Astell&Kern HC3 vs THX Onyx: Which Drives More Headphones?

Astell&Kern HC3 vs THX Onyx

Short answer

Power: about the same. The Astell&Kern HC3 has 125 mW into 32 Ω, the THX Onyx 124 mW. The THX Onyx drives 4 headphones to 100 dB that the Astell&Kern HC3 cannot (Moondrop COSMO, Symphonium Crimson, Symphonium Helios, 1 more).
Hiss with sensitive IEMs: cannot be compared — neither maker publishes a noise figure.
Output impedance: 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).
Price: the THX Onyx costs $21 more; what that buys on these numbers is set out further down, under What the price difference buys.

Both are dongles, $179 against $200.

The THX Onyx reaches 100 dB on 565 of the 567 models here, the Astell&Kern HC3 on 561.

Every published figure, next to each other

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

Not published means the maker leaves it out, so that axis cannot be compared.

Where each power figure comes from

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 the labels mean.

The headphones that tell them apart

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.

Headroom and hiss on 8 models, both sources

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.

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 gap in dollars, against the gap in numbers

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.

Only published figures count here; blank axes are counted separately.