Keep in mind that such circuits are a violation of electrical code. You simply cannot place circuitry in the path of the safety ground. This is why HumX does not and will never have a UL rating despite EbTech saying for years that such a rating is coming.
Another thing to consider is fault currents. Momentary fault currents can be huge (100+ amps) and diodes will blow faster than circuit breakers. In other words, should you have a catastrophic fault, you will be left without any protection whatsoever.
The presentation below was given by one of the world's foremost experts in audio grounding, Bill Whitlock. Bill is an AES and IEEE fellow and a good friend of mine. We have even co-presented on a few occasions. The pertinent information appears around slides 25, 125 and the HumX is specifically mentioned on slide 209. This presentation is probably more than you want to know (it was a half day workshop if I recall) but this covers every possible hum issue.
Thanks, good information in that article, though a lot of it is better applied to premises NEC (509). Care has to be taken to know when NEC (520) or NEC (525) are the applicable standards. This can also be a lesson in HOW to apply standards so that they do in fact, promote safe practice rather than an ideal for the ideal's own sake.
While there's nothing actually wrong in that presentation - it would be wrong to apply it as you suggest. Let's start with the HumX and consider what it is and what it isn't. Whether or not the HumX is UL listed is of no real impart. It's 6 amp capacity (that's 6 amps on the ground leg, the plug itself is a 15A device) makes it a novel appliance not electrical magic. It's rating is consistent with the typical (retail) DJ gear or casual musician.
The HumX addresses an issue that absolute standards don't - which is the intended USER of this device.
In the absence of the HumX what is that user most likely to do? What have they traditionally done? In any meaningful ground fault condition the HumX rectifiers fail to short providing a level of safety to this type of user in conditions where habitually safety was discarded..
UL Listing is an underwriting endorsement, not an electrical code. The device is legal and likely well built within it's 15A design. When you consider what it does for the kind of users I just described - I think it's a win. For all we know it may have already saved someone's life. At the same time - there's likely frustration from user's who have placed it in persistent overload conditions that resulted in some embarrassing moments. HumX doesn't have a corner on that market - excess is often applied to all manner of setups.
Issues with age of the information are relevant when presenting "worst case" scenarios or hand picking numbers without any context. How do you suggest a 100A fault to ground persisting for 2.5s on a modern branch circuit to occur? That's neither momentary nor likely and If that's what surges down my 15A Edison cable a smoking HumX will be least of my concerns. We're not dealing with mains, fuses, feeders, or even a toaster. At 5 to 7 times the nominal current a modern branch breaker is going to win the race with a HumX every time.
Here's the thing - large modern diodes rarely fail open because they are made with solid thru conductors. If they are rated consistent with the stranded wire devices into which they are installed then they will pass exceptional faults to the breaker with time to spare.
So, when DOES the HumX fail and what does that look and sound like? It will fail in a severe fault condition that loads the ground with more than 6A. We're talking however, about the permanent failure of it's noise cancelling "lift" not a failure to pass fault to a GFCI or impede current overload protection at the breaker.
The plug assembly itself is consistent with any other 15A Edison so, if the fault was non destructive - the DJ resets the breaker or GFCI and continues on perhaps unaware that the buzz from any previously blocked ground loop has now returned. A week later he plugs in his HumX to resolve a ground loop but the buzzing doesn't go away. Yanking it from the wall he exclaims
"Piece of crap doesn't even work!" ...and promptly cuts the ground lug off his extension cord.
My goal, is to build a bridge that is on par with the current carrying capacity of the existing internal wiring but with a threshold capable of blocking minor ground differentials from looping through the system. By building the bridge into a specific end point box I'm eliminating the ambiguities around application and rating that make people uncomfortable with the HumX. This ends up positioned appropriately at the appliance end rather than the head end of any AC source lines.
As I said, I'll be doing some testing, and if anyone has done such tests, or experienced failures of such a device, please chime in. One test I will NOT be doing however, is the application of 100A spike to a 15A cable and breakout box. It's just not realistic.