THX Onyx: power output and what it will drive

The THX Onyx is a USB dongle at $200. It delivers 124 mW into 32Ω and 13 mW into 300Ω, which ranks it 198 of 280 for raw power among the sources in this database.

Measured at 100 dB SPL it has comfortable headroom on 512 of the 567 dynamic and planar headphones catalogued here, enough but not generous on 45, only marginal on 8 and not enough power for 2. Power is published at 22Ω; the rest of the table is derived from those figures. Output noise is not published, so the hiss estimate cannot be run.

The manufacturer publishes 180 mW into 22Ω. Everything else in the table above is worked out from those figures, and this table says which is which.

Derivation holds output voltage steady between and beyond the published loads rather than output power, because that is what an amplifier actually does. The full method is written up here.

Source: thx.com, thx.com, thx.com, thx.com.

Ordered from easiest to hardest to drive. Headroom is measured at 100 dB SPL, which is louder than almost anyone listens — a small negative number means the source still plays loud, just without reserve for peaks.

These figures come from each manufacturer's own sensitivity rating. A handful of planars — the HiFiMAN Susvara and the Abyss AB-1266 in particular — are widely reported to need far more than their published sensitivity implies, so treat a bare pass on those as optimistic rather than settled.

The manufacturer does not publish an output noise figure for this source, so the hiss estimate that runs on every other page here cannot be run on this one. Rather than substitute a plausible number, this page leaves the question open.

If you know the noise figure from a measurement you trust, the hiss calculator takes it directly and tells you where that noise lands in your own in-ears.

At 0.2Ω output impedance this source satisfies the one-eighth rule with every headphone in the database, down to the 3Ω in-ears. Nothing here will have its frequency response shaped by the source.

The rule is a rule of thumb, not physics: a source impedance below one eighth of the headphone's keeps the interaction under about 1 dB. The checker works it out for any pair.

Of the 280 sources here, 5 do everything the THX Onyx does as a headphone output: reach 100 dB on every headphone it reaches and stay inside the one-eighth rule wherever it does. The cheapest of them is the FiiO KA5 at $99, against this one’s $200 — $101 less.

The cheaper one also publishes a noise figure, which the THX Onyx does not — so on the paperwork it is ahead, not merely level.

This counts a headphone output and nothing else. Inputs, converters, microphone preamps, balanced outputs, the volume control, the case and the warranty are all outside it, and any of them can be the reason to buy the dearer box. What the count does say is that the three numbers a maker can be held to do not separate them.

Worked out in advance against its closest rivals: vs Astell&Kern HC3 · vs Astell&Kern HC4 · vs ddHiFi TC44 Pro · vs EPZ TP50 · vs iBasso DC07 Pro · vs Shanling UA5. Each of those pages lists the headphones that tell the two apart.

Or put any two side by side in the source comparison tool, or run your own headphones through the amplifier matcher. Or see every THX source in the database, and how much of what this site says about them came from THX’s own documents.