That's not just a Class D thing. It's Ohm's law, and it's the same for any solid-state amp that directly drives the load (i.e. no output transformer).penningtron wrote: Thu May 01, 2025 8:48 am Some Class D amps list that the wattage drops by roughly half when you jump to 8ohms and even more at 16. I think the assumption is that this one works the same way.
The variability comes when you see how the power is specified. Usually it's at 10% THD, and because the output stage has to work harder at 8 ohms than it does at 4 ohms, it will reach that 10% THD threshold a little bit earlier than half power. Basically, the onset of distortion is non-linear with respect to the load.
Tube amps are a little bit different, because the transformer winding ratio takes care of the load matching. The output stage power tubes see the same relative loading on the transformer primary side regardless (assuming you don't mismatch your speaker loads and transformer taps), so they're always working just as hard. The difference is that the reflected voltage on the secondary side is a function of the turns ratio, which naturally needs to change for each impedance tap.