So my USB oscilloscope that I ordered from AliExpress arrived today. I set it up and did some basic measurements using the signal generating app on my Android tablet.
1kHz sine wave looks OK
Square wave however is a different matter:
Also, the PDF manual contains some fantastic English translations that almost make sense:
"Note: The specific speed recorder with computer processing speed, and if use high sampling rate, the situation may break."
"Note: The oscilloscope factory calibration, if you are not satisfied with the measurements, canmanual calibration, the specific reference oscilloscope instructions."
This scope will be a vital piece of test equipment to let me see what's going on inside the amplifier during building.
Next task: reading through the manuals.