Speaker
Impedance stability
Is the amp rated to survive the speaker's deepest impedance dip?
Simple
Speakers don't present a steady electrical load — their impedance dips at certain frequencies, and at those dips they demand more current from the amp. This check confirms the amp is built to survive the deepest dip without shutting down or misbehaving.
The theory
Nominal impedance (the "8 Ω" on the box) is a rough average; the actual impedance curve swings with frequency and can dip well below it. Since current draw is inversely proportional to impedance (I = V/Z), a dip is a current spike. The engine compares the speaker's minimum impedance to the lowest impedance the amp is rated stable into — if the speaker dips below the amp's rating, you're in the danger zone for current limiting, protection trips, or instability.
For experts
The villain isn't just the impedance magnitude — it's the electrical phase angle that coincides with the dip. A speaker can present 3 Ω at 45° of phase, and the combination of low impedance and reactive phase forces the amp to deliver high current while voltage and current are out of step, dumping heat in the output devices precisely when they're already stressed. This is the EPDR (equivalent peak dissipation resistance) problem that makes some nominally-4 Ω speakers behave like a 1.5 Ω load to the output stage. It's why a beefy amp's "stable into 2 Ω" rating matters more than its 8 Ω wattage for hard-to-drive speakers, and why class-AB amps with marginal heatsinking trip their protection on exactly these speakers. The engine's check is a magnitude proxy for this deeper reactive-load reality.
Current draw spikes where the impedance curve dips; a coincident phase angle raises output-device dissipation further — the EPDR effect (Howard, 2007).