In general, headphones that have a Z of more than 100 ohms will likely be
ineffective on today's solid state radios. The military "cans" which were
designed to operate from a 500 ohm source, provided by those vintage
receivers, have even more of a problem getting the level high enough with
today's radios They become voltage driven devices and with typically 13.8 V
being available, thus the monolithic devices will only swing about 7.5 volts
or so with very little current. Lower Z phones in the range of 10 to 30
ohms seem to do a really nice job and won't cause "speaker blowout" when
unplugged prior to turning down the volume.
To the other end of the spectrum, phones that have very low Z values are
thus getting into the current limit range by internal series R values placed
in the output to protect from accidental shorts to the output stage.
73
Bob, K4TAX
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