I ended up buying a portable oscilloscope like the one Dan suggested and I'm now using it to measure a completely different product (Matrix Mini-i Pro 3) that has published output impedance of <11 ohm (single-ended). I must be doing something wrong though because I keep calculating a Zout of about 1 ohm.

Can anyone help me work out my error? Here are my steps:

- Connect oscilloscope to one channel of the headphone output
- Play 1kHz tone through device
- Record RMS voltage from oscilloscope
- Apply known load (6.8 ohm is what I had at hand)
- Record new RMS voltage
- Calculate Zout as (V-VL)/(VL/R)

What am I doing wrong? Is the resistor too low? Should it not be the RMS figures for the calculation?

Here's a photo of the oscilloscope readout in case I'm doing something totally wrong. (By the way, it's set to AC mode, but is showing DC on the screen when connected - not sure what that's about as it shows AC mode correctly when disconnected)