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[John Roberts {JR}]

Here is a little excerpt from a guy who reversed engineered a humX:

"..but inside that slick little adaptor
is a very primitive (but clever) little circuit.  In a
standard AC receptacle, you have three contacts -
hot, neutral, and ground.  The hot and neutral lines
pass straight through the adaptor, although there's
a tap from the hot line (with a resistor) to power the
LED.  The mojo is in the ground line, which has
a pair of 6A/1kV diodes between the wall ground
connection and the adaptor's output socket ground.
These diodes are connected in parallel, with
the anode on one diode connected to the cathode
on the other.

aka anti-parallel

btw there is little reason for 1kV reverse breakdown voltage if there is a shunt diode clamping both directions at 0.5V. 

(And vice versa, obviously.)  The
pair of diodes has a 1K 1/2 W -resistor in parallel
with it; as far as I can see, this resistor serves
to produce a "correct" indication when an outlet
tester is plugged into the adaptor, since without
it, the outlet tester will give an "open ground"
indication."

again a 1/2W resistor is over kill if the peak voltage it sees is 0.5V.

6A diodes sounds better that what I think was inside the early units, they need to survive long enough without vaporizing when passing the 50-100A transient to trip the breaker. 

Easy enough to test. 

JR

[Jerome Malsack]

before using that device,  I would work with one of these to eliminate the hum problem. 

http://www.amazon.com/Ebtech-HE-2-XLR-Eliminator-2-Channel-Jacks/dp/B00101WA4C/ref=sr_1_2?ie=UTF8&qid=1460644933&sr=8-2&keywords=ebtech+hum

[Frank DeWitt]

before using that device,  I would work with one of these to eliminate the hum problem. 

http://www.amazon.com/Ebtech-HE-2-XLR-Eliminator-2-Channel-Jacks/dp/B00101WA4C/ref=sr_1_2?ie=UTF8&qid=1460644933&sr=8-2&keywords=ebtech+hum

Transformers are our friends, and if you use a good one you can't hear any effects. 
http://www.jensen-transformers.com/pro-audio/

Also, the good ones have extra shielding that can block buzz and power supply noise. 

[Al Keltz]

Here's an autopsy we did while I was at Whirlwind. The missing diode is the power LED.

[John Roberts {JR}]

Those look like decent sized diodes and not a 1/2 watt R in between them (looks like 1/4W). 

Was that board exposed to a mains fault? In my experience a mains fault would melt (short) the diode junctions and prevent the R from cooking. Those diodes look intact so if stressed by a mains fault should read a low single digit DC resistance. But that resistor sitting between them might not be so discolored in that case (while they would get extremely hot waiting for the breaker to trip, shorted or not).

If those diodes are open, I do not approve.  My small diode bridge experiments failed as a short circuit every time when hit with a mains fault and that is the typical failure mode for over-temperature with silicon diodes. While too small diodes would literally vaporize, and those diodes are still there. If shorted, or magically still diodes, after a fault that's OK.   

JR

[Al Keltz]

Was that board exposed to a mains fault?

No, it just looks that way because the thing is encased in black epoxy and it took a lot of sawing and side cutter action to get down to the PCB. It was never actually plugged into anything.

I bought it because of tech calls from people hooking up two guitar amps with a switch pedal and getting hum. I wanted to see just what the mechanism was to open the ground but reconnect it in the event of a fault.

Their website says it's " . . . absolutely safe". I've seen tons of shorted power supply diodes and also can't recall power diodes going open but what if?

-Al

[John Roberts {JR}]

No, it just looks that way because the thing is encased in black epoxy and it took a lot of sawing and side cutter action to get down to the PCB. It was never actually plugged into anything.

I bought it because of tech calls from people hooking up two guitar amps with a switch pedal and getting hum. I wanted to see just what the mechanism was to open the ground but reconnect it in the event of a fault.

Their website says it's " . . . absolutely safe". I've seen tons of shorted power supply diodes and also can't recall power diodes going open but what if?

-Al

If it's potted on top of that, that will help provide some more thermal mass... Before I accept that it is "absolutely" safe, show me the UL mark.

FWIW when I was field testing the experimental "stinger cap GFCI" (0.15uF cap in place of safety ground), I had a friend who designs and sells guitar pedals check it out. He said the stinger cap ground, reduced hum/noise in his personal guitar rig (as quiet as lifted ground).

The stinger cap is sized large enough to trip the GFCI in case of a mains fault, while still passing below human safety threshold current for getting stuck to it.

Caveat this stinger approach is not UL approved either.   

