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9th November 2004, 02:30 PM  #1 
diyAudio Member
Join Date: Oct 2004
Location: Israel

Mic input transformer
Hi guys..
have anyone used the LL1538 mic input transformer from Lundahl?? The transformer is 1+1:5 and i wonder which configuration is best for 200600 ohm mic  the primary in parallel or in series... i guess that each primary has an impedance of 400ohm so parallel will give 200 and series will give 800? am i correct? and what is better?? thx here is the link.... 
9th November 2004, 02:32 PM  #2 
diyAudio Member
Join Date: Oct 2004
Location: Israel

Sorry... here's the link....
http://www.lundahl.se/pdfs/datash/1538_8xl.pdf 
14th November 2004, 08:40 AM  #3 
diyAudio Member
Join Date: Nov 2004
Location: Minneapolis, MN and Wiesbaden, Germany

Transformers don't have an actual impedance of their own. They only reflect impedances and multiply them by the square of the turns ratio. If you put the two primaries in parallel, the turns ratio is still 1:5, same as if you only used one primary. If you put them in series, then you get 2:5. This means you can have an impedance ratio of either 1:25 or 4:25, which is about 1:6.25. If you assume a microphone source impedance of 150 ohms, then the amplifier will see a source impedance of either 3750 or 937.5 ohms. Likewise, to provide the microphone with an appropriate nominal load, which is typically about 1500 ohms, then you'd have to load the transformer secondary with 37500 or 9375 ohms, respectively.

14th November 2004, 04:01 PM  #4 
diyAudio Member
Join Date: Oct 2004
Location: Israel

thanks ulysses,
Now I understand thinks better..... 
15th November 2004, 02:02 AM  #5  
diyAudio Member
Join Date: Jun 2004
Location: Sydney

Re: Mic input transformer
Quote:
Better is the LL7902 or 7903, better still is trannies from Cimag and the best is probably Jensen. They make a VHQ 1:5 ratio but you will pay for the quality. There's also one from Stevens & Billington that Kuei W Y has had good experience with. Let me know if you need further info. Cheers, Terry 

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