http://www.bonisaudio.com/soft_imped.html (http://www.bonisaudio.com/soft_imped.html)
{ OUT IMPEDANCE CALC • V 1.00 • Output Impedance Calculator}
This tool calculate the output impedance with a fixed load resistor, the resistor (RLoad) is used for measuring the output voltage first with full load (V1), then without the load resistor (V2).