[CW] A note on headphones

[Ken Brown via CW]

If you do the math, you will see that the higher impedance phones will 
always get a larger percentage of the voltage produced by the generator 
than lower impedance. If you used a smaller series resistance the 
difference will not be as great, yet the higher impedance ones will 
still get the highest voltage across them. What kind of source are you 
using for your tests? An audio signal generator with a 600 ohm source 
impedance? Or something else? Have you tried different sized series 
resistors, including none at all?

DE N6KB