So I have been planning to build the GIC microhpone for quite a while (here is the link to the page about this microphone
http://www.gyraf.dk/gy_pd/g7/gic.htm ) and I already have the capsule and so on... Where I got a bit stuck is the transformer. Sadly I am still just a student and not a milionaire so I cant afford really any transformer that would be quality certified for this job (and before you tell me "you already bought the expensive capsule just pay up the transformer" well no i bought the cheapest possible double sided capsule on ebay which was 30 bucks and thats already expensive for me).
(before you dive deep into this I just want to warn you I am not native and my grammar will probably not be on the spot. Its also 3:40 AM at the time of writing this)
Since buy isnt an option here I went with the "well what can go wrong just wind one" option.
I am not an expert on this so dont burn me please. So I pretty much have the impedances and turns ratio with that.. It would be 15K to 600ohm (and no DC current would ever pass trought the transformer itself). For me to figure out how many turns I need on a given core with a given permeability I need to know what kind of inductivity I need to pass audio. I figured since I already have bought some 1:1 600 ohm transformers from china I should be able to just measure them and go with that value. It turned out about 1.2H (which sound alright to me). I have done some checking arround and I also found other microphone transformers that have the same 600 ohm output impedance which are about 1.8H on average.
I assumed it would be the right inductance to go with.
I was checking arround on TME and I found a pretty decent ferrite core (AL 8700nH or rellative permability 2700) and with this one I would need arround 130 turns of 0.1mm wire to achieve arround 1.8H (I just used a online calculator for this and I know it does not take layer thickness into account and neither the bobbin itself). Since I know the ratio should be 25:1 then i should just multiply 130 by 25 and bam I got my secondary turns ratio which is 3250 turns. At this point I think to myself great math checks out and I have now a number to go with. However I just had a slight thought of how about the primary tube side inductance. I know it will be a whole lot larger but I did not expect it to turn out to be 1200H. I am almost certain I messed up somewhere in my maths and 1200H just doesnt right. Then again I never ran across such a special purpose transformer in my life and didnt measure it so I dont really know.
Best I could figure is to check other tube transformers and so I did check my power transformer for a EL34 push pull amp(I know its wildly not the same as what I am trying to create). It has a 6k6 ohm winding and it turned out to be arround 590H so I regained some comfidence. I dont want to go ahead yet on this because I am a bit worried about this very high inductance and I wouldn want to wind and unwind 3250 turns of wire (nope to happening). I just want confirmation on this if I am doing this right, if the 1200H sound right for you at all. I have never come across anything with that high inductance ever before let alone in a transformer.
All I want to know is if the inductance of the primary 15K ohm winding (which is 1200H) is it sounds right to YOU. I will appreachiate any advice and opinions about what could be done if its right or if not.
Technical parameters I look forward to achieve :
Primary impedance : 15K
Secondary impedance: 600ohms
Bandwidth 20Hz-20kHz
Power: just signal
My results: (RM core AL=8700nH/relative permability 2700, not gapped, code: B65815E N30, link to datasheet:
https://www.tme.eu/Document/4a01e423b117af0ac949e52794f2a2d2/rm_12.pdf )
Primary:
Impedance: 15K
Turns: 3250
Wire: 0.1mm
Inductance (estimated without accounting for layers and bobbin): arround 1200H(!)
Secondary:
Impedance: 600ohms
Turns: 130
Wire: 0.1mm
Inductance (estimated): 1.5H-2H
To anyone who is wondering why I did not buy the transformer that costs 60 dollars. I am sorry I am a student and with limited finances I will gladly go down the route of more effort, besides its gonna be a great adventure to turn that wire round 3250 times... and educational aswell

.
(also if someone could link me a PDF that would contain all the required math for this I would be gratefull. I have only used online calculator tools which are good only so far.)
Adam.