Is this a good idea to connect Stereo System, or anything else that includes amplifier for that matter, via Line-interactive Uninterruptible Power Supply? It seem to sound ok, but I still wonder. Such UPS doesn't produce true syne wave so I'd like to ask if this affects the sound or not.

It does not.

I disagree with pdq's assessment. A ups, like any other component on the AC lines, can have a negative impact. However, I'm talking about "obvious" problems with the sound, not "stereophile" type problems that are fixed via $200 cables. If you don't hear an obvious problem after hooking everything up, you're probably fine.

What kind of problem could you experience with non-sinusoidal power? Well, it depends upon the quality and design of the power supply used in the amplifier, as well as the quality and design of the power supplies used in all of the other audio (or A/V) components that play through the amplifier.

What I typically do in a mixed component (e.g. audio, video and/or computer-based) environment is put the computer and some low-current devices on the surge+battery outlets of the UPS and the rest of the components, including the amplifier on the surge-only outlets of the same UPS. If I notice added audio or video noise on a component connected to a battery+surge outlet, I move it to a surge outlet. Sometimes it is necessary to add an additional powerstrip on the surge-only outlets, but I avoid doing so on the battery outlets to keep the battery drain as low as possible.

By wiring all devices through the UPS, even those not on battery, I can reduce, but do not eliminate, the chances of ground loops, etc.

Some UPS units, while not truly sinusoidal, give a good approximation of mains power with low noise. Others generate more hash than you'd want in an A/V environment. Note also that some UPSs (for higher-end computing and those made for audio or stage environments) are advertised with, and can create, true sinusoidal output.

And remember, if the components don't have well engineered and constructed power supplies, they can transmit noise to or route ground loops through other components via the non-power interconnections (audio/video/remote) even if the signals those components generate don't appear to be affected (e.g. a source of a ground loop that causes another component to fail to operate within specs or very-high frequency noise that, via intermodulation, might end up creating audible noise in the output of another component). It's often necessary to build and test a system component by component when you have these problems and sometimes the component that appears to introduce the problem is actually the victim of a previously added component that appears to be operating correctly.

-brendan

Any UPS that I am aware of draws from the battery, and therefore gives non-sinusoidal output, only when the power goes out. The rest of the time it simply feeds the AC through to the output. Does yours operate differently?

There are three categories I am aware of:

- Some "full time" UPS units rectify incoming AC to DC and then generate AC from that DC all the time. When the AC goes out, they simply switch out the recitified DC and switch in the battery DC to feed the AC generation. These are usually fully sinusoidal units that guarantee a very low level of exposure to outside power problems. The AC to DC and DC to AC portions of these units have to be high quality 24/7/365 components and are therefore costlier.

- There are some mid-tier units that use a hybrid mapproach of actively filtering and voltage regulating (both up and down) full-time on AC power, and then switch to the more standard near-sinusoidal output when outages happens.

- And then there's your standard budget UPS that passes through whatever AC you have (passively filtered, sometimes) unless there's an outage.

I only have UPSs in the the last two categories. There may be some "true sinusoidal" UPSs that have features in-between the first and second category.

-brendan

That is called an Offline UPS. Other UPS types may always convert the incoming power to DC and feed the inverter, which produces the output waveform.

Edit: bhoar was first.

Mine is a little bit different. It has three modes of operation:
1-When there is a power shortage it uses internal battery to keep things going, but this situation is rear. Once a month perhaps.
2-When the AC voltage is "normal" somewhere between 210-238 volts my ups does absolutely nothing and simply passes this voltage to the outputs. At this mode it also makes somewhat annoying noise but since our voltage here is usually around 240 volts this mode is also rarely used.
3-The most often used operational mode- In presence of voltage over 238 the ups performs power conditioning and supplies to the output near sinusoidal voltage of around 205 volts. I guess it sort of shaves off the voltage pikes, so to speak.

Just out of curiosity, where are you, what is your nominal line voltage, and why does your UPS convert 240 volts to 205 volts? The only standard line voltages that I am aware of around the world are 100, 120, 200, 220, 230 and 240, plus a very oddball one that is taken from two phases of a three phase line that ends up around 208 volts.

Also, what line voltage is your equipment rated for?