In message , Martin
Hardy writes
How can you tell that the power will be low into 4 ohms? I don't know much
about this subject, but I would have thought that if you get 90W into 8
ohms, 125W into 6 ohms, then reducing the impedence further to 4 ohms would
have meant the power would go up again- not fall. How does this work?



Usually because the PSU or the output transistors have run out of
current capacity when feeding low impedances.

I'm not sure there's that many speaker (except car ones) that are really
4 Ohm over the whole frequency range though. Certainly there's a lot of
speakers that don't have impedance compensation on the drive units and
so they have some very reactive regions in the input impedance to the
crossover where the Z does fall to this level (or even below) for
certain frequency ranges.
--
Chris Morriss