Thanks Joseph for your answer, I'm preparing a thread with your posts, the beefiest part of the whole discussion and worth to save from oblivion. Have a good time, yours, Moritz

Joseph hi,

thanks for your quick response, but i think it's not your post I mentioned: here the full text with the strange place in bold: 

The reason why I chose the CSE187L current transformer is because it has a metal frame around the laminated core which is a convenient way to connect the ground of a two conductor shielded mic cable. Also connect the ground to one side of the thick wire string loop.

The string loop should be a very low resistance, usually measured in micro ohms to carry enough current to make a good output on the transformer output.

I chose the 500 turn current transformer because it produces an output closer to the range of a low impedance microphone, typically 150 ohms to about 300 ohms max. Most XLR mic inputs are about 2400 ohms actual input loading impedance to act as a bridging loan on the input source impedance of the mic or current transformer pickup.

When using the CSE187L if you bent the primary wires inward to short the primary and silver soldered this small loop together, you get an output moped an electrical on the current transformer of 80 ohms. All you need to do now is to look up the total resistance of the string loop wire. AWG 11 is 105 micro ohms per inch so a 6 inch string loop adds 630 micro ohms. Multiply 630 micro ohms by the turns ratio squared and get 157.5 ohms. Add this to the impedance of the transformer loop portion of the string loop and you get 237.5 ohms which is right near the upper limit to feed an XLR input. If you were to use a 1000 turn transformer you would have an output close to 1000 ohms and would be severely loaded down by the XLR input impedance.