Uninterruptible power supply units have become commonplace in offices, studios and homes alike. No longer relegated to complex and expensive units, UPS systems that are available are smaller, more efficient and less costly. In fact, many of these can easily be purchased at your local office supply store. While wholly appropriate for ensuring sensitive office equipment, desktop computers, servers and other gear, the inexpensive units are designed only to supply backup power. These UPS systems will produce power that has nearly the same voltage and nearly the same current as the power from a utility, but in many cases it will not be as clean.
UPS systems use a battery or a bank of batteries to create a DC voltage that is then turned into the necessary AC voltage. This is done by generating some sort of waveform and then shaping it to produce a standard sine wave--or a wave approximating a sine wave. Herein lies the trouble. Shaping that waveform requires large and therefore heavy and expensive passive components such as inductors and capacitors. Cutting corners at this stage will produce AC voltage that is good enough to drive a switch mode power supply, like those found in nearly all computers, but voltage that may contain enough waveform irregularities that they will affect the performance of certain audio gear.
While switch mode power supplies are very common in digital audio equipment, they are not used in all audio equipment. Gear that operates in the analog domain will in many cases rely upon linear power supplies, which are designed to filter low frequency hum components. But, it may not be as good at removing higher frequency noise that may be present on the output of some UPS systems. If you notice strange whines or other noises in the background of your audio signals, check to see if any gear that accepts and/or produces analog audio is fed by a UPS--this might be the root cause of the noise.
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