Hummmmmmmm from speakers

[CESIL]

OK folks here is today's challenge...

 

Setup is:

Promethean ActivBoard

Powered speakers

Visualizer

Laptop

 

If audio lead is connected between speakers and laptop there is a loud (very loud) howl from the speakers.

If laptop is on battery noise is still there.

[EDIT] If usb leads are removed from laptop and audio still connected then noise goes away [/EDIT]

If laptop is removed and a person (me) holds the audio jack plug and the IWB usb connector the noise is the same.

Same if holding the visualizer usb plug or even touching the metal arm of the visualizer camera while holding the audio plug.

 

This happens is several rooms with the same board/speakers but different audio cables.

 

So there is clearly some sort of grounding issue or similar, but where do I start?

 

The board and visualizer are powered through double insulated PSUs so two core cable, the speakers are (I believe) three core mains powered.

 

If I can't fix it the school involved will need to call in someone who can but should this be an audio expert or electrician?

Edited October 24, 2013 by CESIL

[newpersn]

I'm thinking RCA Ground loop?

 

Does sound more of ground problem.

[LosOjos]

Is the USB lead replaceable? Could be a grounding issue...

[fiza]

I was just about to open a thread with a similar issue.

 

We have had a system put in the gym (projector, speakers and screen). When we plug in a laptop there is a hum from the speakers. Unplug the laptop and run it on batteries and there is no hum.

Installer said it is because the laptop is not earthed?? I dont know enough to know if thats what the issue is or whether they should sort it as part of the install.

[Koldov]

This might help...

 

How do I get rid of humming and buzzing sounds (ground loop)?

 

We had similar issues a long time ago, but not a howl just a very audible buzz. Ours was to do with the laptop power supply I think as it was audible when the laptop was plugged in, but not when it was on battery.

 

Maybe instead of the laptop power supply (as in our case) it is the visualiser power supply?

 

Kol.

 

Edit: There are ground loop devices you can buy to plug inbetween to help eliminate the hum.

 

Also, I think the power supply thing is because they may be unearthed (as in class II) devices?

 

'A Class II or double insulated electrical appliance is one which has been designed in such a way that it does not require a safety connection to electrical earth (ground).'

Edited October 24, 2013 by Koldov

[CESIL]

Thanks all, you have come up with all the ideas that I have had and found online

 

The thing that is confusing me is that the noise is there if I provide the connection between devices instead of the laptop so which part needs to be isolated?

 

In our school we have an amplifier by each board and all audio goes through that to the speakers which are not mains powered, I guess that is why we don't get the same irritating noise.

[Andrew_C]

As Sysman says, an isolating transformer is the right fix for one of a ground loop. They can be caused by power supply design and nothing else will fix it.

However, a "loud howl" is unlikely to be a ground loop. Is the mic on the laptop on? You could have an "audio loop" causing feedback.

[crc-ict]

Get yourself one of these. C2G Stereo Audio Isolation Transformer (80449) - www.misco.co.uk

 

Good call on the solution, but heck that is expensive!

 

Try this one from CPC:

A-1095 - UNBRANDED - GROUND LOOP ISOLATOR. | CPC

 

Much more bearable price :-)

[Chris_Cook]

Does the visualiser power supply use a two pin figure-8 connector or two pin mains plug with an adaptor? Sometimes you can fix this sort of thing by plugging the connector in the other way round.

[DMcCoy]

It's going to be difficult to get a ground loop from devices with only two pins

 

Double insulated power adapter can also help, you can tell if they aren't, they will have 3 pin connector.

[Koldov]

It's a common problem...

 

http://www.edugeek.net/forums/av-multimedia-related/99979-how-eliminate-hum-noise-laptops-connected-speakers-pa-equipment.html

 

Kol.

[mac_shinobi]

It's going to be difficult to get a ground loop from devices with only two pins

 

Double insulated power adapter can also help, you can tell if they aren't, they will have 3 pin connector.

