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.
| Load | Astell&Kern HC3 | THX Onyx |
|---|---|---|
| 32 Ω | Upper bound | Derived |
| 150 Ω | Upper bound | Derived |
| 300 Ω | Upper bound | Derived |
| 600 Ω | Upper bound | Derived |
Neither column is a power rating, and the two are not even weak in the same way. The Astell&Kern HC3 figures are ceilings worked out from a maximum output voltage, on the assumption that the amplifier holds it into the load. The THX Onyx figures are worked out from figures its maker did publish — power at other loads, or the voltage printed beside it. The second method understates; the first flatters. That is the wrong way round for reading the table straight across, so a gap in favour of the Astell&Kern HC3 is the one to doubt.
What each of these labels means, and why a derived figure is still worth printing, is set out in how this data was built.
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.
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 Campfire Audio Andromeda 2020: THX Onyx has +13 dB of headroom there against +9 dB. 3 dB is about 2.1 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for Astell&Kern HC3 and THX Onyx because neither maker publishes no output noise figure. That is a gap in the specification, not a quiet amplifier, and it is left blank here rather than filled with a plausible number.
| Headphone | Impedance | Needs | Astell&Kern HC3 headroom | THX Onyx headroom | Astell&Kern HC3 hiss | THX Onyx hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +29 dB damping | +33 dB | not published | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +9 dB damping | +13 dB | not published | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +19 dB | +19 dB | not published | not published |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +14 dB | +14 dB | not published | not published |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +11 dB | +13 dB | not published | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +7 dB | +7 dB | not published | not published |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +3 dB | +3 dB | not published | not published |
| HiFiMAN Susvara | 60 Ω | 50 mW | +1 dB | +1 dB | not published | not published |
The THX Onyx costs $21 more than the Astell&Kern HC3. Out of the 567 models here, that difference buys 4 more models driven to 100 dB and 89 more models left alone under the one-eighth rule.
Nothing runs the other way: there is no axis here on which the cheaper of the two does better.
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: Astell&Kern HC3 and THX Onyx. Or put either against your own headphones in the source comparison tool.