I am trying to measure the output impedance of a commercial headphone amplifier, but the results seem to change depending on how I probe it. When I plug a 1m cable into the device and the other end into an adapter to a terminal block on a breadboard, I get 5.5Ω, when I use a shorter cable I get 4.4Ω. The results also change depending on where I'm measuring the voltage on the breadboard.
This is what the breadboard looks like:
I'm connecting the terminal block to the amplifier, playing a 1kHz sine wave and measuring the voltage with and without that resistor across (I put the resistor into a terminal block because I have to unplug it many times), then calculating the impedance as
Z=R*((V1/V2)-1). My measurements seem to be pretty good, I even did all the uncertainty propagation maths and it ends up being around ±0.6Ω, after doing 30 measurements at different voltages (also tried a few with different resistances and frequencies just to see what's up). The only problem is that I got a different, but equally accurate value when repeating everything the next day (including building a new setup with slightly different cables and connectors), and it's outside of the previous day's measurement's error bars.....
Am I not accounting for some other source of impedance? The cable, or the connectors maybe? What am I missing, and what's the proper probing technique for a measurement like this?
I have now opened it up and I'm measuring directly on the PCB:
The result is closer to my second attempt (seems to be a bit lower even, but I only did a couple of measurements). I still don't have a lot of confidence that I'm measuring the impedance correctly, but at least now I understand that I have underestimated the effect of the series resistance on the voltage under load.