Humming issue on whiteboard/audio install

[jay_t]

Recently had some smartboards, projectors and audio systems installed. There is a low level buzz coming from the speaker systems which is tolerable but when a laptop is plugged in and powered from the mains, the noise increases 2 or 3 fold. If the power supply to the laptop is disconnected, the buzz dies down again.

I'm assuming some kind of earth issue, we used to get it with PA systems years ago with turntables and if you disconnected the earth fly lead from the turntable to the mixer, it usually stopped the buzzing.

Anyone have any experience of this or any suggestions for a cure? The audio/video/vga feed to the projector/speakers is all run via a common faceplate so all connections are fairly close together.

 

[ABC Technical]

Transmission line fault?

 

[Robojin]

Likely you have an earth loop problem, quit common on low cost or poorly designed audio kit

Check the cables by wiggling (scientific term) at the face plate to see if the loop alters, also disconnect each bit of kit to the Amp one by one to see if a particular piece of kit is the cause, if no source is connected and the speakers still buzz, then the Amp design is a likely cause unless the speaker cable runs are close to any florries?

I'd look at the earth shielding, check the back of the face plate for a poor shield connections, also at these points the shield has little or no effect if a poor design of connector has been used, which might explain why you pick up the Laptop PSU

Any transformers close to the face plate?, does the additional buzz induced by the Laptop PSU diminish with distance?

Can you supply the laptop from a different circuit temporarily, to see if this is mains born interference?

 

[ABC Technical]

I like that scientific term.