You're right, the range is not infite (on my Furman is 97 to 141 volts, though they have models that will handle above and below that). And you're also right in that it filters out noise, which is what my Furman does in addition to power conditioning. This is what it says about it on the Furman site, should anyone be interested:
Description
The 15 amp AR-1215 AC Line Voltage Regulator is intended to protect audio, video, computer and other electronic equipment from problems caused by AC line voltage irregularities-sags, brownouts, or overvoltages that can cause sensitive digital equipment to malfunction, or, in extreme cases, to sustain damage.
The AR-1215 is designed to provide a steady, stable 120 VAC output. It accepts any input voltage from 97V to 141V and transforms it to a constant 120V, ?5V. Voltages beyond that range may also be converted to usable levels, depending on how far out of range they are. The AR-1215 can handle loads totaling up to 15 amps as long as the input voltage is above 124 volts. For voltages below that level, its capacity must be derated at approximately .15 ampere per volt.
The AR-1215 has been designed specifically with the unique needs of audio and video in mind. Its technology differs from that of computer-oriented voltage regulators in many important ways.
For Example:
The AR-1215 does not use a ferro-resonant transformer, which would be heavy and bulky, radiate a large magnetic field, and be too frequency-sensitive to be usable with generators. Instead, it uses a use a design based on an eight-tap toroidal autoformer. The toroidal design assures minimal leakage of stray magnetic fields.
The AR-1215 circuitry monitors the incoming line voltage with each cycle, comparing it to an extremely precise voltage reference, accurate to ?0.15%. If a voltage fluctuation requires that a different tap be selected, the new tap is electronically switched exactly at the zero-crossing, to avoid distorting the AC waveform. (Most commercial voltage regulators using multiple-tapped transformers switch taps at uncontrolled times, thereby creating voltage spikes, and often creating clicks that can leak into the audio.) Hysteresis of 1.5V in the switching circuits avoids ?chatter.? The design is not sensitive to small errors in line frequency, making them ideal for use with generators.
The AR-1215 also features power conditioning that is truly in a class by itself, thanks to the quantity, quality and configuration of the overvoltage suppression devices used. These include MOV's, high voltage inductors and capacitors, and precise high-inrush magnetic circuit breakers. This unique combination can safely divert large spikes as well as filter audible high frequency noise.
An additional feature, Extreme Voltage Shutdown, senses dangerously high or low voltages and shuts down the output before any damage is done. The output remains off until the overvoltage or undervoltage is removed, with an LED indicating the shutdown condition. This invaluable feature provides positive protection to your equipment from accidental connection to incorrect line voltages (for example, 220V when 120V is expected-- a not uncommon hazard in the entertainment industries.)
The AR-1215 has eight outlets on the rear panel, and one on the front panel. All outlets are regulated, spike-suppressed, and filtered against RFI with a 3-pole filter. There are no controls except the circuit breaker/on-off switch. A bar-graph meter comprised of 10 LED's indicates input voltage, while another LED indicates 'In Regulation' status (i.e., that the output voltage is within ?5V of 120V.) The unit is housed in a compact, single-space rack-mount chassis, 1.75? high and 8? deep and weighs only 12 lbs.
It works for me!
Alan
