I emailed designer Philip Gruebele: "Now that a few years have passed, the 'USB improver' black box has become an accepted member of the audiophile club. There are numerous choices. Except there's still not a completely hard-core invariable explanation about what's going on, exactly. How can DACs with galvanic isolation on their USB inputs benefit when they can't see the computer ground? How can DACs with 'femto' clocks or some insane ASRC, all claimed to be utterly immune to jitter, audibly improve because the signal now arrives pre-clocked or re-clocked by an intermediary box? There's no doubt that these devices work. It's also apparent that as DACs improve (particularly their USB transceiver implementation and isolation of what follows), the efficacy of these boxes diminishes. Still, it seems embarrassing that 'audiophile' designers still haven't figured out how to render your type device redundant because they put something inside their DACs which already does the same thing. Good for your business for sure. Somewhat of an indictment about their design chops, too. Could I motivate you to pen something from your perspective that gets properly technical; that explains why 'double clocking' (in your box and then again in the DAC) can make improvements; and how computer-generated noise traveling over the USB ground can still infect the signal even if the USB input on the DAC is isolated?"

Part of reviewing has writers play at Grande Inquisitor. Designers aren't obliged to divulge anything. Approaching them as not Joe Public can simply loosen tongues. Occasionally. I love these opportunities to learn more from real experts. Getting nosy isn't an attempt to strip them of their IP. It's to add to our hobby's knowledge pool. Of course there's a fine line between adding to that and giving away the farm. "In a nutshell, I think that USB->SPDIF converters basically act as filters which can filter PC switching noise, avoid ground loops and filter clock phase noise. DACs of course are also designed to act as such filters but all a filter can really do is to reduce noise. It cannot remove it completely. So adding additional filtering in series may result in additional sonic improvements. Older DACs were quite ineffective filters. Changes to their S/PDIF input signal noise greatly affected the analog output signal. Designers have been paying more attention to this. Resultant newer DACs are much more effective filters." So I'd learnt nothing new except that even the author of such a device felt that newer/better converters should have much less need of them.

Decrap haven? With Audiobyte's battery-power Hydra X+ and Intona's USB-power HiSpeed Isolator on hand, I could polish up my USB pipeline three different ways. Of my not dirty dozen but clean triplets, one output via USB exclusively (Intona), two instead converted to BNC (Audiophilleo, Audiobyte), one added coax and AES/EBU (Audiobyte). This lot slotted into our usual rig of iMac/PureMusic, Aqua Hifi discrete R2R Formula DAC, Wyred4Sound STP-SE II preamp, Linnenberg Audio Allegro monos and Albedo Audio Aptica speakers. For twists, I had two €1'250 designer USB cables from new Polish brand StavEssence. Those might explore whether extra hifi funds were best spent on premium USB cables; or Philips's generics with his filter. At least that was the plan.

Rolling these filters on the Formula was an exercise in most subtle degrees of smoothness gains. Of the three, I couldn't tell the Audiobyte and Audiophilleo apart at all. Using Philip's two generic cables, the Intona was different from them - even more suave and 3D sculpted. As a USB/USB bridge, the Intona was also alone in passing 384kHz to any capable DAC though for the occasion, I did not upsample in 64-bit PureMusic software. With the others outputting S/PDIF, 192kHz was their limit. Assessing the triplets came versus running Philip's longer leash direct into the Formula's USB input. To be sure, these differences were minor. They played out as essentially a reduction of remnants of pixilation around lead vocals and strings and the occasional small whiff of glare. Even then I had to listen for it and spin tracks that had the 'correct errors' (what a concept). After spending a sunny afternoon swapping every which way, I thought that except for the €344 Intona, the delta of difference for the costlier decrappifiers was far too minor to get excited over. The €13'800 Italian DAC with its optocoupled heart seemed to be one of those "newer... much more effective filters". Also, the Aqua didn't play nice with the StavEssence cables' alligator-clipped ground wires. It either couldn't see the iMac at all; or distorted with heavy clicking until the iMac paused itself. It was time to declare the Formula immune from these shenanigans and find a more responsive DAC. Our COS Engineering D1 was next. Clipped to its actual ground post, the StavEssence cables were an instant go to promise more fruitful explorations involving them.