I’ve been looking at clipping performance of several preamp designs and noticed that the waveform that is displayed bu the QA401 application is 180 degrees out of phase with the waveform I see on the scope. So if I’m seeing the positive peak clip on the scope, the QA401 tells me it’s the negative peak, Just as an idiot check, I’m driving a series circuit of a 1k resistor into the Anode of a small-signal diode, and looking at the output waveform at the anode. As I would expect, the scope shows the positive peak clipped at about 680mV. The QA401 is telling me that the negative peak is clipped however. I’m using the Left channel + output and measuring on the Left + input, with the - input shorted.
I’ve checked my cables and verified the polarity is correct, and the scope is not inverted. The two signals are in phase on the scope - so if there was a phase reversal in one cable, I would see it.
The Right channel of the QA401 does the same thing.