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Testing Ground

THX Onyx Test: a remarkable USB mini DAC with a powerful headphone output

By Alban Amouroux |

Mini USB DAC amplifiers have been multiplying in recent years. Their primary purpose is simple: to improve the original headphone output of smartphones. This is not that difficult to achieve, as mobile phone manufacturers do not really have the quest for audiophile sound quality at the top of their to-do list. With the THX Onyx, the world-famous THX brand offers a compact, high-performance solution for amplifying music on headphones, both on the move and in the office.

THX, three letters recognised throughout the world for their link with surround sound in the cinema, later became available to home cinemas. THX is involved in the sound signature of many home audio products. The company also developed processing and enhancement technologies for HiFi (THX AAA) and 3D sound (THX Spatial Audio). Recently, THX decided to demonstrate its know-how to headphone users with a mini USB DAC amplifier called the THX Onyx.

In the case of the Onyx, it is high-resolution HiFi quality audio that is targeted thanks to the THX AAA amplifier circuit. This allows the brand to develop the recognition of its technologies in a field in which it was not necessarily expected. However, many HiFi equipment manufacturers have already integrated the AAA circuit into their headphone amplifiers or even directly into the headphones. This is the case with Razer, known for its many gaming products, from laptops and headsets to ergonomic seats and game controllers. A close partner of THX, Razer sells the Onyx DAC on its own website.


Price : €209.99
Type : USB DAC amp
D/A Conversion : ESS ES9281PRO
Amplification : THX AAA-78
Impedance Range : 22-1.000 Ohms
Output Impedance : 0.25 Ohm
Dynamic Range : 118 dB
Distorsion (THD+N, 1 kHz) : -110 dB
Power Output Per Channel (22 Ω, <1 % THD+N) : 180 mW
Input : USB-C + including USB-A to USB-C adapter
Output : Microphone compatible stereo mini-jack (non-Apple)
Dimensions : 208 x 12 x 7 mm
Weight : 18g

Overview of the THX Onyx

The portable mini-DAC category is a recent development. Up to now, components could not be miniaturised enough to integrate a digital/analogue converter and a pre-amplifier/amplifier section into a unit scarcely larger than a USB stick. Now it's possible, using real quality components. The THX Onyx uses the ESS9281PRO chip. It is mainly found in USB DACs because of its specific characteristics such as its minimal size and low power consumption. Another special feature is that this chip can decode MQA and DSD, the two other Hi-Res audio formats apart from PCM. Its compatibility is therefore universal.

It will not have escaped your attention that THX features in the product name. This certification originated in the cinema world and has gradually been developed in other sound related fields. Home cinema of course, and, more recently, headphone amplifiers. This product has THX AAA technology, for Achromatic Audio Amplifier, built into its product. It delivers the most faithful sound possible by extending the dynamic range. Thanks to this technology, the THX Onyx delivers 180 mW per channel at 22 ohms to provide the power needed for the most demanding headphones. Even some sedentary format HiFi devices do not go this far in terms of pure power.

This USB DAC has a really small format. It ends in a USB-C plug about ten centimetres away from the electronics housing. They are joined by a flexible cable allowing the THX Onyx to be easily folded. What is more, when the cable is folded, the two ends are held together by a magnetic clasp thus taking up even less space. If your equipment doesn't have a USB-C socket, THX provides a compact adapter to USB-A.

On the electronics side, the DAC ends in a 3.5mm mini-jack output. As we have seen, its power will be sufficient for most headphones. This output is not balanced. If this feature is important to you, you should look at one of the competing models. In the output end, it delivers up to a maximum 2v, meaning that this DAC can be used upstream of an amplifier or a pair of active speakers. For those who favour minimalism in all things, you can imagine a simplified quality HiFi system: a smartphone, the THX Onyx DAC and amplified speakers.

