You are welcome.
For output impedance measurement you need
- a multimeter that can measure AC voltage
- a resistor (I use a 47Ohms resistor, we note its value as RL)
- a cable that you plug into the headphone-out of the amp you want to measure. I use a cable with one 3.5mm jack on each end.
- some more cables, preferably with clamps
First, give some sine signal on the amp. I use a 100Hz sine. Use your usual listening volume, in doubt a bit more, but stay away from clipping. I went for around 0.5V.
Measure the output voltage with the multimeter between right/left channel out and ground (i.e. for a 3.5mm jack: measure AC voltage between the first and third ring).
Make sure your multimeter correctly measures the AC voltage: change the frequency of the sine signal, the voltage measure should stay constant if you don't change the volume.
Write down the voltage you read with the multimeter - that's U1.
Now connect the resistor in parallel to the multimeter. In my example I would connect one end of the resistor to the first ring and the other end to the third ring. Cables with clamps come handy here. Keep the multimeter connected the way it was. Write down the voltage you read with the multimeter now - that's U2.
The formula for the calculation of the output impedance Ri is
Ri = RL * ( (U1 / U2) - 1)
or use this web page (in german, but it is easy to use)
http://www.sengpielaudio.com/Rechner-EingangsAusgangsWiderstand.htm.
For my Sonata, I had these values:
RL = 47Ohm
U1 = 0.494V
U2 = 0.477V
and I got Ri = 1.78Ohm.