I'm helping a friend with this circuit:



It uses dual 6418s operating in single-ended as a "stereo buffer", not all that sure what it means. Anyway, he wants to use it as a headphone amp, but gets a bad impedance mismatch when hooked up to low-ohms headphones...obviously needs a matching transformer.

How do I figure out the characteristic impedance of the circuit, so I know what impedance ratio transformers to tell him to get?

Ignore the hand-drawn notation, we were adding a battery power option.

That circuit is not meant to be anything but a high-impedence preamp, obviously to feed another amplifier.
Tell your friend to invest in something else for a headphone preamp.

Yeah, nowhere near the power to drive headphones. Bump up B+ and put some matching transformers and you'll be alright. As for the values of that transformer... I'd plot that out on the datasheet to find the best value.

To drive headphones one could use that tube as a preamp then use a 12AU7 as a cathode follower to drive the headphones.

I once had a device that fit in a 5 1/4" drive bay in my computer that had a 12AU7 tube in it which could drive headphones or function as a line output, but it did have a slight drop in gain though.