In response to a recent Tech Tip we did (TTOTD – 3/19/2002) on UPS systems and how they relate to digital audio gear we’ve seen a rash of questions about UPS systems in general. Seems this is a very confusing subject to many. Since it is a broad topic where do we start?
Sweetwater Sales Engineer, Rob Bowers, has kindly illuminated us with some of the pertinent issues to think about. These questions don’t all come with obvious answers, because those depend on your specific needs, but at least we can better understand the issues.
There are at least four types of back-up AC power available in terms of the output waveform (the price and complexity rise further down the following list):
–Square Wave – ICK! Not bad for (non-fluorescent) lighting and resistive heating, but…this is just no good for any kind of music gear.
-Modified Square Wave – This system plays games with the duty cycle of the square wave. This works. I wouldn’t want to think long about what kind of junk is getting through to the active components of a piece of electronics, but it will basically get the job done without wrecking anything permanently.
-Stepped Square Wave (a.k.a. modified sine wave). This is actually getting close. GOOD analog gear should do all right with this level of power, assuming their power supply is of equally good quality. Unfortunately, designers of digital equipment often give far less thought to pfower quality than they should. It’s also the lowest form that will work for motors. A refrigerator will run on this kind of power without destroying the unit. Generally, if a unit’s power supply is high quality, it should have no problem with this.
–Sine Wave Power. The best (and most expensive) systems supply power that is indistinguishable from the local power company, and is often MORE pure. There are no welding units down the block so EMI via the mains should be a thing of the past in a studio setup with such a system. But you’ll pay through the nose for it.
The downside to the better solutions is lower system efficiency. The better the waveform purity, the lower the power out for given power in. Non-sine wave units are going to range from 80% to better than 90% efficiency. Sine wave units are in the 50% efficiency range. These are essentially high power audio amplifiers running @ 60 Hz. Lower efficiency means shorter run times and limitations on output current, if the battery is not able to source the current at the rate needed.
Here are some basic questions to pursue with power back-up units:
1) Running time. How much? In order to determine this you’ll need to know about the gear you plan to connect to it, and how much power that equipment requires. UPS systems generally have ratings of the maximum load as well as how long they can hold out under a variety of loads (see #2).
2) How does running time “derate” with load over time? Does it tend to overheat if stressed? And what happens then?
3) Can running time be extended with additional batteries and how far?
4) How stable is the voltage output? Will it sag when a new current load is added, e.g. when the studio mini-fridge starts up?
5) How stable is the 60 Hz signal? Some equipment actually depends upon 60Hz as a “clock” of sorts. The power company has a pretty tight control of the frequency such that it only varies a maximum of a few cycles per day. My generator, on the other hand (nice smooth sine wave), runs at a calculated 62 Hz. This is not an accurate source of 60Hz.
Linear power supplies (with transformers) will not “like” dirty power. A transformer will work best with a clean, frequency stable, sine wave. They will get really warm (even hot) and are really inefficient when fed poor power – Pure Square wave is not your friend. You want at least stepped power. The up side of running dirty power through a transformer is that the transformer will filter a great deal of the noise out – anything that is off frequency to the design frequency of the transformer will be attenuated.
So, a transformer will filter, but get hot and the rectifier front end of a switching power supply will change it all to DC – we hope. Square waves are harmonic rich, even when stepped, and supplies are not designed to deal with frequencies other than 50 or 60 Hz. This is sub-optimal.
Here’s another rub: many stepped units that put out a fairly close approximation of a sine wave at low current levels will “distort” under heavy load, and begin to put out something more akin to a square wave. So, if you want to try to get work done while using the UPS you need to make sure you get one big enough to “easily” handle the load you place on it.
For the vast majority of users, UPS should be seen as a protective unit, not one that will allow work to continue. The APC brand units we sell will (if maintained) protect equipment plugged into them just fine. You want to use your UPS time to shut the studio down, however, not continue to try to work. Moreover, serious power regulation, as opposed to a UPS, should probably be used to protect equipment that does not store data as part of its regular operation (preamps, compressors, etc.). Many UPS systems are not even in line until power is interrupted and the power they offer isn’t all that clean anyway. Good regulation will be of more benefit on a daily basis.
There are solutions, which are not simple, but very effective to provide back-up for about as much load as you can create. The question is: “What is it worth to you?”
There are other important distinctions among UPS systems we’ll cover in the future.