An inSync reader had the following follow up question from our recent tip on using line filtering devices with UPS devices (see TTOTD 11/25/98).

"I use a UPS all the time in the studio. Sometimes the local grid will be off for five or six hours and we use a 1000-watt generator with a Roland VS-880 and Alesis amp/speakers. When the voltage drops and the UPS kicks in I get a square wave and bad noise in the system. If another filter like the Juice Goose isn't the right thing to use, what is?"

There are actually a few questions hidden in there. There are some differences between the way generators and battery powered UPS devices work that can come in to play. The 11/25 tech tip was really directed more at people using the latter (though it is valid for both). The square wave output that many of these devices produce is not a problem for the switching power supplies that are found in most computers. And since it is usually computer equipment that most people want to protect or keep powered, the square wave is widely considered an acceptable output. It is possible to purchase supplies that produce a true sine wave or stepped sine wave output, but they tend to cost quite a bit more. This usually only matters with audio or video equipment, but as your system proves, it can be quite an important point.

Sometimes filters like a Juice Goose can work just fine with these devices, but they can also be smoked by them. It depends on the specific design of the UPS and the filter. The advice of the tech tip was to contact the maker of the UPS before hooking up such filters. We're talking about an interaction between two components here, so the specifics of each one really have to be handled on a case by case basis. Further, if you need such a filtered output it is usually better (though it will not be cheaper) to get all of the protection you need in the one device (in this case the UPS). Hooking up multiple devices of this nature is generally frowned upon due to possible anomalous side effects.