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What's causing my PSU to make crackling/buzzing noises?

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I just put together a new gaming PC with a Corsair HX1000i platinum-rated (80+ & Cybenetics) PSU that is hooked up to my APC Back-UPS BR1500G-IN UPS. As long as the UPS is simply transferring power from the wall (it's line-interactive), or when the PSU is plugged into the wall directly, it is fine. But whenever the mains power goes out & the UPS switches to battery mode, my PSU starts making crackling/buzzing/vibrating noises.

I've heard some PSUs don't like to work with the modified/stepped sine wave outputted by most UPSs and that this noise is just the PSU's circuitry compensating for that. Is that correct? Corsair officially recommends either a Pure Sinewave or Simulated Sinewave UPS for their PSUs (so I guess what my UPS outputs on battery which is what APC describes as "step-approximated sinewave" doesn't qualify as simulated?). What's surprising is that my previous PSU, a 80+ gold-rated Corsair RM850x, was also plugged into this same UPS and it never made a noise like this, and it had gone on battery dozens of times back then. Aren't both these PSUs supposed to have ActivePFC? Or is the HX1000i just more sensitive as it's supposed to be more efficient?

Despite the noise I'm not having any stability issues with my PC itself, even when running Cinebench R23 on battery (but the PSU does seem to make a bit louder noise of the same kind when under load). Under realistic use it would only have to be on battery power for a minute or so before I shut it down (or my building's diesel genset comes on, which typically happens about ~40 seconds following an outage).

Also, I'm using a 6A to 16A adapter to plug this PSU into my APC BR1500G-IN because this UPS only comes with 6A sockets and my PSU has a 16A plug. Where I live the voltage is ~240v so if I'm right the Amperage should never even hit 6A even if I'm pulling over a 1000W which my system really can't. That said, do you think the adapter is causing this? If so, why would the noise only happen when the UPS is outputting battery power instead of all the time?

Is this noise indication of any harm being caused to my PSU (and as a consequence, the rest of my system)? Should I just get a pure sine wave UPS or is the noise safe to ignore? Is this even due to the waveform? The system is otherwise perfectly stable but this noise coming on whenever the power goes out is making me nervous for my new PC.  :-BROKE

P.S. I did talk to a Corsair support rep regarding the issue. While he did recommend getting a pure or simulated sinewave UPS (reading off Corsair's official recommendation I reckon), he also said that if the PSU won't be staying on battery for more than a few minutes the buzzing shouldn't be a big deal. But he also wasn't able to give me a straight answer regarding whether APC's step-approximated wave qualifies as "simulated" or not as per Corsair's own measure, so I'm not entirely sure if he fully knew what he was talking about.

P.P.S I also talked with a local UPS retailer and he said the problem was the voltage drop when the UPS goes on battery mode (city's input being ~240v but the UPS drops to about ~214v when on battery), and that I need to get an On-line Double Conversion UPS (like APC Smart-UPS SRC1KI-IN) to make this problem go away, rather than just a pure sinewave UPS (like APC Smart-UPS SUA1500I-IND) which is still line-interactive like my BR1500G. But aren't these modern PSUs supposed to work on any voltage between 120-240v (HX1000i is indeed rated for that range)? How could the slight drop to ~214v cause it to make that noise?

Thought I should bring this matter to the electronics guys on this forum to have a crack at diagnosing my PSU's problem!

It doesn't seem like I can upload video to this forum so check out this post of mine from the LTT forum where I posted a recording of the noise:

Use headphones. Wait for the UPS beeps to end, then turn up the volume.

And yes, I assure you the noise is from the PSU, not the UPS.

m k:
Maybe not a thyratron, nor rotor carbon brushes.

UPSes often generate a very dirty sinewave. Or maybe put better: "sine-adjacent" waveform :D
The Back-UPS Series of APC seems to be a mixed bag in that regard. Lover spec ones have really bad waveforms, the one higher spec Back-UPS in this list has a pure sine output though.
Look at this list for a couple of examples:

I was not able to find a measured output for your exact model though.

Some inverters are not so good with non linear loads. Maybe a filter would help.

See this video.


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