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Then synchronicity struck this assignment. Fred Crane, US importer at StereoDesk, inquired whether I'd like to review the Hydra X+ which customers seem to take off his hands in goodly numbers. Having owned the simpler Audiophilleo without the later battery-power option and still using SOtM's very best two-box USB bridge with battery power and super clock, I was intrigued. Going off board with USB-to-S/PDIF conversion routinely improves the sound of converters which have, or ought to have, solid USB transceiver solutions of their own. By outputting AES/EBU whose signal is about 10 x stronger than coax to get rather more robust, the Hydra would play nice with both my Metrum Hex and AURALiC Vega DACs and the customary VdH professional 110Ω AES/EBU leash that would connect it. Finally I could try its I²S port into the WaveDream. That connection is capable of DSD512—not that I own any such files—and pure DSD if one had Nicolae's matching transport. Of course one wouldn't expect the WaveDream to be improved over its own USB port which sports the same FPGA tech and clock quality. But my sand box was certainly ready for playtime.

Nicolae 'Nucu' Jitariu
Unlike my SOtM dX-USB-HD/mBPS-d2s whose twin batteries power-cycle for 24/7 play, the original Hydra's single battery only lasted ~9 hours before taking 90 minutes to recharge. The + version replaced Toslink with BNC; adopted a larger battery; and improved its power management. I'd have to ask whether that had any effect on prior play and recharge times. But exhausting the battery doesn't mean mute. Fully automated charging simply switches to wall power should you run down. That ought to be hard to do if as recommended you leave the charger connected at all times. Of course having its i/o on opposite ends means you'll want to turn the Hydra sideways and run your cables clean unless you don't mind one right out the front. Sideways orientation of course eliminates seeing the LEDs for power status and DSD. And needing a charger obviously means a li'l SMPS wall wart. This could have purists moan in agony. But agony is part of their job description. That said, I much prefer all i/o on the back panel for a clean fascia. So groan in mock pain.

Hydra X+ and original X version in silver

Original Hydra X board with Toslink rather than BNC output; ARM processor for USB transaction followed by Texas Instruments SiO2 5KV noise isolators; then the FPGA Spartan6 audio core capable of outputting raw DSD including, via ASIO, at 11.2 and 22.5MHz aka DSD256 and DSD512; CCHD-957 Crystek clocks at 45.1584 and 49.152MHz respectively; LiPo-nano battery.

Hydra X+ retailers by the way report good workings of its I²S output into the equivalent port of Wyred4Sound's DAC2se-DSD converter (alas not without small ticks for DSD). Sonore and PS Audio also support I²S over HDMI. Such parallel-data ports can thus be used not only from a spinning transport like Paul McGowan's PerfectWave but also with this USB bridge (hence such devices are also known as CATs or computer audio transports). Whether that expense including a short HDMI leash will be worth the sonic uptick over going with a DAC's own USB solution should depend on your DAC and the overall resolution of your system.

Here we see Nicolae's block diagram for USB isolation and why he believes his is superior to the two standard types. The next diagram shows his TI silicon dioxide isolators which he prefers to common opto-couplers. From their application notes we learn that "these devices have a logic input and output buffer separated by TI’s SiO2 isolation barrier providing galvanic isolation of up to 4000 VPK. Used in conjunction with isolated power supplies, these devices block high voltage, isolate grounds and prevent noise currents on a data bus or other circuits from entering the local ground and interfering with or damaging sensitive circuitry. A binary input signal is conditioned, translated to a balanced signal then differentiated by the capacitive isolation barrier.
Rockna Audio 'network-enhanced' WaveDream transport/server (can do DSD and WAV written to DVD or Blu-ray discs)

"Across the isolation barrier a differential comparator receives the logic transition information, then sets/resets a flip-flop and the output circuit accordingly. A periodic update pulse sent across the barrier ensures proper DC level of the output. If this DC refresh pulse is not received every 4µs, the input is assumed to be unpowered or not being actively driven and the failsafe circuit drives the output to a logic high state. The small capacitance and resulting time constant provide fast operation with signaling rates available from 0-150Mbps. The A-, B- and C-option devices have TTL input thresholds and a noise filter at the input that prevents transient pulses from being passed to the output of the device."

As these photos show, the '+' version Hydra has the clearly bigger battery over the original.

It now piggybacks above the main board on its own bed with standoffs. With this battery upgrade and attendant altered power management, play time of the Hydra X+ is over 40 hours. That's 4 times longer than the original. A full recharge now takes 9 hours i.e. overnight.