I don't believe cables make a difference, any input?

[cbdb]

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] "

[simon7000]

Originally Posted by doug20

Could you explain this a little more. When Im doing measurements with my notebook I have to unplug it from the wall because of serious interferance. Its been recommended I buy a Jensen isolation transformer (or something like that). Im not happy about spending > $100 to fix noise.

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.

[event horizon]

Originally Posted by Steve Eddy

No, they sense the amount of current flowing in hot and neutral and if they're not equal, they'll trip.

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

[simon7000]

Originally Posted by gedlee

A ground fault interuptor does not issolate the ground. I don't see how that would do anything. Yes, issolation of the audio lines is one solution, but, as mentioned not the ideal. I don't see how you can issolate the power ground and still have it as "ground". I understand issolating the signal grounds.

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.

[gedlee]

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.

[Steve Eddy]

Originally Posted by event horizon

Fair enough The thing is though you must have some kind of fault condition to make this happen.

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

[gedlee]

Originally Posted by Steve Eddy

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"?

[simon7000]

Originally Posted by gedlee

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.

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.

[Steve Eddy]

Originally Posted by simon7000

The two prong on consumer gear is allowed because very little is "Listed" (I.E. UL approved)...

********.

Every piece of consumer gear I have here with two prongs and that I'm conveniently able to access is UL listed.

se

[DDF]

Originally Posted by doug20

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.