Both are dongles, $179 against $200. 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?
The THX Onyx reaches 100 dB on 565 of the 567 models here, the Astell&Kern HC3 on 561.
| Specification | Astell&Kern HC3 | THX Onyx |
|---|---|---|
| Power into 32Ω | 125 mW | 124 mW |
| Power into 150Ω | 27 mW | 26 mW |
| Power into 300Ω | 13 mW | 13 mW |
| Power into 600Ω | 6.7 mW | 6.6 mW |
| Output impedance | 2.0 Ω | 0.2 Ω |
| Output noise | not published | not published |
| Price | $179 | $200 |
| Headphones it reaches 100 dB on out of 567 | 561 | 565 |
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.
4 headphones reach 100 dB on the THX Onyx but not on the Astell&Kern HC3 — Moondrop COSMO, Symphonium Crimson, Symphonium Helios, 1 more.
89 headphones are left alone by the THX Onyx under the one-eighth rule but coloured by the Astell&Kern HC3 (64 Audio Aspire 1, 64 Audio Fourte, 64 Audio Nio, 86 more).
561 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.
Each has a page of its own with every power figure traced to what the maker published and a full headphone list: Astell&Kern HC3 and THX Onyx. Or put either against your own headphones in the source comparison tool.