Things I have learned about THD+N measurements:

1. use a logarithmic scale
2. let the preamplifier warm up again between measurements

This second point is very important as I was informed by the Pass Labs engineers that I will see a 10-fold increase in THD+N if I measure immediately after the settings have been changed.


The above plot of MC THD+N demonstrates this 10x effect (XONO factory traces are not shown as they are not public images).


Here is the MM THD+N vs. Amplitude. Again, we are seeing a 10-fold increase in distortion+noise. A logarithmic scale would draw out the subtlety of the effect given lower signal.

I intend to retake these measurements when I use the AP again.