Sub blowing Christmas lights?

The Sub power supply is sloppy and the fluctuating power needs of the sub are messing up the voltage in the A/C supply. The mains are being drawn down and bouncing way over the standard voltage by the sub.

I think that you may very well be pointing in exactly the right direction, Elizabeth. I'm thinking that what is known as an inductive kickback effect, from the power transformer of the sub's amp, may be putting a large voltage spike into the lights.

Inductive kickback, which results in what can often be an extremely large voltage spike, is what happens when the current through an inductance is abruptly changed. Power transformers have lots of inductance. The abrupt change might be related either to the sub's "fluctuating power needs," as you put it, or conceivably to the sudden change in line voltage that may occur when the lights are plugged in and start drawing current.

"Back emf," though, is probably not the best term to use for this. The back emf voltage produced by a woofer or other speaker driver results from a different effect -- the motion of the voice-coil continuing for some amount of time after the input signal has stopped or changed, which causes the driver to act as a generator until its motion gets back in sync with the signal.

Sprink, it would be helpful to know some further details: Does the fuse blow precisely at the moment the lights are plugged in, or at random other times? When it blows, is the sub turned on, and is it playing music?

Regards,
-- Al

electrical folks (AL?) could this be a grounding issue? Even with the sub doing something to the circuit a bad ground seems like a possible cause?

That's a logical question, Paulsax, but I can't envision how either a missing ground or a miswired outlet could cause something like this. No matter how the outlet were miswired or ungrounded it would seem that no more than the normal 120V or so line voltage would be put across the light string (unless, as Elizabeth and I suggested, the sub is adding a transient voltage spike on top of the 120V).

Regards,
-- Al

If it happens when the sub shuts off,there could be a spike from it.When I unplug the walwart for my cellphone,I get a spike.In the bedroom,it even interferes with the tv by unplugging it.I checked it with my Fluke meter,and sure enough a small spike.

Also beside the sub turning off/on doing it,there could be a loose connection in the outlet amplifying the problem if that is the cause.