Noisy electricity supply in music studio (possibly from air-conditioner?)

[sir_sauce]

Hi all,

Thought I'd ask for help in this forum as part of my efforts to get to the bottom of this annoying issue.
I run a small home studio, in a room with a air conditioning unit. Brand is Gree, it uses an inverter and rated at12000 BTU I believe.

Whenever the AC is cooling, it seems to be emitting harmonics into the electricity supply, which can

1) Be heard just by being in the room
2) Be heard clearly through headphone/speaker outputs of some (but not all) of the studio's music equipment

Check out this video describing the problem.

I observed these nasty harmonics to be starting just below 5KHz, and continuing up to 15KHz. Below you can see them showing up in a recording of the noisefloor. I also have a step-down transformer going from 230V to 120V (I am in Malta where the mains voltage is around 230V) which is causing a hum around 50Hz. It's a mild annoyance but much less of an issue than the high end harmonics - those make it nearly impossible to focus and get into the zone.

There is an archived thread on this forum here, where the OP had a similar problem to me which turned out to be caused by inadequate grounding. I've had an electrician come round and check the earthing on my plugs. He found no issues, except that the resistance was just slightly higher than average (~70 Ohms if I recall correctly). Perhaps someone can confirm whether this is OK.

The noise gets even louder when I turn on an analog synthesizer. Not only can the harmonics be heard in headphones/speakers, but they can also be heard when speakers are off and headphones unplugged, like a "phantom" in the electricity supply.

Through testing I have observed some interesting things that could potentially help get to the bottom of this saga... Following a friend's suggestion, I plugged in ALL intruments + equipment + speakers (except air conditioner of course) into the same wall outlet, using multisocket extensions where necessary. His idea was to "avoid any ground loops". I noticed that it significantly reduces the harmonics showing up in the headphones & speakers. If I take the speakers and plug them into a different wall socket, boom - the harmonics are back in the speakers and headphones at full volume. So while I have a way of combatting the noise in speakers and headphones, the "phantom" noise in the electricity supply can still be heard loud and clear, especially when that analog synthelsizer is on.

I'm considering trying a "power conditioner" such as this one, as there are reviews saying it eliminated noise in various scenarios. However, if possible I would like to understand the cause of this issue before throwing money at it. Your thoughts, comments and feedback would be greatly appreciated guys.

Kind regards,
Sir Sauce

 

[Externet]

Welcome.
Are all the electrical outlets and equipment grounded ? Air conditioners cycle the cooling; if noise coincides with the cycling, the air conditioner is emitting the noise. Can be trough mains wiring or can be via air as radiofrequency.
Turning off equipment one at a time can determine the source of the noise. A plugged microphone can receive noise.
Use a methodic elimination of each piece of equipment to find the offender.

 

[Pommie]

(~70 Ohms if I recall correctly)

For an Earthing reading, I would say this is very unusual.

Mike.

 

[schmitt trigger]

Indeed! 70 ohms is way too large.
Would running an independent grounding wire from the offending A/C to the ground /earth rod help?

 

[Lightium]

He found no issues, except that the resistance was just slightly higher than average (~70 Ohms if I recall correctly). Perhaps someone can confirm whether this is OK.

Indeed, anything above 60 ohms is considered a resistance.

 

[Diver300]

What are you running from the 230-120 V transformer? Is the output of the transformer grounded?

 

[ZipZapOuch]

Your inverter is likely not generating a "true sine wave" and the non-sine wave will generate a wide range of harmonics. Typically, a nice and clean 60Hz sine wave will make a nice and easy to filter 60Hz hum but the huge range of harmonics and may pass through the filters but will definitely create audible noise at the steel in the transformer (magnetostrsiction). This noise will get louder with greater load on the transformer.

 

[sir_sauce]

Thank you all for the replies, I am following up this issue with the electricians... it's tough to get a hold of them as they're always occupied with larger projects.

Regarding the ~70 Ohms resistance of my earthing/grounding - apparently this is OK here in Malta because the ground contains a lot of humidity, and is mostly limestone rock with high amounts of calcium carbonate. Still, I am a bit concerned that you collectively agree it is a high reading... For now I will try not to argue with the electricians.

Welcome.
Are all the electrical outlets and equipment grounded ? Air conditioners cycle the cooling; if noise coincides with the cycling, the air conditioner is emitting the noise. Can be trough mains wiring or can be via air as radiofrequency.
Turning off equipment one at a time can determine the source of the noise. A plugged microphone can receive noise.
Use a methodic elimination of each piece of equipment to find the offender.

I ran some tests and decided to document them the best I could on video.

Started with the aircon + 1 active speaker (no audio cable inserted) just plugged into the wall. No audible harmonics observed in the speakers.

Aircon still on, I inserted an audio cable into the speaker and connected it to my analog synthesizer. Specifically, the cable is a TRS (balanced) cable on the synth's end and an XLR male (also balanced) on the speaker's end. I'm not sure if this matters, but the outputs of my synthesizer are all unbalanced. This time, audible harmonics were observed in the speaker.

Weirdly, whenever I tapped my finger on the XLR end of the cable, it induced even more noise in the speaker. Also, the noise in the speaker goes away if I do not allow the speaker to ground itself. This can all be seen in the video. And since this weirdness is happening - something must be wrong. But I don't quite understand what's going on. Questions on my mind are:

1) Could there be a ground loop somewhere?
2) Would placing a DI box between the synthesizer and speakers stop the harmonics from reaching the speakers?

