The meaty part in the Audiobyte is two-fold. First, it’s one of the handful of modern devices that includes I2S connections through HDMI. If you think I2S was just a digital flash in the pan from the 1990s, remember that it’s basically the native tongue of digital audio, avoiding any kind of transcoding necessary to package digital for transit from device to device. Done properly, I2S rips out several stages between codec and ear, and if you have the devices that can connect this way – Audiobyte and Rockna, for example – this is the must-do way to connect.
Next, the architecture is developed in house and fed as firmware to a Xilinx 7-series Field Programmable Gate Array. This is not dissimilar to Chord Electronics programming its own DACs into FPGA chips instead of using custom or off-the-shelf DACs. It means the one-bit DAC and signal processing are all unique to Audiobyte and – even though the DAC currently has a sampling rate close to DSD512, if more is needed, more can be provided through firmware updates. This also means the DAC is capable of some very heavy lifting; Using 80 DSP cores running at 200 MHz and 68-bit processing, the Hydra.VOX has internal audio file capacity of up to 35-bit, 768kHz rates. Also, a unique feature of the Hydra.VOX is the seamless integration between analogue and digital filters in order to preserve accurate phase of the audio signal. The digital filters inside FPGA are programmed to exhibit a “mirrored” phase response, thus cancelling the phase error from the analogue domain, creating a very accurate phase response over the audio band.
There’s a ‘category error’ that is easy to fall into with regard to the Hydra.VOX/ZAP combination. Because they are three-quarter sized and come with a headphone socket, you naturally pigeon-hole them in ‘personal audio’. Don’t. While the pairing represents an excellent headphone amp and DAC system in their own right (although in that context, the absence of balanced headphone outputs might sway against them), they are also so much more than that. In fact, it was in a conventional audio context that they really show what they are capable of, and as a first-rate DAC, they are in the top tier. The sound can perhaps best be summed up as ‘organic’ and ‘fluid’. It’s a very dynamic and detailed presentation but that coherence across the frequency range is what first jumps out at you.
A solo piano can be a surprising torture test for a DAC, because it’s such a known and pure sound, even the mildest digital hash quickly turns a Steinway into a Segway. The ‘La Campanella’ from that audiophile classic Nojima plays Liszt [Reference Recordings] is a fine example of just how easy it is for good digital to shine over expensive-yet-mediocre digital audio; here the upper registers are played with precision and clear delineation… and none of the raspy grittiness that many systems portray as ‘high-frequency information’. There’s a lot of good frequency extension and the notes are easy to hear and pick out, but that ‘digital’ sound is gone.
The clarity of piano is swiftly joined by the sheer scale of the soundstage. The Hydra.VOX/ZAP create a wide and deep soundstage with some – rare in the digital domain – stage height. For this, some of those classic Decca SXL recordings turned digital – with, admittedly mixed success – are useful. My oft-played overture to The Pirates of Penzance by the D’Oyly Carte company from the early 1960s is a textbook test that the Hydra.VOX aces. If it makes my Wilson Duette Series 2 produce an even wider stage than usual, that’s a very good sign.