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Load | ddHiFi TC35Air | Hidizs SD2 |
|---|---|---|
| 32 Ω | Published | Published |
| 150 Ω | Derived | Derived |
| 300 Ω | Derived | Derived |
| 600 Ω | Derived | Derived |
Both makers publish a power rating. ddHiFi TC35Air publishes at 32 Ω; Hidizs SD2 at 32 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.
8 headphones reach 100 dB on the ddHiFi TC35Air but not on the Hidizs SD2 — HiFiMAN Susvara, Moondrop COSMO, Moondrop PARA 2, 5 more.
556 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: ddHiFi TC35Air has +10 dB of headroom there against +5 dB. 5 dB is about 3 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for ddHiFi TC35Air and Hidizs SD2 because neither maker publishes no output noise figure.
| Headphone | Impedance | Needs | ddHiFi TC35Air headroom | Hidizs SD2 headroom | ddHiFi TC35Air hiss | Hidizs SD2 hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +32 dB | +27 dB | not published | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +12 dB | +7 dB | not published | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +21 dB | +16 dB | not published | not published |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +16 dB | +12 dB | not published | not published |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +13 dB | +8 dB | not published | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +10 dB | +5 dB | not published | not published |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +5 dB | +1 dB | not published | not published |
| HiFiMAN Susvara | 60 Ω | 50 mW | +3 dB | -1 dB | not published | not published |
The Hidizs SD2 costs $4 more than the ddHiFi TC35Air, and on these three axes the money buys nothing: among the 567 models here there is not one that the Hidizs SD2 handles and the ddHiFi TC35Air does not — not on power, not on the one-eighth rule, not on hiss. The traffic runs the other way: the cheaper ddHiFi TC35Air drives 8 models to 100 dB that the Hidizs SD2 does not.
Only published figures count here; blank axes are counted separately.
Each in full: ddHiFi TC35Air and Hidizs SD2.