Power Supply Conditioning for AV Equipment

[dhicks]

Hello All,

 

We have a hall with an AV setup that consists partly of a couple of speakers driven by an amp and a projector and motorised drop-down projector screen. The amp and the projector screen are plugged in to the same double socket in the wall. Whenever the motorised screen starts or stops there's a loud popping sound from the speakers. I assume this is something to do with a power surge of some kind produced by the motor in the drop-down screen - does that sound right? If so, how do I stop it happening? Do I need some kind of specialist AV power-conditioning setup? A quick Google search suggest APC have a range of this kind of thing, but a look at their current websites turns up one 6-way surge-protected socket. Is that all I need - a surge-protected socket? Or is there a more heavy-duty bit of kit available to smooth out power for AV equipment?

[Fazza]

It sounds like you use the same electrician as we do! Personally I'd have a chat with the electrician you use to see if there's anything that they can do for you. They might be able to isolate the power going to the motorised screen. I'm not sure a surge protector will make any difference, APC make UPS's and although they offer surge protection they also output a completely clean power line and 'soak up' any kind of electrical noise that it sounds like you are experiencing.

 

Until recently we had an electrical hum on the speakers in our hall and our head of music insisted it was the phono sockets we'd installed next to the VGA connection for laptops etc. so I called his bluff and gave him a new back box, front panel, leads and our drill so he could move the 'faulty' sockets and put them somewhere else! Now he had to investigate properly (rather than just try and blame IT Services) and it turned out their amp was buggered as he could see the power arcing inside it! Once the faulty bit of kit had been replaced the hum has gone!

[Andrew_C]

Radio frequency interference; shouldn't happen, (IF the kit complies with those damned European undemocratic rules) but does. It may be possible to suppress the pop with a "snubber" across to motor. But - that requires someone to design it, and fit it. Not a "bloke off the web".

[southhamster]

Radio frequency interference; shouldn't happen, (IF the kit complies with those damned European undemocratic rules) but does. It may be possible to suppress the pop with a "snubber" across to motor. But - that requires someone to design it, and fit it. Not a "bloke off the web".

Is it worth asking the screen motor manufacturer if they have any ideas.

You might find replacing the switch with an electronic one would fix it but not all motors will be happy with that.

[dhicks]

Is it worth asking the screen motor manufacturer if they have any ideas. You might find replacing the switch with an electronic one would fix it but not all motors will be happy with that.

 

Good points. Update: I've just tried using a UPS to power first the motor and then the amp, so isolating both items away from the mains. Made no difference - there was a still a nasty (really quite loud) popping sound when the screen started and stopped. The problem, therefore, can't be a power surge on the mains circuit, but must either be RF interference as Andrew_C suggests above, or it could simply be a current being induced by the power cables to the motor in the signal cables to the speakers, i.e. the cables are running too close together. I probably need to spend some time carefully re-running cables around the stage.

[Andrew_C]

Re-routing won't help much if at all.

 

Try running a LONG extension from 'somewhere else' to supply one or the other. That, if it clears the fault, would indicate mains born, if it doesn't, RF.

[dhicks]

Try running a LONG extension from 'somewhere else' to supply one or the other. That, if it clears the fault, would indicate mains born, if it doesn't, RF.

 

Surely running the motor off a UPS, completely disconnected from the mains, and still getting the same effect indicates it's not a mains issue?

[Andrew_C]

Sorry, I didn't realise you were running on battery. Thought you were just using it to filter.

[dhicks]

Sorry, I didn't realise you were running on battery. Thought you were just using it to filter.

 

Yes - I tried running both just the motor and then just the amp off the battery of the UPS, no mains connection - in both cases I still got a popping sound when starting the motor up. If this implies RF interference, what can I do to reduce that - shield the amp's box with conductive paint / tinfoil?

[Andrew_C]

shield the amp's box with conductive paint / tinfoil?

A tinfoil hat? Nope. It will probably be being picked up in the cabling. Make sure all low level* leads are screened, no connector shells connected to the screen, and use balanced connections where ever possible.

 

 

* Microphone, then line level. Speakers leads - not important.

[difinity]

It's usually cables not devices that pick up the EMI. Power, speaker and interconnects.

Does the volume of the pop increase/decrease depending on the volume of the amp ? If its not then it would indicate its entering through the power or speaker cables.

[PotNoodleTech]

This can be caused by speaker cables being run in the same conduit/area as mains cables. It's worth having a look at the speaker to amp cables to see where they're run.

[dhicks]

Make sure all low level* leads are screened, no connector shells connected to the screen, and use balanced connections where ever possible.

 

How do I make sure leads are screened - wrap them with copper tape? Seemingly, it does also repel slugs, which is bound to come in handy.

[dhicks]

Does the volume of the pop increase/decrease depending on the volume of the amp ?

 

Yes, the higher the amp is set, the louder the pop.

[Andrew_C]

Looking inside the panels/connectors. Foil screened is best*, as you get a better coverage. Speaker cables are MUCH less of a problem, because the signal is not subsequently amplified.

 

*for installed cables.

[dhicks]

This can be caused by speaker cables being run in the same conduit/area as mains cables.

 

Indeed. I think I ned to spend a few hours re-routing some of the cabling so that the power cables are well away from anything else.

[difinity]

Yes, the higher the amp is set, the louder the pop.

 

That suggests its the inputs cables or components picking up the EMI and not the speaker cables.