[Elecraft] Headphone jack impedance

[kevinr]

I am just starting my search for a good set of ear buds to replace those 
I'm using.  Is there a standard headphone jack output impedance for the 
family of K2, K1, KX1, K3, KX3, and KX2 transceivers?  I plan to buy a 
set of head phones or ear buds and want them to be the best match.  I 
know I could find all this data by opening and doing a ctrl-f on all the 
manuals but I'm lazy.  Or is it more efficient?

    Kevin.  KD5ONS