Chord Electronics Mojo portable DAC/headphone amp

Headphone amps and amp/DACs
Chord Electronics Mojo

But praiseworthy though the Hugo was and is, the device posed certain drawbacks and challenges. First, although portable, the Hugo was by no means pocket-sized (the Hugo is roughly the size of paperback book), meaning that it was not really an on-the-go device. Second, the Hugo was undeniably (and for some, prohibitively) expensive; the best is never cheap, but for obvious reasons, even the most ardent of headphonistas likely will think twice before dropping four-figure sums of money on a portable device. Third, the Hugo featured user controls that, although cleverly and attractively implemented, were sufficiently unorthodox so as to prove confusing, at least for some listeners.

What was needed, Chord reasoned, was a product that in virtually every way offered Hugo-like performance, but in a smaller and more ergonomically satisfying package, and at a dramatically lower price. That, in a nutshell, is precisely what the Mojo aims to be.

The Mojo is roughly the size of (or perhaps a bit smaller than) a deck of playing cards, so that when viewed alongside the original Hugo, the little Mojo seems positively Lilliputian (it’s about a third the size of its illustrious big brother). Its finely sculpted, black anodised aluminium chassis features subtle curves and gently scalloped recesses for three pleasingly dome-shaped control buttons: an on/off switch and a side-by-side pair of up/down volume controls. Adding an almost whimsical touch of colour, the three buttons illuminate from within; the hue of the power button indicates the sampling rate of the files in play, while the vibrant colours of the volume buttons give visual confirmation of the output level settings chosen. Most of all, though, the Mojo simply feels pleasing in the hand—much like a favourite talisman. This is very much by design. In fact, according to John Franks, the Mojo’s general shape and surface textures were inspired in part by the look and feel of the smooth beach pebbles his daughter likes to collect.

Before talking about the Mojo’s sound, let me first provide a summary of the unit’s basic features and functions, and also spend a bit of time discussing the sophisticated design philosophy that guided the Mojo’s development.

The Mojo provides four basic inputs: an optical digital input, a coaxial digital input, and two USB inputs (one for playback and the other for power charging). In turn, Mojo offers two variable level headphone outputs implemented via 3.5mm mini-jacks—the train of thought being that friends can thus share the Mojo listening experience if they so desire. Interestingly, the headphone outputs can, by following a Chord-specified start-up sequence, be configured as line-level outputs for those wishing to use the Mojo as a standalone DAC. Even so, my sense is that Chord expects most Mojos will be used primarily to drive headphones or earphones.

The DAC section of the Mojo supports PCM files from 32kHz to 768kHz and DSD files ranging from DSD64 to DSD256—capabilities that handily exceed those of the original Hugo, which supported PCM files at up to 384kHz and DSD files up to DSD128. As with the Hugo, the Mojo DAC’s interpolation filter is implemented via a Rob Watts-designed WTA filtering algorithm running on a computationally potent, but low power consumption FPGA (field programmable gate array) device—in this case, a Xilinx Artix 15T chip. According to designer Rob Watts, this powerful FPGA actually handles a number of functions for the Mojo including, “S/PDIF decoding, USB time, DPLL, WTA filtering, DSD decoding and filtering, volume control, thermal protection, battery status, noise shaping and DAC (functions).” The Mojo is compatible with iPhone, Android, or Windows smartphones and with Mac, PC, or Linux computers (though a Chord-supplied device driver will be needed for use in Windows environments).

Chord says that the, “Mojo shares Hugo platform FPGA code but with half the power consumption.” Indeed, the Xilinx Artix 15T device used in the Mojo is in principle more computationally powerful than the earlier-generation Xilinx FPGA used in the Hugo, but to accomplish Chord’s low power-consumption objective, the Mojo’s FGPA is deliberately run with its clock speed turned down (lower clock speed = lower power draw = lower operating temperature).

One critically important point is that the design of Chord’s Hugo and Mojo DAC sections are radically different to most other desktop or portable DACs on the market. The theories involved can get a bit complex (and frankly exceed the limits of my own technical knowledge base) so I will try to provide a simplified—but hopefully accurate—layman’s explanation. According to designer Rob Watts, certain fundamental aspects of digital audio sampling theory are either misunderstood or simply overlooked by most designers. Chief among these overlooked elements is the notion that—and please stick with me here—with digital audio files of CD resolution or better, it is always possible to fully and perfectly recreate the original analogue waveform provided you have digital filters of infinite (or virtually infinite) tap lengths. Ah, but there’s the rub.

Most off-the-shelf DAC chips provide perhaps a couple of hundred digital filter taps, which Watts says is not nearly enough to properly recreate certain crucial timing aspects of music. For this reason, Watts uses FPGA-based filters and his own WTA filter algorithm, which support tens of thousands of digital filter taps, leading to markedly better resolution of timing (that is, says Watts, “the starting and stopping of notes”). Chord does not directly specify the number of filter taps supported by the Mojo, though from conversations with Franks and Watts I suspect that the Mojo may actually offer more filter taps than the Hugo, but with the proviso that its FPGA clock speed is turned down to reduce power consumption. One difference between the Mojo and Hugo, however, is that the Mojo uses very subtly different filter parameters to achieve a sound thought to be friendlier to mid-priced headphones or earphones. In any event, the Mojo stands as one of the most sophisticated DACs that money can buy, despite its diminutive size.

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