Mic Preamp w transformer input: changing impedance

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Hi all,
this is my first post here. I'm quite new to mic preamp design...I'd like to design a mic preamp with transformer input (probably JT-115K-E) and the first thing I have to understand is how I can change the input impedance. Looking at this schematic:

http://www.jensen-transformers.com/as/as016.pdf

how can I determine the input impedance? How can I say 150Ohm when switch S2 is "up"? The impedance that the mic will see is related also to the specific transformer I'll use and (reflecting the impedance from secondary to primary) to the following stage. I'm I right? I'd like to try 100Ohm (to match my AT4040), 500Ohm and 1 or 2K...

Thanks for your time, I know that this is probably a very stupid question, but I want to try to understand things more than copying and trying...

Regards

Stefano
 
This circuit does not lower the input impedance to "adapt" to a mic with a very low impedance, but try to keep the mic impedance to a "standard" level (about 150Ohm) if you connect a mic with very low impedance. If I'm not wrong, I'd have to change the N2/N1 parameter (increasing it) to lower the impedance seen by the mic, leaving everything else the same...but for this I'd need another (different) secondary winding. I'm I right?
Thanks
Stefano
 
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