The speaker click device the A2 uses simulates capacitive coupling because otherwise it was impossible to mix its output with a Mockingboard AY-3 or the GS's ES5503. After all, the definition of a speaker click device is that the voltage is either 0 or 5 volts, nothing in between.

Can we get back to what exactly made the AP2000 work? It was the NMI-at-startup and then something with the ADCs?