Line filter for amplifier to block noise from air conditioner - Page 1
Forum section: electronics / general

author: okw
author_rank: Frequent Contributor
timestamp: January 17, 2025, 02:47:32 pm
content: My friend is experiencing humming + beeping noise in his class A amplifiers when the heat pump (air conditioner) is plugged in. And the intensity increases when the pump runs. He is neighbour to a factory / industrial building, so there is alway a slight beeping sound in the amp even on zero volume.He has 3 amplifiers, and would like me to design a filter (250V, 16A max and 4 outlets - encapsulated).I successfully made a filter many years ago, to stop my TV from blinking when I turned on the relay controlled lights (Nexa wireless). But that was only 3A.What's the typical noise here and what is the best approach? Is it a surge protector, a DC block, EMI filter, etc?He only needs 4A on each outlet, so maybe put a 4A (plus margin of course) filter on each outlet (fused), instead of one 16A filter on the input?I am designing, not buying off-the-shelf. Anyone with suggestions for designs/schematics/etc or critical parts (e.g. chokes)?

author: madires
author_rank: Super Contributor
timestamp: January 17, 2025, 04:04:00 pm
content: I'd guess it's not just EMI but also mains voltage sags and surges. An EMI filter might be able to mitigate things to some extend, but won't resolve the issues completely. You could start with a standard EMI filter and see if it helps. In the worst case the solution could be a power conditioner or online UPS.

author: okw
author_rank: Frequent Contributor
timestamp: January 17, 2025, 04:38:42 pm
content: Quote from: madires on January 17, 2025, 04:04:00 pmYes, I think I need to add a few filter blocks in the filtering/conditioning. I'm trying the analytical approach as I'll design and produce myself, not buying and testing off-the-shelf products.

author: mtwieg
author_rank: Frequent Contributor
timestamp: January 19, 2025, 05:07:17 pm
content: Line filters may help with high frequency emissions and harmonic distortion, to an extent. But if "sags and surges" (i.e. flicker) is the problem, then no amount of line filtering will help. Would need to use a fairly sophisticated power conditioner (like a double conversion UPS), or would need to change how your mains distribution is done to mitigate it at the source. But honestly if line flicker affects your amplifiers then IMO that is a serious defect in the amplifier itself.Could you describe the issue in more detail, especially the "beeping"? Does it just happen when the pump starts/stop, or does it happen continuously?

author: temperance
author_rank: Super Contributor
timestamp: January 20, 2025, 01:18:19 am
content: Sound more like demodulation of signals which are being picked up at the input circuitry of the amplifier or somewhere else.Why I think that:The power supply in the amp is probably a standard transformer > rectifier > buffer cap supply. With this supply being an excellent low pass filter it is nearly impossible to inject audible noise into the amplifier trough the mains plug all the way into the amplifier power rails.Try the amplifier with nothing connected at the input. If it goes away, the problem is higher up in the chain. If it doesn't go away, try shorting the amplifier inputs.