Terrapin Audio output transformer for Tube or FET Condenser Microphone 12:1 Cobalt Amorphous core Perfect for AKG C12 \ Telefunken Ela m251 replacement of T14/1 or for DIY model recreation built Neumann U47 FET, U87 Replace Bv11 Toroid Cobalt Amorphous Core Extra low Hysteresis loss, excellent inductance per turn over wide frequency range and low DCR makes it best core material for low level signals with exceptional transient response, Frequency response: 20Hz-20kHz +0\-0.3db with Minimal Phase Shift and THD Primary inductance: 350H Dimensions: ~23mm diameter with 11mm height Output impedance: 150Ω with ~20k primary load (6072 \ 12AY7 tubes) Output impedance depends on the Ri and Rp of the tube type used, so it may vary in the range of ~100-300Ω depending in the tube, That is fine for all microphone preamps,  ECC81, EC92, 6072, AC701, 12AT7, 12AY7 - and many other tubes would match output impedance nicely, For some tubes with low Ri - Lower Ratio transformer can be used - like 6.5:1 for U47 \ 5:1 for LOMO 19A19 etc., Feel free to contact me regarding compatibility with other tubes or with any tech questions about tube mic build, Other ratios and transformer types are available - including microphone preamp input \ output and Ribbon mic output, moving coil MC transformer, Power amp SE \ PP outputs etc.