Intermittent hum on XPA-5 gen 1

[unbeliever]

I am a happy original owner of a now 8 year old XPA-5 gen 1.

Recently the amp has started to hum intermittently (from the amp itself, not the speakers). It sounds like a 50Hz hum for 2 seconds then it's silent for 6 seconds. Periodically, like clockwork.
I'm an electrical engineer and has designed lots of powersupplies and amps but I cannot for the life of me understand this behaviour... 

So, my question is, is this something to worry about? The answer will probably be yes but since I live in Sweden, it's a lot of work to send it in to Emotiva for a checkup.

Also, what is the general feeling of speaker protection from these amps? If an internal rail or something equivalent goes, will it disconnect the speakers or is there a risk of sending the woofer cones into orbit?

[vcautokid]

Protection on the XPA is very good. It is microprocessor controlled. Monitoring for shorts and DC offset and so on. The buzz sounds like DC components in your AC causing the buzz possibly. Since you are versed in the electric arts I assume you did the basics already. I know that there was a sorbethane pad that could be installed under the transformer to quell the buzz or resonance. DC will cause buzz in a Torroid and Lam rattle and or resonances in C or EI transformers too. In our ever polluted electrical environment of computers and so on, it just gets worse. Power conditioning may help.

[unbeliever]

Ahh, of course, didn't think of that. Hysteresis-"abuse" in a large toroidal => all kinds of bad!

I have no measuring equipment at home other than a DMM. The phase the amp is on is at 220V, 15V lower than the other two phases. I shut down the main breaker to our house and measured on the un-loaded incoming mains, same result. The missing 15V could very well be due to distortion/asymmetry. 

They are doing heavy construction down the road, might pollute the mains. I will wait a few days and see what happens!

Thnx for the excellent reply!

[leonski]

Hum associated with anything? Fridge running? Neighbor running table saw? A Doctor running some specialty gear?

Ssounds like someone else is drawing hearily on the 'low voltage' phase.

[vcautokid]

Another way to look at your AC power is with an Oscilloscope with the correct value leads to see how clean the AC waveform is. Also if any noise or DC components are present.

[leonski]

I've seen O-Scope attachments / software for PC and MAC.