 

Am sure I posted something some time back with regards to having a ground loop on the Lenovo screens we have at work here and one of the blogs / articles I found mentioned to disconnect the earth wire from a 3 pin plug or get an adapter to convert the 3 pin plug into a 2 pin plug ( but this was definitely not advised as is dangerous because you are disconnecting the earth wire so someone could potentially get a nasty electric shock ) from either the laptop or the external screen ( not both )

[Koldov]

Am sure I posted something some time back with regards to having a ground loop on the Lenovo screens we have at work here and one of the blogs / articles I found mentioned to disconnect the earth wire from a 3 pin plug or get an adapter to convert the 3 pin plug into a 2 pin plug ( but this was definitely not advised as is dangerous because you are disconnecting the earth wire so someone could potentially get a nasty electric shock ) from either the laptop or the external screen ( not both )

 

Think this is the 'ground lifting' mentioned, some of the more expensive boxes have a switch that does it for you it appears.

 

I'm still concerned it is described as a loud 'howl' by the op though, then a 'hum' in the thread title...

 

Kol.

[CESIL]

Well I went and had another look at this problem at lunchtime.

 

By pushing the phone plug tight into the laptop audio socket the noise stopped! Releasing the pressure at all made it return.

 

So I had a closer look at the various cables that have been displaying this problem and those that have a problem have the insulation around the plug flush with the flange on the back (bottom?) of the plug. On the cables that are better the flange sits just proud of the insulation.

 

I have also found that the problem is better/worse depending on the laptop used with newest batch being worse possibly due to the way the socket is mounted in the case. I have had a similar problem with some of these laptops with the VGA connector that is set such that it is difficult to get a cable to 'lock' on securely.

 

Just to confuse matters I went to one classroom where the teacher said she was very happy with the new laptop because it had made audio noise go away!

 

I am going to take another look later and will try removing some of the insulation on the plug (the audio jack plug not the mains one!) to see if it improves things...

Edited October 24, 2013 by CESIL

[CESIL]

Think this is the 'ground lifting' mentioned, some of the more expensive boxes have a switch that does it for you it appears.

 

I'm still concerned it is described as a loud 'howl' by the op though, then a 'hum' in the thread title...

 

Kol.

 

Well Hum or Howl...it is difficult to describe the sound in words

 

It is not just the usual low level background 'hum' and not a feedback squeal. I guess I could try recording it later?

[CESIL]

OK so I cut about 0.5mm of the insulation from around the base of the pin and hey presto...noise is gone!

[mac_shinobi]

OK so I cut about 0.5mm of the insulation from around the base of the pin and hey presto...noise is gone!

 

Base of what pin - earth pin on the three pin plug or what exactly ?

[CESIL]

Base of what pin - earth pin on the three pin plug or what exactly ?

Nothing that drastic...

 

0.5mm of insulation from the base of the jack plug pin on the audio cable...

 

Not even I would mess with the mains plug

[john]

Good call on the solution, but heck that is expensive!

 

Try this one from CPC:

A-1095 - UNBRANDED - GROUND LOOP ISOLATOR. | CPC

 

Much more bearable price :-)

 

I have used the above in many installations and temp setups with ground loops and all problems with audio humm were solved, you don't need an £80 box for that job usually

[mac_shinobi]

Nothing that drastic...

 

0.5mm of insulation from the base of the jack plug pin on the audio cable...

 

Not even I would mess with the mains plug

 

Any chance of some photos @CESIL ??

[Koldov]

I second the photo request @CESIL and just out of interest, may I ask what brought you to that conclusion?

 

Kol.

[chazzy2501]

I've experienced the hum, 'sometimes' if you don't have the manufacturers software installed the sound card won't change to the correct output mode. it will treat every device on the audio out as "headphone" but with the manufacturers software, Realtek in my case, you can see it switch the output to "line". When it does this the ground loop issue goes away.

 

I've seen this with sigmatel as well.

 

This issue has hit me more recently as we've moved to windows 7 that normally has the right sound drivers in already. So we didn't bother with the manufacturers version. Whereas on XP we had to install it.

[CESIL]

Sorry for the slow reply folks, but I was not at work last week...

 

For those who wanted to know what the fix was I have produced a natty illustration which will show it better than a photo...

@Koldov...I found that pushing the plug very firmly up against the laptop socket stopped the noise and that suggested that that there may be a contact problem...

[Koldov]

Ah I can see now what the earlier description meant (I was thinking you were stripping insulation from the cable, hence the wut?). Luckily, your natty little illustration explains things very clearly (a picture paints a thousand words...) Thank you.

 

Kol.