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Re: AC Power Filtering units?

Posted by racerguy on July 12, 2003 at 07:58:23:
I work for a large-scale-computing manufacturer, and have access to lots and lots of computer power equipment. I've done some personal tests with computer power conditioning equipment, and found that it works poorly with a hi-rez audio system.
One thing to consider with what most of the world calls a "UPS" is that it really isn't. UPS stands for Uninterruptible Power Supply, but 90% of the units available, and certainly the inexpensive ones (i.e. below $10K), are really an SPS - Standby Power Supply. IOW, they are not active all the time, they are in standby and switch to battery power shortly after a power failure or sag is detected. Because of this confusion, manufacturers of _real_ UPS' now call their devices "Online UPS." An online UPS is always running your equipment off its battery, instead of having the battery sit in standby. AC is used to constantly replenish the battery in an online system.
So there are a few problems with the standard SPS-type UPS. First of all, they generate incredible electrical noise. Nearly all claim to isolate EMI/RFI, but it doesn't matter because they make their own. Switching power supplies and digital circuitry in computers don't care much about AC line noise, so it doesn't matter in a computer environment. It DOES matter with non-switching power supplies, analog circuitry and discrete components in the audio world. Another issue is that most SPS-type UPS' don't output a very good approximation of the input AC wave, which again doesn't matter in the computer world except for some very large systems, but does matter to audio gear.
One thing switching power supplies ARE very sensitive to is over-voltage or under-voltage situations. Well guess what? The non-switched power supplies in a lot of audio gear are NOT very sensitive to over-volt or under-volt by comparison. But, since the computer equipment is sensitive, the UPS manufacturers build circuitry into them to adjust voltage within the computer PS tolerances. So the unit will clip over-volt and momentarily switch to battery to boost under-volt. Clipping the AC waveform is something non-switching PS' don't like. All that switching on and off of the battery makes noise - line noise, RFI, EMI.
The last knock against computer-specific SPS-type UPS' is that they tend to run a bit hot, especially when on battery, so they are equipped with fans. Since the designers expect that these units will be put in computer rooms or other out-of-the-way closets or the like, they don't pay much attention to how noisy the fan is. That's not exactly what an audiophile wants, is it?
OTOH there are the real "on-line" UPS'. I did some tests with one very expensive unit, and found that if I had a place to put it (they are large and even noisier than the SPS units) and could afford it (they are very expensive, then careful selection might yield a device that would condition my power without too much in the way of sonic degradation.
It just isn't worth it though.