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Old 26th July 2002, 12:49 PM   #1
SteveG is offline SteveG  United States
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Default Shielding a transformer

What is the best way to go about shielding an EI core type transformer, so that it dosen't induce noise into an amplifier?
Also, I aquired some cheap transformers that have a piece of copper wrapped around the windings, and what looks like tin wrapped around the laminated part. Is this effective at all? It was so cheaply done that I wonder if it will buzz when the transformer is under load. I am thinking of cutting it off (wouldn't be difficult) and making a more solid shield either on the transformer itself, or in the enclosure.
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Old 26th July 2002, 01:20 PM   #2
tiroth is offline tiroth  United States
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A previous thread on this topic stated that those copper foil shields actually were quite effective.

Then there is always the mu metal planet10 is selling.
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Old 26th July 2002, 02:18 PM   #3
SteveG is offline SteveG  United States
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which thread are you refering to? I looked at a lot of different threads and never found anything about those copper foil shields. BTW, what is more effective for 60hz shielding, steel or copper? I would assume steel.
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Old 26th July 2002, 02:22 PM   #4
SteveG is offline SteveG  United States
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Forgot to ask... where is the magnetic field radiated the most from a standard ei core trans?
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Old 26th July 2002, 02:53 PM   #5
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The noise that a transformer will induce in an amplifier is usually due to stray magnetic fields leaking off the core/windings.
A copper shield will act as an electrostatic shield- it will prevent voltages from being capacitively coupled, but will do nothing to stop the magnetic field.

The problem is solved by using a toroidal transformer which has much lower flux leakage than an EI core transformer and/or shielding the transformer with mu metal.

Usually just using a toroid and paying careful attention to lead dress and component placement is sufficient to kill hum induced by flux leakage. Twist all power leads (DC and AC) tightly and use a star ground. A final tweak when installing the transformer is to try rotating it until one finds position with the lowest induced hum, then tighten the mounting bolt (assuming you've left leads long enough to allow rotation).

Of course, there are some sensitive types who can hear the color of the insulation on their cables. They will require either a couple layers of magnetic shielding and/or putting the power supply in a separate box physically removed from the amp. This will provide the side benefit of yet another set of cables to obsess over...

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Old 26th July 2002, 04:21 PM   #6
JohnG is offline JohnG  United States
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Just a small correction, the copper strap present on many EI core transformers does reduce the effective magnetic flux leakage from the transformer.

The copper strap presents a shorted turn to the leakage flux, and an opposing magnetic field is generated by the current induced in the copper strap. This opposing field reduces the effective magnetic flux seen away from the transformer.

Note that the copper strap is entirely outside the EI core, so that it only cancels the leakage flux, which is small compared to the magnetizing flux within the core and winding structure (at least in a properly designed transformer). If the copper strap were within the core where the windings are, it would short the transformer.

Electrostatic shields are usually placed between transformer windings. They are constructed such that they do not form a shorted turn.


P.S. One more thing: If you are mounting a toroidal transformer, and you think about shielding it in a metal can, BE CAREFUL! If you form a conductive path that goes through the center of the toroid and completes the path on the outside, this will also form a shorted turn. This is a good reason to make sure the mounting bolt of the transformer never touchs the top of the chassis, or sparks WILL fly!
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