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Old 3rd February 2004, 02:34 PM   #11
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Yes the main fields (if not the only fields) are magnetic.

But noise on the primary gets coupled capacitively. (And I suppose are to small to be coupled via electromagnetism)

Just like a capacitor passes high frequencies...So does the transformer....

the electrostatic shield is like adding a shield in between the two windings that cut down on that capacitive coupled noise.

Similar to the electrostatic screen on your monitor.

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Old 3rd February 2004, 02:43 PM   #12
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Just like a capacitor passes high frequencies...So does the transformer....
At the voltage involved, the capacitive current can be quite large, despite the small coupling capacitance in question.
That's why there are sometimes iprovements depending on which way you plug your equipment into the mains sockets.

I once had to learn this the hard way ........

It is a pity that x-formers with a shield between primary and secondary are not that common anymore nowadays.


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Old 3rd February 2004, 02:59 PM   #13
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If obtaining ES screened transformers is a problem make an isolation transformer by connecting two identical transformers secondary to secondary with a copper screen or sufficient space between them.
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Old 3rd February 2004, 03:07 PM   #14
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As you say, it seems rather difficult to get transformers with
electrostatic shields (I have two, but they were industrial surplus).
However, as I understand, an R-core transformer should also have
a very low capacitive coupling. Although a bit expensive, they seem
to be available nowadays. It has been discussed on the forum
before and I think there is at least one french and one german
company selling R-cores. Can't remember where right now, though.
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Old 3rd February 2004, 07:38 PM   #15
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If you can't get a transformer with an ES screen between the primary and secondary it isn't a major problem. Bypass the RF noise to Earth via some very small capacitors. Without checking, I think I remember seeing something around 10nF being used for this, with one from each end of the secondary, effectively making it 20nF to the noise.

Morgan Jones does this in one of this supplies. He also goes a step futher and adds a pair of low value induction coils. I think he only uses this method for the LV supply however. At the front of his supply, before the transformers, he uses Class ?X2?, mains safe, caps and another pair of low value coils to futher remove RF noise.

This method would probably cost less than $10 in the US.

Morgan Jones mentions that the more serious problems are found when there is no ESing between the secondary coils themselves. If this is the cases, common mode noise from the HV supply can be coupled and superimposed on the differential output of the other supplies, such as the bias and heaters.

Best wishes,

edit - just got the book, here's the method he suggests. if you don't have his book, buy a copy!

.......................15T 1.6mm on ferite
Mains L--------- ~~~~~------------ To mains trans
.............class x2.........class x2
............470n cap.......470n cap
Mains N----------~~~~~------------ To mains trans
........................15T 1.6mm on ferite

Ignore the dots. The Caps MUST be Class X2 for safety.

The LV filter is the same but remove the caps and wind the coils as 25T of 1mm on a 30mm core each. Attach a 10n (FKP2) cap to the + and - on the right of the coils and tie the end of these caps to Earth.
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Old 26th March 2013, 12:33 AM   #16
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here is your answer

read the whole doc .................

Earthing (Grounding) Your Hi-Fi - Tricks and Techniques
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Old 27th March 2013, 09:37 PM   #17
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Unfortunately the answer came 9 years late! But the paper is still well worth reading.
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