Reg, G4FGQ wrote:
"I would like to know, crudely, the audio input level to 2000-ohm, non
diaphragm head phones for a nice, comfortable, not too difficult
intensity level."

First problem is sensitivity of the phones. These vary widely.

Satisfactory reception also depends on competing noise in addition to
signal strength. According to the FCC in the U.S.A., you may need 1/2
millivolt for satisfactory reception.

A kilowatt radiated by a medium wave transmitter may produce about 200
millivolts/m at one mile. It depends on antenna and path. Field strength
is proportional to the square root of the watts per square meter times
377.

I used to live about 15 mile4s from a 50 KW station. The signal strength
at a mile from the station could have been 1.5 volts/ m, and at 15 miles
could have been 0.1 V/m. Point is, my crystal set drove a dynamic
loudspeaker directly through the output transformer mounted on the
speaker. It was clearly audible. Impedance presented to the crystal set
was closer to 10,000 ohms at 1 KHz than to 2000 ohms. The speaker and
its cabinet were from a battery vacuum-tube set. The audio power may
have been less than a microwatt, but as a kid my ears were acute.

My 2nd edition GE "Transistor Manual" has 2000-ohm headphone amplifiers
with 2 milliwatts maximum power output.

Chinese stereo headphones claim 20-20KHz response, 32-ohms impedance,
100dB/1mW sensitivity, 140 mW rated input, 400 mW max. etc. Much of this
spec. is target and untrue. Wide variations exist between samples of
these phones claiming similar specs.

The GE book is believable. Class A power output is from the familiar.
First formula says maximum power is 1/2 the product of the peak voltage
and peak current. Load resistance is then the peak voltage divided by
the peak current. So, Load resistance=Epk squared/2Po

When GE prints the circuit of a 2 milliwatt amplifier driving 2000-ohm
phones, I believe it works.

Best regards, Richard Harrison, KB5WZI