there are usu. three kinds of UPSs, off-line, on-line and line-interactive.
off-line UPSs usually are cheaper, lower efficiency solutions. they deliver electricity using the batteries only when there is none from the mains.
line-interactive UPSs are a compromise on cost and quality and are ok for computer use. the crux is the switch-over time (optimally needs to be 5-6ms or then the comp. may reboot, data losses may occur or damage to the electronics is possible).
on-line are the best (among UPSs), for computer and other sensitive equipment needs. this is coz. differential o/p is generated from the batteries, irrespective of the fact that electricity is on or cut, meaning there is waveform correction (square to sine).
so, no AVR needed in on-line. PWM (pulse width modulation) us used.
in on-line UPSs, AC is converted to DC (say input of 230V AC to 180V DC) then AC is regenerated from DC (say input of 180V DC to 220V AC). so, o/p AC is cleaner and constant.
UPSs (incl. on-line) however, are bad choices for audio (since they use a switching frequency of 14.4KHz to 16KHz that could introduce harmonic distortion and noise).
the off-line and line-interactive UPSs need AVRs but the quality of the AVR (or the total lack of it) is a price/market dependent variable.
AVR work using relays for step-up or step-down (physical moving devices).
Line-interactive UPSs usually go upto 1.5KV. Only APC goes upto 5KV.
APCs used to have a lot of issues in India since the designers in the US were not aware of electricity issues here. However, over time, they seem to have customised their offerings for Indian conditions.
coming to servo, correction time, owing to the physical movement of the motor/brush, is approx. 35V/sec where correction in the on-line UPSs is in millisecs. servo also does not do any waveform correction.
consequently, IMHO, for audio use, CVTs are the best option (when not incl. power re-generators in the list of options, like Power Plant from PSAudio). esp. when from companies like Bhurji, that are and have been at the epitome of power conditioning for many years.
CVTs handle harmonic distortion. They handle waveform correction. They work on the saturated core principle so are able to deliver best, clean and stable power. the manufacturer's credentials and R&D are critical to a good CVT design as 'balancing' is a key requirement, esp. since there are 04 coils. and, of course, use of the highest quality of materials.
A CVT, by virtue of its design, is 85% efficient (approx. 15% heat, etc., loss).
Secondly, owing to the large transformers therein, there is an avoidable hum.
Consequently, there are CVT usage best practices, viz.,
1. Try and install the CVT in a room/area other than where the audio eqpt. is, to avoid the irritating hum.
2. Try and keep the CVT atleast 3m away from the audio eqpt. to avoid harmonic distortions.
3. Try and keep the CVT in a well ventilated area to dissipate the heat generated.
4. Depending on the distance of the CVT from the equipment, quality and gauge of the power connecting wire/s need/s to be calculated carefully.
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