Bob from Florida
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Take a look at my math for your example of 50 ohm headphones.After thinking about it I think he just put a meter on it and measured in //.
To measure actually output Z you need to drive the amp over known loads and measure V and I, that will give you an equivalent R, and back calculate output Z.
Maybe do the same with a short.
The actual is the amp output Z (mine is ~0.03 Ohm) in // with the 800 Ohm.
Which is ~0.03 Ohm, same as for the speakers.
If in series and headphones are 50 Ohm at full out 31 V, I = 36 mA, so P = 0.26 uW
Will drive a headphone?
800 ohm example
50/(800+50) X 31=1.82 volts RMS across 50 ohms at max volume. Power = (1.82 x1.82)/50=0.067 watts or 67 milliwatts
80 ohm example
50/(80+50) X 31=11.92 volts RMS across 50 ohms at max volume. Power = (11.92x11.92)/50=2.84 watts
Double check me