Lehle High Impedance 1:1 Line isolation Audio Transformer.
Lehle LTHZ transformer is a central part of many of Lehle’s pedals. You can now buy it separately from Custom Boards Finland. This transformer is made specifically to handle source impedances of 10 kOhm like from an electric guitar.
The LTHZ transformer is probably the best you can find on the market today, reliable and solid, mumetal shielded. It can also treat stereo signals.