A quick follow-up to my previous post.
Since I had an assembled 650 Hz CW filter kit handy, I decided to use the QA400 with a 650 Hz tone to make a simple THD/THD+N measurement.
Below is an image capture of the baseline filter board powered-on with no signal input. As you can see, the Bandpass characteristic of the filter will allow noise within its passband to exit the output stage. Beyond the passband, the baseline noise floor sits about -142 dBFS, i.e. where the QA400 nominal noise floor level sits. The center frequency of the filter passband shows up in this image at 662 Hz, which is what I previously had measured by other methods, so this cross-checks that result very nicely.
Now I add a 650.39 Hz tone (as close to 650 Hz as the QA400 software allowed me) @ a level of -18 dBFS to achieve the nominal measurement output level of -10 dBFS.
As you can see, the THD+N figure of 0.063% is very close to the THD reading of 0.062%, which is expected since the TL972 is a low-noise Operational Amplifier and significantly newer than the LM386 I previously tested.
Next I will construct a TL972 audio amplifier circuit as a direct replacement for a typical LM386 audio circuit, then measure and post those results.