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A Question for Kevin. ????

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Kevin, While surfing I came upon these instructions for Balanced Power Transformers.


I was wondering what your thoughts were on his directions? I also am curious about his comments about Toroidal vs I-E and how I shouldn?t have used a Toroidal.

His Comments

Some might wish to use toroidal transformers instead of the recommended E-I core style transformers. DON'T! Toroidal transformers tightly couple the AC line to the secondary in a wideband manner, This means that more noise, not less, will get through to the secondary windings. Also, toroids tend to saturate much more abruptly than E-I core transformers, and need to be over rated by a much larger factor to avoid compressing the dynamic range of a power amp or other electronics.



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There are a number of ways of making a toroidal transformer, one way is you wind the primary all the way around the thing, then

afterwards add the secondary also around the entire core. This definitely adds capacitance between the primary and the

secondary. The otherway is to wind the primary on one part of the core (say about 40%) and then do the same thing

for the secondary. Way less capacitance and basically equivalent to an EI core except the magnetics are more efficient. The

torroids i use are split primary and secondary this way. Not sure about the others.

As far as balanced vs fully isolated, i use both, but in any case they are wired as fully isolated. So when there is a center

tap on the transformer i don't use it. Instead i use a .75 inch diameter solid copper ground rod 10 feet long and pounded

into the earth. Then a significantly large copper strap runs inside the house where it is attached to the ground plugs of

the power strips that distribute the power.

The idea is to get rid of all conducted and radiated noise on the neutral leg.

Starting with 220 which is already balanced definitely helps.

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When you're adding snubbers, I find that if you use a split-bobbin transformer, the interwinding capacitance is so low you can actually leave it out of the calculation (I'm measuring on an LCR meter).

I'm refering to Hagerman's PSU snubber calculations: http://www.hagtech.com/pdf/snubber.pdf

I can see the ringing on a scope even with soft-recovery diodes (rectifiers generate oscillation in the LC circuit of the transformer parasitics at turnoff). Any ringing of course is elminated by the filters after rectification, but since it's high frequency, it's radio and capacitively coupled to the rest of the circuit, and though it's not in the audio band, it's modulated by the mains AC swing.

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