JR

[Greg_Cameron]

Here's Bill Whitlock's take on the Hum-X:



Hi Greg,

It's good to hear from you. Most technicians are aware that disconnecting safety grounds will "solve" most serious ground-loop problems. Usually the voltage differences that drive the ground-loop currents are under a few hundred millivolts, so it's an appealing idea to insert diodes (paralleled in opposite directions because it's AC) in series with the safety ground connection. Because a silicon diode requires some 600 mV before significant current flows, this keeps the circuit open at low voltages. But the catch is that, under fault conditions when this path is critically important to avoid electrocution, these diodes must conduct extremely high currents until the circuit breaker on the branch circuit opens. In order to safely handle such high currents, each of the diodes would need to be the size of a hockey puck!

In my most recent seminars, I mention the HumX in particular when I talk about power-line safety. I've attached the slide used in my Sept 09 seminar for CEDIA. The question "Will it survive ..." is a rather rhetorical one because the tiny diodes inside (rated at around 5 A, judging by their size when we tore one of these units down), would certainly become a puff of silicon vapor should an actual fault occur ... and I certainly won't bet my life that they'll fail in a shorted condition!  Based on my years of experience as a repair technician, most rectifiers fail by opening like a fuse under serious overcurrent. IMHO, this unit is a poster child for marketing gone wild and commerce without conscience!  For the past several years, I've stopped at their booth and asked about the UL listing ... their answer is "We're working on that."  I'm a member of UL's advisory panel for professional audio equipment (UL 1419) and feel sure UL would never grant such an approval.

Beyond the safety issues, this "solution" ... just as disconnection of safety grounds ... doesn't solve the real problem anyway.

[John Roberts {JR}]

Here's Bill Whitlock's take on the Hum-X:



Hi Greg,

It's good to hear from you. Most technicians are aware that disconnecting safety grounds will "solve" most serious ground-loop problems. Usually the voltage differences that drive the ground-loop currents are under a few hundred millivolts, so it's an appealing idea to insert diodes (paralleled in opposite directions because it's AC) in series with the safety ground connection. Because a silicon diode requires some 600 mV before significant current flows, this keeps the circuit open at low voltages. But the catch is that, under fault conditions when this path is critically important to avoid electrocution, these diodes must conduct extremely high currents until the circuit breaker on the branch circuit opens. In order to safely handle such high currents, each of the diodes would need to be the size of a hockey puck!

In my most recent seminars, I mention the HumX in particular when I talk about power-line safety. I've attached the slide used in my Sept 09 seminar for CEDIA. The question "Will it survive ..." is a rather rhetorical one because the tiny diodes inside (rated at around 5 A, judging by their size when we tore one of these units down), would certainly become a puff of silicon vapor should an actual fault occur ... and I certainly won't bet my life that they'll fail in a shorted condition!  Based on my years of experience as a repair technician, most rectifiers fail by opening like a fuse under serious overcurrent. IMHO, this unit is a poster child for marketing gone wild and commerce without conscience!  For the past several years, I've stopped at their booth and asked about the UL listing ... their answer is "We're working on that."  I'm a member of UL's advisory panel for professional audio equipment (UL 1419) and feel sure UL would never grant such an approval.

Beyond the safety issues, this "solution" ... just as disconnection of safety grounds ... doesn't solve the real problem anyway.

Not to quibble with an industry icon but my experience is that properly sized diodes will short and survive long enough to take out a breaker.  I make no assertions for their diodes.

I used a 4 diode bridge (2x2 in series anti-parallel) that also had a metal heat sink package. The heat sink benefit was more for thermal mass since the transient event was too brief to move significant heat out of the package.

I had Peavey's agency guy approach UL about this back in the '80s and they were receptive to opening file (code for give me $10k). My sense was that they did some preliminary bench work that was promising, but I have no evidence of that. I abandoned it as unnecessary with proper I/O design, so why bother or throw money at it?

I believe it "could" work, but is not addressing the root problem (pin 1 or unbalanced grounds corrupting audio signals).

If you absolutely must lift a ground for emergency sound surgery, use one of those portable GFCI power drops to keep the meat puppets safe. 

JR

[Frank DeWitt]

Here's Bill Whitlock's take on the Hum-X:

Beyond the safety issues, this "solution" ... just as disconnection of safety grounds ... doesn't solve the real problem anyway.

I think this is the important part.  Ground loop.  Remove it in the sound path (lift pin 1) or use a good transformer.

I always carry a 1:1 transformer and a good Jensen equipped DI to solve ground loop problems.  Hasn't failed me yet.  Well there was a projector.  It needed a different transformer.

 

[ProSoundWeb Community]