Hi Mojo and mdrew,
I suspect it's a thermal sensor that's shutting down the Yamaha when it gets really hot. But it could also be a current sensor into the output stage: when the low impedances of the M80s and paralleled M22s demand more current from the power supply, it senses that and, since it can't supply it, it just shuts down.
As to your Denon, Mojo, it may have a kind of "soft clipping" circuit in its output stage that rounds off the clipped square waveform and lessens the clipping distortion. It's a trick that NAD has used for years on its amplifiers which prevents the amplifier from producing the harsh, edgy distortion of a hard-clipped signal. Don't get me wrong; it's still distorting, but it's not as irritating as it would otherwise be without the soft clipping circuit.
One of the Denon AV receivers that we measured (actually, Tom Cumberland, Axiom's electronic engineer, did the bench tests) at Axiom's lab produced exactly its rated output into 8-ohm loads and the same output into 4-ohm loads. Class A/B amplifiers normally would produce much more output power into 4 ohms than into 8 ohms, so we concluded there was a circuit in the Denon that "clamped" or limited its current flow into a 4-ohm load and even when you turned it up, it wouldn't get louder. So in effect, it was a built-in limiter that prevented the Denon from going into hard clipping when presented with a 4-ohm load.
Of course, although your Denon doesn't shut down, it's still a limiter, preventing the true dynamics possible if you had a much more powerful separate amp.
I know my H/K AV receiver runs out of juice driving my 4-ohm M80s at high concert levels with really dymamic classical music. It doesn't overheat or shut down into 4-ohm loads, but on really dymamic material, the loudest orchestral peaks get a bit edgy and compressed
. I think it's likely some kind of soft-clipping circuit in action. Can't wait until I get my own sample of the new Axiom multichannel amp.
Regards,
