Terrapin Audio output transformer for Tube or FET Condenser Microphone 6.5:1 Cobalt Amorphous core Perfect for Neumann U47 replacement of BV8 or for DIY model recreation built 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: 200Ω with ~10k primary load (VF14 / EF13 tubes) Output impedance depends on the Ri and Rp of the tube type used, so it may vary depending on the tube and application in the range of ~100-300Ω, That is fine for all microphone preamps,     VF14 - EF14 / EF13 / EF42 / EF732 / EF80 / EF800 / EF802 / EF860 / EF95 / 6J1P / 5840 - and many other tubes would match output impedance nicely, For some tubes with low Ri - Lower Ratio transformer can be used - like 5:1 for LOMO 19A19 etc., For tubes with higher Ri - higher step down ratio required - like for C12 \ M251 - 12:1 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.