Single-board computers (hereafter SBCs) have been instrumental in the ever-growing success of DIY streaming platforms. The Raspberry Pi is, without doubt, the most popular and has been widely adopted by the audiophile community to run streaming servers and clients in conjunction with dedicated software such as Volumio, MoOde, PiCore and Roon et al. On the hardware front, a great many accessories have been developed specifically for audiophile use including DAC hats from HiFiBerry and IQAudio among others.
Chinese-founded Khadas, owned by Shenzhen Wesion Technology Co Ltd manufacture SBCs primarily targeted towards streaming media players. ‘Khadas’ being rooted in the word ‘Khata’ which is a traditional ceremonial Tibetan scarf symbolising purity and compassion. Though their main business is in OEM and ODM product manufacture, they offer a product range under their name available to the open-source community and the commercial industry alike. Yet as far as I know they remain one of the only SBC manufactures to produce a dedicated hi-fi DAC. More impressive still is that the DAC is no afterthought or box-ticking accessory, but a real high-end board with a specification the equal of commercial products many times its cost.
Physically the Tone Board resembles a typical SBC hat; that is, a board designed to stack on top of an SBC and interconnect via its GPIO (general-purpose input and output) header, or custom headers available on some SBCs. The ‘generic’ edition Tone Board arrives with provision for the header and the connector provided in the box for you to solder yourself. There are also an additional 20-pin header and the requisite standoffs, nuts and other mounting hardware required to stack the Tone Board with an SBC for an integrated media streaming solution.
The 20-pin header is used for debugging the Tone Board and updating the firmware of the XMOS microcontroller. There is also an I2S connector, an extension connector for the XMOS microcontroller and provision for an OLED display. Pinouts for the connectors are at the time of publication available Here.
Where the Tone Board differs from most hats however is in the ability to use it externally. Its hat functionality is intended to be used with a Khadas SBC. To that end, it connects to everything else via a class-compliant USB connection and XMOS XU208 controller. Class compliance means that mOST operating systems are supported including Windows, Mac, Linux and Android systems, and iOS too with a suitable connection cable. The board can be powered via the USB connector or via any 5V external power supply, for which there are onboard connection pads should you wish to directly connect a 5V power supply and integrate the DAC fully into custom equipment or a more traditional case.
The Tone board provides a USB C connector and includes a braided USB C to USB A cable in the box. It supports PCM sampling rates up to 768kHz and DSD up to DSD256. Fitted to the opposing edge are high-quality RCA jacks, a cut above your typical board-mounted sockets. These provide the stereo analogue audio outputs and a S/PDIF coaxial digital input which becomes active when USB data is not being transmitted. The coax connector can also function as a digital output when a USB input is active.
I was kindly provided a sample of the Tone Board for evaluation by KJF Audio. Also supplied was an acrylic case, designed to transform the Tone Board into a simple yet high-quality external DAC.
The case is supplied with a choice of clear or black top and bottom panels and black sides and is slotted and screwed together with the included hardware. It’s an acceptable solution, but I feel most competent DIYers could build a vastly better enclosure without much difficulty. The case looks like a DIY effort from the protrusions designed to act as feet, to the way the board is mounted which positions one of the mounts in a location that is impossible to access without removing one of the pre-soldered RCA connectors.
A screw at that point is unnecessary and better positioned beside the USB connector where additional resilience is required, but where there is no hole drilled. It’s a half-baked, half-hearted effort in case design. But let’s not let it detract from the Tone Board itself.
For the board, when measured, demonstrates superlative performance. Total harmonic distortion + noise is measured at 0.000337% by Khadas, I measured the sample at 0.000334% using an identical Audio Precision APx555 analyser that I used for the measurements in this review. Audio Science Review (ASR) also measured the board with the same model of analyser and got 0.000331% THD+N, so performance appears highly consistent across samples. In terms of jitter and noise, any spikes are well below audibility. Dynamic range on my sample fell short of the specified 120dB but only by 2dB, with just over 118dB measured. This too was consistent with findings from ASR. Rather than fill this article with further measurements, I’ll direct you to the ASR review, where you can see graphs of the above as well as linearity, crosstalk and channel separation, all of which are equally superb.
Sonically, the fact that there is little to say is a good thing. One of the reasons I seldom review new DACs on the market unless they have unusual features or a greater number of features in their design is that I believe a well-implemented DAC should sound as good as any other.
Given the exceptionally low noise floor, high dynamic range and exceptional stereo separation, all of which are far beyond what is audible to us as humans, the Tone Board should – and does – sound clean, clear with largely without any character of its own. What you’re hearing is your amplification and loudspeakers which will differ in every use case. Depending on your point of reference you may find the Tone Board to offer extra detail or analytical tendencies of which it is perfectly capable.
This will be especially noticeable if your reference is a DAC with a lesser output stage. Compared to a higher-end DAC but one equally as competent technically such as my Musical Fidelity M6s DAC, the Tone Board suffers a little dynamically but otherwise performs admirably and there is little to tell the two apart besides the Musical Fidelity having a healthier output and better load driving ability, which in turn gives the impression of better dynamics, better staging and an overall less strained listening experience.
Taken on its own however the Tone Board is a clean, clear, neutral DAC. It takes in digital audio and spits out a true analogue representation that is faithful to the source. It’s a marvellous little board for all kinds of usage scenarios from a simple desktop DAC to integration with a headphone amplifier, DIY streamer or even a custom integrated amplifier. I can’t recommend it highly enough. You can purchase one from KJF Audio, who I thank once again for making this review possible.
