Earl, I dont think so. GFIs are to make 2 prong plugs as safe as 3 prongs . Other wise a 2 prong plug woudnt work in a GFI. ( like my razor in the bathroom, up here there standard recepticles in bathrooms)
More from bills paper:
". If a proper safety ground isn’t available, always use a ground-fault circuit interrupter or GFCI. A GFCI works by sensing the difference in current between the line and neutral conductors. This difference represents current in the hot conductor that is not returning in the neutral - the assumption is that the missing current is flowing through a person. If the difference reaches about 5 mA, an internal circuit breaker is tripped. The GFCI shown at left is unusual because it has a retractable ground pin that allows it to be used with a 2-prong outlet. [5] "
More from bills paper:
". If a proper safety ground isn’t available, always use a ground-fault circuit interrupter or GFCI. A GFCI works by sensing the difference in current between the line and neutral conductors. This difference represents current in the hot conductor that is not returning in the neutral - the assumption is that the missing current is flowing through a person. If the difference reaches about 5 mA, an internal circuit breaker is tripped. The GFCI shown at left is unusual because it has a retractable ground pin that allows it to be used with a 2-prong outlet. [5] "
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The switching wall wart power supply makes noise. A transformer can balance, isolate and provide some bandpass filtering improving your measurements. If you want to try one a cheap mouser unit such as 42TU016-RC will do 600 ohm to 600 ohm.
I think you will find unplugging the power supply is best.
Fair enough
The thing is though you must have some kind of fault condition to make this happen.Why can't you lot over there have 3 pin plugs & 240V, it'd make life so much easier
Earl,
The theory is that the safety ground and the neutral are only connected at the building's entrance for the power service. Even when this is done correctly there are currents induced into the safety ground from capacitive coupling.
The common approach is to try and get all of the safety ground terminals at the same voltage. This is done by using in effect a single outlet. There is still a potential above earth reference with this method.
The balanced power approach is to use a 120 to 120 center tapped transformer and connecting the center tap to safety ground. This has the advantage of the capacitance coupled noise is greatly reduced.
Just using an AC isolation transformer provides improvement.
By using a common ground method I see 60 hertz noise and the harmonics around -120dbm in my bench top tests. I can go another 40 or more when I start to use isolated AC power and battery pre-amps.
The basic circuit in a GFI is a differential transformer on the hot and neutral. It works because in the US the neutral is earth grounded and any leakage has a place to go. In the UK the power lines are not grounded and such a circuit would not be the best bet, so they look for any current where it does not belong.
When you use a safety ground adapter to remove the third wire and things clean up it means that current was flowing from that safety ground terminal through your shields to another safety ground terminal. This of course can only happen when they are at different potentials. So another option is to open a shield at one end. Of course if you do lift a ground and a fault occurs it will probably sink to earth through the shields on your test cables. What it damages is more interesting.
Yes, I understand all that about single point grounds - not the GFI sense technique however. So the GFI would not care about no safety ground - makes sense. Hence, if I do lift the ground on the mic preamp and use a GFI then there is no hum and I am still safe. Correct?
I bought some Chinese plate amps and everyone of them hummed if grounded, even if everything was on a single plug. Never did quite figure that one out.
I bought some Chinese plate amps and everyone of them hummed if grounded, even if everything was on a single plug. Never did quite figure that one out.
Fair enough
Of course.
Why can't you lot over there have 3 pin plugs & 240V, it'd make life so much easier
Hehehe. I'd be perfectly happy with 2 pin plugs and 120V.
That's what gets me. All these high end audio manufacturers think themselves so vastly superior to cheap mass-market stuff, but they apparently still haven't figured out how to design a chassis to meet Class II specs and rid themselves of the safety ground that causes no end of noise problems.
se
You know, thats very true!! My latest receiver only has two prongs, no safty ground. Al computers have the safety ground. Is this just "easy"?
Yes, I assumed you knew the basics, but restating them reduces questions from other readers, as to lifting the safety ground on a mic preamp you are correct, no hum and the interconnect shield will still provide an unapproved safety ground path. Just don't plug it into a 30 amp circuit.
