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I have some computer speakers which suffer from the following problem: when switches flip in certain devices in the house, like the thermostat on a radiator or the refrigerator, the speakers will emit a loud popping noise, caused (I believe) by changes in the voltage on the mains.

Would an isolation transformer between the speakers and the mains alleviate this problem?

If so, what capacity isolation transformer should I buy? The speakers are rated at 500W RMS (Logitech Z-5500). I've read that an approximate conversion of 500W to VA comes out to 500*sqrt(2) = 707. If I don't play the speakers at anywhere near maximum volume (certainly less than half the maximum), would a 600VA isolation transformer suffice?

Thanks for any assistance.

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    \$\begingroup\$ Do you have access to portable power meters such as those kill-a-watt devices? Some of them can measure complex power and the best way to be safe would be to use one of those and measure exactly the power used by your speakers. Also the formula you provided shouldn't be used on the speaker power but on the number that is printed on the power supply. You'll see something like input XXX V and XX A (or mA). Multiply the current by voltage to get the active power and then apply the formula you found. \$\endgroup\$ – AndrejaKo Jun 19 '12 at 6:32
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    \$\begingroup\$ I had a similar problem, but with my monitor messing up the picture for a second. These things can best be solved near the source of the problem, by instaling a snubber network across the mains switch. en.wikipedia.org/wiki/Snubber I solved my issue with a capacitor in series with a resistor across the mains switch of the device that made my monitor 'plop'. Can't remember how I did the sizing of this, hence a comment instead of an answer. Check this question: electronics.stackexchange.com/questions/18321/… \$\endgroup\$ – jippie Jun 19 '12 at 6:58
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I've never had much luck in using isolation transformers to clean up power. I've always had to use an isolation transformer plus something else (filter network, ground breaker, etc.).

If you're looking for overkill, how about hooking up a power conditioner? There are cheap power conditioners that are a waste of money. But if you spend a decent amount on a good name brand, it should provide all the isolation you need from any of the nasty stuff going on elsewhere. If you've got the money, I defiantly suggest using a power conditioner on critical sound applications.

To take it one step further, I find that opening up the speakers and placing ferrites around the speaker wires that actually go to the voice coil helps reduce the noise induced by GSM cell phones. So that might be worth exploring as well. And it's a whole hell of a lot cheaper.

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I'd wager that it's most likely some high-frequency noise showing up on the mains, and not a brown-out. A full-blown isolation transformer may be overkill. You may be able to find a power bar with some line filtering components in it, which may help attenuate those line glitches.

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    \$\begingroup\$ True, but I'd like something that is guaranteed to work near perfectly, even if it is overkill (it's kind of a critical application - I play some noise as a sound screen to help me sleep, but the pops wake me up!) Are my assumptions about the power use correct? \$\endgroup\$ – Mike B Jun 19 '12 at 4:05

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