Amplifier noise from AC Mains - Page 1
Forum section: electronics / projects

author: gnif
author_rank: Unknown
timestamp: June 27, 2016, 02:22:12 am
content: Hi All,I have slowly built up a pre-amplifier to drive a bi-amplified pair of speakers using Linkwitz-Riley active crossovers. Everything works great, but at random times in the day, I get the very strange buzzing from the AC mains in the circuit, it is very intermittent, and can be heard in other equipment throughout the house. It is barely audible elsewhere so is no concern there, but in my pre-amplifier is is very annoying. It is certainly not the 50Hz ac mains hum, the frequency seems much higher (100-120Hz), and pulses on and off in some kind of modulated fashion for a few seconds, then then stops, only to reoccur a few hours later.I have spoken with some friends in the area and they have confirmed that they have also detected this odd buzzing from their equipment, so I am coming here asking if anyone knows what this could be, and the best way to filter this kind of noise.My pre-amp is using a linear power supply, +-15VDC (no switching electronics) and I have confirmed there is not a ground loop issue.

author: lapm
author_rank: Frequent Contributor
timestamp: June 27, 2016, 02:25:09 am
content: Sounds more like someone is feeding some dirty electricity to grid, if even your friends have picked this up... Better call power company and let them track it down...

author: gnif
author_rank: Unknown
timestamp: June 27, 2016, 02:51:07 am
content: Possibly, and I will follow this up, but even still I have a consumer power amplifier in my living room that does not exhibit this behavior, so my question remains as to what my power supply is missing to filter out this kind of noise?

author: richard.cs
author_rank: Super Contributor
timestamp: June 27, 2016, 12:18:17 pm
content: How sure are you that it's coming from the mains? Your description sounds rather like 217 Hz GSM TDMA noise.This claims to be a recording of that but I haven't listened to it:https://soundcloud.com/av_artifact_atlas/electromagnetic-interference

author: Audioguru
author_rank: Super Contributor
timestamp: June 27, 2016, 03:14:01 pm
content: The recording of GSM buzzing is exactly 217Hz. Mains is 60Hz in north America and harmonics are 120Hz, 180Hz and 240hz so will be at a different frequency from the GSM buzz.

author: richard.cs
author_rank: Super Contributor
timestamp: June 27, 2016, 03:26:16 pm
content: Quote from: Audioguru on June 27, 2016, 03:14:01 pmIfit really is mains-borne noise, yes. But nothing in his post gives any evidence for it being mains noise, maybe he knows something he didn't tell us or maybe he just jumped to a conclusion - that's why I asked.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 27, 2016, 03:54:23 pm
content: I had the GSM "noise" for quite a while in a radio. It only come something like 50 times an hour, but not at a constant rate. How often it comes can be different in other areas. For me the sequence was also shorter, maybe 1/4 the length of the recording.There are other possible radio / mains based signals going around. Usually a linear supply should filter out most of it, especially the LF part coming over mains and longer distance.A common source of RF interference is power line networking.

author: StillTrying
author_rank: Super Contributor
timestamp: June 27, 2016, 04:51:16 pm
content: Quote from: richard.cs on June 27, 2016, 12:18:17 pmSomedays I hear this multiple times, and I don't think the description could be much closer. I especially like this bit.I have spoken with some friends in the area and they have confirmed that they have also detected this odd buzzing from their equipment,For testing purposes rather than waiting 2 hours, you can make a GSM device do it's high power TDMA noise by switching it off, switching it on, ringing it's number from another phone, sending it a text...I don't think there's anyway of filtering it completely.

author: SeanB
author_rank: Super Contributor
timestamp: June 27, 2016, 08:11:43 pm
content: If not GSM then old power line control signals, used to do things like changing distribution transformer taps and such. Induced in the MV feed at a high level and then received by the local substation to operate a tap changer to keep voltage within regulation limits as load varies.This was often also used as a control signal to switch street light groups on and off, so that the step load as night fell was reduced to a rate the power stations could slew production to match. This was generally discontinued as they went to local group photocells and on luminaire photocells, that would give a more gradual increase or drop in load as night and day controlled the switching level.

author: gnif
author_rank: Unknown
timestamp: June 28, 2016, 05:57:14 am
content: Thanks everyone for your input so far. I can confirm with no doubt that is is not GSM, I am very familiar with how that sounds running through a poorly shielded amplifier. It does sound like some kind of control/comms on the wire. To confirm it is coming from the AC mains I have powered the amplifier from a UPS (obviously running off battery), which completely eliminates the noise.

author: tombi
author_rank: Regular Contributor
timestamp: June 28, 2016, 07:55:41 am
content: Could it be the signal used to enable off-peak stuff? (Around 1kHz)https://en.wikipedia.org/wiki/Zellweger_off-peakTom

author: gnif
author_rank: Unknown
timestamp: June 29, 2016, 07:48:27 am
content: Quote from: tombi on June 28, 2016, 07:55:41 amI think you are dead on, I will try to capture it and compare, but I think that this is exactly what I am hearing.