[Eeyore]

Good Music wrote:

Question: Impedance matching:

How can I make an adapter to allow connection of old 8 ohm headphones to a
modern personal audio source (MP3 player, etc) expecting the newer kind of
30-40 ohms or so (?) headphones?

Via google, I saw mention of using audio transformers for other impedance
matching applications: Are there such audio transformers that I could use
one for each headphone channel?

Or if that's not sufficient, where might I find plans for whatever kind of
battery-powered IC circuit I may need to build to perform this function?

(Replacing the original 8-ohm drivers in the headphones is not an option for
this application.)

Don't even bother. Many headphones claiming to be 8 ohms aren't anyway !

Just connect it and see if it sounds ok. You won't hurt anything and in any case
a new pair of headphones will be the cheapest and best option if you're not
happy with the results anyway.

Graham