The two prong on consumer gear is allowed because very little is "Listed" (I.E. UL approved) Computers have 2 issues, getting past the FCC and being used in multiple occupancy spaces are only allowed by insurance inspectors if "Listed".
As to the happily humming amplifiers, what can be happening is that for RF reasons they connect small caps from each side of the AC line to the case. Since the values are never exactly the same it mandates you will get current traveling on the case or shield and thus the hum. Even without caps there is leakage from the power transformer windings to the case, both magnetic and "electrostatic."
Bill Whitlock has a number of papers on how to handle these problems and the solution is always use a transformer! Being creative also works.
I do have a B&K preamp that always had a hum issue, finally got around to looking inside the case, someone had repaired it just plain wrong. Changed a resistor value replaced the wrong filter caps and now it works great.
********.
Every piece of consumer gear I have here with two prongs and that I'm conveniently able to access is UL listed.
se
DBTs are not peer reviewed
Call up Harman International maybe someone like Sean Olive can help explain this better to you.
I used to be professionally intimately involved with audio DBTs for Telecom. their results would be use to set ITU-T standards, IEEE standards, or to pick the codecs you use on your cell phones or in VoIP.
Every DBT for these purposes was peer reviewed. Even Olive's and Tooles papers were reviewed by acceptance boards at the AES, gedlee may even have sat on of these.
Unapproved because its not safe. A lot of those foil shields will melt like a fuse way before your breaker trips, and if your signals ballanced your shields probably only connected at one end. (read Bills handout, one of its main concerns is saftey)
http://www.jensen-transformers.com/an/generic seminar.pdf
I do not understand why no one has addressed phase coherency distortion between the upper and lower tones due to various cable designs.
After the things actually connect stuff together, this issue seems to be one of the biggest buggers with varying cabling types, to my ears. Anyone who has ever sat in his back yard, surrounded by crickets should be able to identify with this problem in reproduction and you don't really need a cricket recording, either.
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What kind of speakers do you listen too?? Do you think speakers have better phase coherency than cables??
Just how bad can you screw up a piece of wire so there is an audible difference in phase coherence?? Is it even possible??
Here is a measurement or a 3ft interconnect. It's a 20 year old Diskwasher Goldens cable. Looking at the Red curve there is about 1/2 degree of phase shift at 22K. The Green is a sweep sine frequency response.
Rob
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What kind of speakers do you listen too?? Do you think speakers have better phase coherency than cables??
Just how bad can you screw up a piece of wire so there is an audible difference in phase coherence?? Is it even possible??
Here is a measurement or a 3ft interconnect. It's a 20 year old Diskwasher Goldens cable. Looking at the Red curve there is about 1/2 degree of phase shift at 22K. The Green is a sweep sine frequency response.
Rob
Of course speakers are a nightmare, but you do get used to hearing what you have. I would doubt seriously that anyone can hear 1/2 a degree difference over six or eight octaves. Attenuation of one end of the spectrum or the other seems more likely in a badly screwed up piece of wire.
Thanks for posting the graph.
Steve,
There are different classes of listing. Musical equipment used to be allowed to have a Hot chassis, didn't do well in Baptist Churches.
Also parts may have listing stickers, but that is OK only as long as there is no assembly that does the same thing that has been listed.
Finally there are an amazing amount of fake stickers. I had somebody buy a scad of nice thick power cords with stickers, bad news all plastic the conductors were 18Ga!
That is why if you have to do this for test purposes use a GFI. Also as mentioned do not use a high current circuit as the trip current required may be higher. A GFI can trip at a few ma.
There is a big difference between what you do in a testing lab and what is good for the general public.
I used to have the picture of the guy who made a floating extension cord so he could watch a TV on his lap while he floated in his pool!
Not intentionally it wasn't. And the key phrase here is "used to."
Every one of the components I have here have the listing sticker on the chassis itself, not on individual parts inside.
Well sure. There's tons of stuff coming from China these days that's fake in more ways than UL stickers.
But I can assure you that the UL sticker on my 1976 Pioneer integrated amplifier is not fake. Nor were they fake on millions of other pieces of similar equipment both at that time and today.
Bottom line, your claim that two prong consumer gear is allowed because very little of it is listed is simply ********.
se
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