Filter out audible frequencies in mains power - Page 1
Forum section: electronics / projects

author: okw
author_rank: Frequent Contributor
timestamp: September 29, 2025, 08:31:08 am
content: I'm making a filter based on the below schematics. The author of this filter has since removed one DC block (D2/C3/C4) and one choke (L3/C6/C7/R2/C8), as it was the excessive. But it gives the basic idea of the filter.It's some sort of a jack of all trades filter, for both filtering, spikes, protection and so on.But I also wanna tackle a problem I'm having with the air-conditioner. When the AC is running, a tone can be heard in the hifi-system (maybe from the fan motor or compressor). How do I add suppression of certain frequencies within this filter? And how do I calculate the appropriate passives based on a certain frequency, or a wider band?I was planning to use an online audio analyzer and get the frequency/frequencies of the noise, and specifically suppress it/them.Or is it better to suppress the whole audible range, and will this be as effective?

author: moffy
author_rank: Super Contributor
timestamp: September 29, 2025, 08:51:15 am
content: The problem with the noise in the HiFi is that it could be comming through the ground somehow, in which case the filter shown will be ineffective. Identify the tone frequency, quite possibly the inverter PWM frequency, then start with the HiFi disconnected from everything else but the speakers, I am assuming that the speakers are not powered, and see if the noise disappears.

author: tggzzz
author_rank: Super Contributor
timestamp: September 29, 2025, 08:53:25 am
content: Quote from: okw on September 29, 2025, 08:31:08 amI certainly wouldn't put an poorly understood circuit from a random source on a high energy power supply (i.e. the mains).Having an electrolytic capacitor with rectifier in series with the AC supply is not something I've seen before, but that is a weak statement since I am not an expert. What is the justification and safety?In general it is preferable to suppress noise and interference before it emerges from the source, rather than filtering at the victim.QuoteThe first step is to characterise the problem; numbers not adjectives.Capture the noisy hifi system output, and use audacity to look at the noise. Especially use its analysis functions to determine the frequency spectrum of the noise.EMI/noise may or may not be coupled through the power supply. Move parts of the installation around to see what increases/decreases the noise.

author: Uunoctium
author_rank: Frequent Contributor
timestamp: September 29, 2025, 09:34:29 am
content: my 2ctAt first you should do a search, if local electricity supplier is prohibiting any AF filtering. In Germany a so called "Rundsteuer" is still in use. A centralized remote to switch off and on various loads from grid eg road lights, electric home heating.This is done by AM modulating the 50Hz mains with very low bitrate in telegrams multiple times per hour. Generators are electromechanical machines with up to several 100kW power. For Cologne the frequency ist set to 1350Hz. And they are not amused about any parallel filtering or shorting their signals to ground

author: Zero999
author_rank: Super Contributor
timestamp: September 29, 2025, 09:41:49 am
content: I suspect the air conditioner has an inverter for one or more of the motors. Is it a new unit? If so, contact the supplier. It shouldn't be doing that.

author: tszaboo
author_rank: Super Contributor
timestamp: September 29, 2025, 09:56:22 am
content: For you it's the air conditioning? For me it's my solar inverter (solaredge), when it's at low power in the afternoon.Audible buzz coming from my speakers. Let me know if you found something, I was about to take my stuff apart to try to quiet the speakers.

author: Terry Bites
author_rank: Super Contributor
timestamp: September 29, 2025, 04:45:19 pm
content: Overkill doesn't solve problems, a measured approach is required.I think you need to track down the sources of interference and fix that rather than over cooking the Hi-Fi mains input.Its normal to bond neutal and ground in your breaker panel so components wired across N and E will probably do nothing useful.Norway would almost certainly use this wiring system, I'd be amazed if it didnt!The Hi-Fi issue is most likely caused by a ground loop.ie the ground potential at the amp differs from the ground potential of the AC.seehttps://help.campbellsci.com/CR6/Content/shared/Maintain/Troubleshooting/ground-loops.htmFigure 2 shows exactly the problem that might apply to your situation.Note that LC filters with high Q can ring causing a whole set of new interference problems for your Hi-Fi.Ping your filters with a sharp pulse and watch the waveforms on your scope. A simulation will show this too.

author: okw
author_rank: Frequent Contributor
timestamp: October 01, 2025, 04:16:34 pm
content: Quote from: tggzzz on September 29, 2025, 08:53:25 amThanks. If I produce the PCB before measuring (I don't have the amp here, but need to get my hands on the filter asap), which component(s) should I put in and where to make a placeholder for a possible filter (and tweak the values later when I have the measurements)? Even if I don't populate it, and bridge where needed, I just need a placeholder.

author: tggzzz
author_rank: Super Contributor
timestamp: October 01, 2025, 04:44:35 pm
content: Quote from: okw on October 01, 2025, 04:16:34 pmYou seem to be following a traditional design anti-pattern, summed up as "ready, fire, aim".Measure noise.Analyse noise.Design filter to remove that noise.Implement filter.In that order.

author: okw
author_rank: Frequent Contributor
timestamp: October 01, 2025, 05:32:28 pm
content: I know :-) but I need to quickly order and receieve this filter for other reasons, not primarily for filtering the audible noise. So I hoped there was some kind of equivalent to the pi/T filter for matching the signal path to antennas. Just leave some empty footprints before production and populate after measuring.

author: MariuszD
author_rank: Regular Contributor
timestamp: October 01, 2025, 09:25:15 pm
content: The fact that you hear interference from the speaker doesn't mean the interference band is limited to the audio range. Most often, interference penetrates into distant circuits because it has a very wide bandwidth.

author: IconicPCB
author_rank: Super Contributor
timestamp: October 01, 2025, 10:39:14 pm
content: A local ( Australian ) company , Gayrad transformer is manufacturing these kinds of filters to eliminate interference from various switchmode power supplies ( printers ,other office equipment etc ).Contact them they may have what you need.

author: okw
author_rank: Frequent Contributor
timestamp: October 04, 2025, 02:11:39 pm
content: Do I have to record with Audacity or is it sufficient to record a wave file (say builtin macos or windows program)?I'm helping a friend and he (and his Amp) is far from me.

author: tggzzz
author_rank: Super Contributor
timestamp: October 04, 2025, 02:29:54 pm
content: Use audacity (or a similar program) for analysis of the audio file.It doesn't matter what creates the audio file.If the audio is compressed (e.g. mp3), it would be prudent to ensure the compression neither introduces artefacts nor removes useful detail.