Installing the THX Onyx

The 3.5mm headphone jack is still very common on computers, tablets and smartphones. Although in the latter case, it is disappearing as the USB-C has become more generally used. However a USB-C to mini-jack dongle is sometimes provided by smartphone manufacturers. But the output is usually of inferior quality. This is not really so bad, as the current trend is towards Bluetooth headsets. Although very convenient, Bluetooth is not the best way to have direct access to very high-quality audio.

The USB DAC allows you to receive digital sound and convert it to analogue. This THX Onyx is compatible with all devices capable of delivering sound through a USB port. This is the case for desktop and laptop computers, tablets and smartphones. The USB DAC runs on Windows and macOS. If you connect a headset to the THX Onyx, the microphone function is not supported under macOS. If you are using an iPhone, you will need an intermediate adapter, from the proprietary Apple Lightning plug to USB-A. Apart from these two exceptions, you should have no doubt that the THX Onyx works with everything.

Once connected, the THX USB DAC remains in sleep mode. Only one of its micro LEDs is lit in blue. It will not appear in the list of devices. To activate it, you must connect headphones or a HiFi amplifier to the mini-jack output. The DAC then makes itself known on the device being used. It will appear as THX Onyx USB Amplifier.

When music is played, the three micro LEDs light up blue if the source is CD quality. In high-resolution PCM, the LEDs change to yellow. DSD is indicated by the colour red, while MQA is identified by pink. This makes it possible to check the audio quality at a glance. THX's website has available for download a comprehensive user guide covering all scenarios, according to the hardware, music service or software used. The site also states that the THX Onyx is compatible with Qobuz Studio on all devices. For our test, we connected the USB DAC to a Sony Xperia Android smartphone and Beyerdynamic Amiron Home headphones, all via the Qobuz app.


We started by listening to Makaya McCraven and his last album, Deciphering the Message. The THX Onyx treated us to a wide soundstage in which the different instruments of this octet are distinctly isolated from each other, with a very noticeable depth and distance effect. With analogue headphone output, this sense of depth is lost. The sound also seems less alive, more closed in, more compacted. With the THX Onyx, the bass offers more depth, the notes are sustained longer. There's no doubt about it, the contribution of this mini USB DAC is significant. This was confirmed listening to the track My Girl, one of the Royal Philharmonic Orchestra's covers of Motown classics. Everything is smooth with this DAC. Micro-information appears with that little reverberation effect that indicates there is a world behind the instruments. But above all, voices are clearly discernible from the orchestral mass. Compared to the original headphone output, the midrange is better defined with the THX, and the snapping of fingers sounds more natural.

The ambient details behind Juliette Armanet's voice are fine and precise when listening to Brûler le Feu. Her voice is separated from the piano, the latter is lower down the scale while the voice is higher. The finesse in the upper midrange is remarkable without becoming aggressive. Compared to the basic headphone output, the result offered by the THX Onyx is like removing a veil from the music. We moved on to the electro of Darius on his track Rise. The round, percussive bass that forms the backbone of the piece gives its full measure without ever sagging. Controlled, it is even explosive when it is alone at the beginning and end of a piece. The airy aesthetic is perfectly rendered with the lead vocal extending from ear to ear. Keyboard gimmicks are reproduced outside the head to provide a pleasant fullness.

Pros :
Compact and Ultra Light
Compatible with PC/Mac/Smartphone/Tablet
Midrange/Treble Finesse
Open Sound Feeling

Cons :
A little more expensive than the competition


THX rarely markets electronic products under its own brand, however, with the Onyx USB DAC, the company has pulled off a major coup. This small USB DAC is invaluable for the sound quality it delivers. Everything is well thought out, there is absolutely nothing to criticise. The technical choices tilt the balance in its favour. The excellent separation makes it easy to enjoy all types of music. The latter feature bears no comparison to the basic headphone output of a smartphone, tablet or computer. If you own a quality headset, once you have experienced the THX Onyx you'll never again settle for the original outputs. It will quickly become inseparable from your favourite headset. The price may seem high compared to the competition offering equivalent models, but the compact size and the improved quality will more than compensate for this difference.