dougey, don't try to move the goal post. I don't know you qualifications, but you may want to lose the attitude. You haven't shown you are qualified to have one.
Testing methodology is literally real world, i.e. normal equipment configurations, as customers would have in their homes, using AP, or custom acquisition equipment to measure amplifier output gained back to the DAC in the first case with connected equipment, measuring both SNR and THD+N under computer loading, identifying issue and tracing back.ย Line conditioning will not help unless the conditioning equipment has separately conditioned sections, and even then could be conditional on filter configuration. In both cases circuits were modified.
Second case is in the lab where we used a variety of NI and Labjack USB products for control and acquisition for automated testing. Quick experiments are often done with a laptop, and depending on the end equipment, even if transformer isolated, capacitive ground loops caused EMI that could be broken just by running off battery power.
If you are a electronics engineer, you probably have hooked a non-isolated USB debugger up to AC powered equipment (on the DC isolated side), and had connectivity issues due to noise. This is not a new phenomenon nor rocket science.