 

[sir_sauce]

Your inverter is likely not generating a "true sine wave" and the non-sine wave will generate a wide range of harmonics. Typically, a nice and clean 60Hz sine wave will make a nice and easy to filter 60Hz hum but the huge range of harmonics and may pass through the filters but will definitely create audible noise at the steel in the transformer (magnetostrsiction). This noise will get louder with greater load on the transformer.

ZipZapOuch, this makes sense to me. I think specifically the analog synthesizer does nothing to remove the harmonics, because I hear its own circuitry "whining", probably once the harmonics enter it. Do you think powering all my studio equipment through this "power conditioner" will work to eliminate the harmonics in the main electricity supply? TIA

 

[ZipZapOuch]

will work to eliminate the harmonics in the main electricity supply?

It will reduce but not eliminate (make zero). The answer can only be, try it to see if you get a good enough result. It may be reduced to near zero and you'll call it zero, and that would be great. The problem may arise that synths use some very high gain circuits to create some effects. Those effects may re-amplify some of the harmonics that are greatly reduced by the line conditioner (possible but unlikely). I'd try it because I can't think of another option other than buyi a so-called "pure sine wave" inverter. Note that some inverters claim to be "pure sine wave" and the power looks like a pure sine wave when no load is attached but turns to some ugly waveform with a load. Be careful what you spend your money on and/or make sure you can return the device after testing. Some electronics suppliers don't allow returns on open boxes.

 

[Lightium]

I watch your video. Sorry didn't hear anything unusual.

 

[sir_sauce]

It will reduce but not eliminate (make zero). The answer can only be, try it to see if you get a good enough result. It may be reduced to near zero and you'll call it zero, and that would be great. The problem may arise that synths use some very high gain circuits to create some effects. Those effects may re-amplify some of the harmonics that are greatly reduced by the line conditioner (possible but unlikely). I'd try it because I can't think of another option other than buyi a so-called "pure sine wave" inverter. Note that some inverters claim to be "pure sine wave" and the power looks like a pure sine wave when no load is attached but turns to some ugly waveform with a load. Be careful what you spend your money on and/or make sure you can return the device after testing. Some electronics suppliers don't allow returns on open boxes.

I just ordered the power conditioner, it is expected sometime early October. I will keep you updated on this, thanks for the help all.

 

[sir_sauce]

I watch your video. Sorry didn't hear anything unusual.

Thank you for watching. If you turn up the volume at 2:56 timestamp for example can you not hear the noise?

 

[ZipZapOuch]

It's hard to hear your hum/buzz in the video because of noise cancelling on most newer phones. The audio must be turned to max to hear anything.

 

[Pommie]

Where do you (or the electrician) take an earthing reading? The probes (to measure resistance) must be connected to something. Earth rod to where?

Mike.

 

[ZipZapOuch]

Ideally, at the ground (earth) bus bar in the fuse box. At that point, the potential should be the same as the neutral bus bar and a fat cable should be connected to that bus bar and your grounding rod.

 

[Pommie]

Ideally, at the ground (earth) bus bar in the fuse box. At that point, the potential should be the same as the neutral bus bar and a fat cable should be connected to that bus bar and your grounding rod.

So the resistance is from the local earth (grounding) rod to the earth rod at the transformer. Shouldn't "the ground contains a lot of humidity" (quote from #8), make the resistance lower but it sounds like the electrician used that fact as a reason it was high.

Mike.

 

[sir_sauce]

Where do you (or the electrician) take an earthing reading? The probes (to measure resistance) must be connected to something. Earth rod to where?

Mike.

Ideally, at the ground (earth) bus bar in the fuse box. At that point, the potential should be the same as the neutral bus bar and a fat cable should be connected to that bus bar and your grounding rod.

The electrician measured the earth/ground resistance at the wall sockets - he had some kind of multimeter device with a standard UK socket which plugged right into the wall like a normal appliance. Using that, he tested a couple of the sockets in the room but not all. No readings were taken at the actual grounding rod. So in this case, I presume the connection made was between the socket's ground/earth input and neutral input (or live, not entirely sure).

 

[ZipZapOuch]

The electrician measured the earth/ground resistance at the wall sockets - he had some kind of multimeter device with a standard UK socket which plugged right into the wall like a normal appliance. Using that, he tested a couple of the sockets in the room but not all. No readings were taken at the actual grounding rod. So in this case, I presume the connection made was between the socket's ground/earth input and neutral input (or live, not entirely sure).

It would be neutral to ground since they are likely connected at the panel to make a closed loop.

 

[ZipZapOuch]

You should have only one ground rod. If you have one at the generator/solar panel/wind turbine/battery pack and one connected to your home's breaker box, then you'll likely have a ground loop that always maintains a true sine wave and a buzz/hum at 60Hz.

If you have a wide array of noise frequencies that can be caused by an inverter that can't generate a pure sine wave as you load it, then you must deal with harmonics and the filtering built into normal appliances and sound systems can't quite handle filtering the range of frequencies generated and passed through to your audio system.

Lastly, if you have a non-true sine wave from your inverter and multiple grounds, you'll have to address the second ground rod. You should only have one ground rod connected to a bus-bar close to the breaker box and all cables run to the bus-bar (no daisy-chaining of solar panel ground to your back yard outlet and back yard outlet to breaker box - run one wire from the solar panel to the breaker box bus bar).