Hi Raindance,

If you examine the M80's measured impedance curve (thanks to Chess for posting the link), it's quite smooth and "linear"; i.e, there are not dramatic peaks and dips in impedance. The M80s curve hovers around 4 ohms between 100 Hz and 500 Hz and is above 4 ohms over the rest of the audio range.

That is considered a relatively "easy" 4-ohm load, because it's smooth and linear, however, if a receiver does not have a robust power supply capable of supplying enough current into a 4-ohm load, then it isn't "easy" for that receiver. This is why previous Yamaha receivers have 4-ohm impedance switches that reduce the voltage drive in the 4-ohm position, thereby limiting power output and avoiding excessive current flow and heat build-up (and shut-down). By contrast, a "difficult" 4-ohm load might have fluctuations to 2 ohms or less followed by peaks in impedance.

Gene's tests of the new Yamahas on his Audioholics site are very thorough, and, as I suspected, the auto-EQ circuits are no panacea. An Axiom customer complained to us suggesting that his speaker had been wired with the polarity reversed. Not so. The Yamaha auto circuit mistakenly identified the speaker as being "out of phase" with the rest of the system, just as Gene DellaSala experienced in his tests of the Yamaha 2400.

It's true I've not used the new NAD receivers, however, a number of Axiom customers have reported that they easily drive M80s. Moreover, I checked with a very experienced engineer and amplifier designer who consults to AXiom (he has designed amplifiers, both solid state and tube, for Macintosh, Luxman, Harman/Kardon, and others, and is entirely familiar with the circuit design of the new NADs). He confimed that the NAD receivers do NOT impose current limiting (thereby reducing the power output) when set to drive a 4-ohm load.

Most other receivers that have rear-panel 4-ohm impedance selector switches do impose current limiting when set to the 4-ohm position. As to whether the new Yamahas will drive the M80s when left in the 8-ohm position, you'd have to try it at higher volume levels and see if the Yamaha's protection circuitry dectected the lower impedance and shut it down.

Regards,

Alan Lofft,
Axiom Resident Expert (Retired)