Beware of NAD M3 Fire Hazard

"Could be anything, but a shorted (Zener?) diode in the bridge passing AC and cooking the capacitor would be my guess."
I agree with the above.

The same thing happened with my NAD integrated amp about 7 years ago, I don't remember the model number but it was 150 or 200 watts a channel and luckily it happened when I was in the room listening to music- flames/sparks shooting up through the top plate ! NAD replaced  the unit for free and I don't recall if it was even under warranty at the time. 

I'd contact them first to see what they'll do; my guess is they'd service or replace it at no charge. 

The only likely cause for a power filter cap blowing like that is a power surge issue or a mechanical short.  The only incident like that I have experienced in decades of commercial installations was when high voltage showed up between neutral and ground on an AC outlet.
Check your power outlet with a ground fault detector, on of those little 3  light plugs.
With a volt meter test outlet voltage between hot and neutral, then hot and ground. Both should be the same and less than 130v max. Then test from neutral to ground. Ideally there should be no voltage, but often there is a little bit.  To blow a cap like that it would have to see a reverse voltage spike significantly higher than its rating, typically more than 250 volts. Lightning surges coming in on grounds are a common cause. Surge protectors don't typically protect against reverse voltage on ground.

If this was a serious problem with NAD products we all would have heard about it long before this.


Its very possible that the cap failed without provocation (such as from a failed rectifier); once having developed a source of heat internally it becomes a vicious cycle that goes into thermal runaway until something gives (which is usually at the other end of the cap).


So it might be that replacement of this part and cleaning up the amp is all that is needed, but I would not run it again without having replaced all four filter caps. Caps that are downstream of this particular part may well be just fine- much depends on what the failure mode is (so that should be investigated first- see if there is a failed rectifier), but even if a shorted rectifier allowed AC on the cap without shorting out the power transformer's winding, downstream caps might be just fine as resistors are in the circuit that could have limited current.

I've owned three NAD pieces in the past.  All died premature deaths.   For me, NAD now means:

Never Again Daddio