Odd hum problem on install

[fredfish]

Having been slightly perplexed by a stubborn hum problem on a boardroom install I thought I'd pick the brains of the Blue Room for ideas!

 

The set up is as follows: a stereo unbalanced audio signal can be sent from a minijack socket in a boardroom table to the audio input of a polycom VC unit. The cable is routed through the floor via another minijack termination in a floor box, then through a kramer VGA/Audio switcher (which seems not to be affecting any problems) and in to a minijack input in to the VC unit.

 

As soon as I switch to the PC input on the VC system, I get a substantial mains type hum coming back through the VC unit's output. When I plug a laptop in to the audio input at the boardroom table end, the hum changes to nasty buzzing more akin to power supply noise (this occurs whether the laptop is connected to the mains or not).

 

I have tried using an RDL AV-HK1 (a passive, unbalanced 1:1 audio isolation transformer) before the VC input, but this just passes the problem straight through.

 

I have, however, had success in solving the problem by taking the signal just before it enters the VC unit, balancing it using a PC balance box (basically a passive DI box) sending it as a balanced signal through a Canford passive isolation transformer, then unbalancing it again using another PC balance box the other way round. (still following me?)

 

My question is, why should this latter solution work, but not the former? And can anyone suggest some way I haven't thought of to solve the problem directly rather than applying a rather inelegant (not to mention expensive) series of additional boxes in the signal chain.

[sleah]

Might be obvious and you may have tried it-

Have you tried bypassing the installed cable completely by running a cable direct from source to the VC unit?

Could be the buried cable introducing the hum......

[pjrails]

Give one of these solutions a try, connect the audio screen and the VGA screen together (usually in the faceplate) this is something that works in my situation and another solution is to unearth the PC mains to the PSU.

 

HTH

 

PJ

[Shez]

another solution is to unearth the PC mains to the PSU.

A solution to endangering life perhaps...

[fredfish]

Could be the buried cable introducing the hum......

 

Indeed it is the buried cable introducing the hum, the question is why should it be introducing the hum? I don't see how replacing like with like should solve the problem.

 

 

Give one of these solutions a try, connect the audio screen and the VGA screen together (usually in the faceplate)

 

I hadn't thought to do this, since I assumed that both the audio and video grounds should be common anyway if coming from / going to the same bits of kit. am I wrong? I'll try it next time I'm on site anyway.

[pjrails]

another solution is to unearth the PC mains to the PSU.

A solution to endangering life perhaps...

 

A Case of to many earths causing an earth loop - break the loop and the hum is gone and earth is still picked up through screen, if it works then you need a proper mains earth lifter for safety.

Although many Laptops only have L & N but on a desktop needs to be done properly.

 

PJ

[themadhippy]

A Case of to many earths causing an earth loop - break the loop and the hum is gone and earth is still picked up through screen

and ere can you confirm the screen earth will saflety handle the fault current when the inevevatable happens? sorry but this is a highly dangours practise that should be stomped on from a great height,perferable whilst stomping on the persons head who carries out such things

[p.k.roberts]

another solution is to unearth the PC mains to the PSU.

A solution to endangering life perhaps...

 

A Case of to many earths causing an earth loop - break the loop and the hum is gone and earth is still picked up through screen, if it works then you need a proper mains earth lifter for safety.

Although many Laptops only have L & N but on a desktop needs to be done properly.

 

PJ

 

Could you explain what you mean by a 'mains earth lifter'? It's not a term I've come across before.

[wmcalin]

Come across a similar issue before, try a di box on the signal send, hope this helps

[KevinE]

I can't believe people are still recommending the lifting of mains earths. Surely we live in a more technically enlightened world on the BR!

[pjrails]

another solution is to unearth the PC mains to the PSU.

A solution to endangering life perhaps...

 

A Case of to many earths causing an earth loop - break the loop and the hum is gone and earth is still picked up through screen, if it works then you need a proper mains earth lifter for safety.

Although many Laptops only have L & N but on a desktop needs to be done properly.

 

PJ

 

Could you explain what you mean by a 'mains earth lifter'? It's not a term I've come across before.

 

It's a simple unit that disconnects the earth for you very much like a 'Ground isolater' would on an audio feed.

[Shez]

It's a simple unit that disconnects the earth for you very much like a 'Ground isolater' would on an audio feed.

Does anyone actually sell something like that? Any chance of a link?

[pjrails]

It's a simple unit that disconnects the earth for you very much like a 'Ground isolater' would on an audio feed.

Does anyone actually sell something like that? Any chance of a link?

 

Hi Again

The only I can find nowadays only seems to be in the states link but I have seen one over here somewhere in my past.

 

Cheers

PJ

[sleah]

Could be the buried cable introducing the hum......

 

Indeed it is the buried cable introducing the hum, the question is why should it be introducing the hum? I don't see how replacing like with like should solve the problem.

 

Errrm... to prove if it was the physical cable, maybe there is a break in the screen or similar. You didn't say you'd proven the physical cable was OK!

[Brian]

Could you explain what you mean by a 'mains earth lifter'? It's not a term I've come across before.

It's a simple unit that disconnects the earth for you very much like a 'Ground isolater' would on an audio feed.

For anyone stumbling across this topic what IS NOT being talked about (I hope) is disconnecting the mains safety earth connection feeding the equipment.

 

Instead, what you want to do is break the ground connection between two pieces of equipment and hence break any unwanted 'earth loops'.

 

Where it gets confusing is when the connected equipment is Class 2 - ie it doesn't have a separate mains earth connection. So how do we end up with an earth loop?

 

A picture is worth a thousand words...

 

http://www.prosoundweb.com/images/uploads/Figure2ClearPathGroundLoops.jpg

 

...told you.

 

Without the Ground Isolator there is a non-obvious earth loop via both transformer interwinding capacitances and the mains feeds. So we have an 'earth loop' which is part 'earth' and part live mains. What's important is that as far as your audio is concerned it's a loop capable of carrying an audio signal. And hum.

 

The interwinding capacitance is a 'feature' of most transformers and the way they are made.

 

When the equipment has Class 2 switch-mode supplies, either internal units or external wall-warts, then the interwinding capacitance is usually less of a problem at audio frequencies. HOWEVER, manufacturers then go and spoil it by deliberately fitting capacitors where the interwinding capacitance would be. These are fitted to reduce high-frequency interference and usually make a much better hum conductor than an old-fashioned transformer ever did.

 

So even on systems without an obvious earth connection you can still get hum.

 

Referring back to the picture we can now see how the ground isolator/hum eliminator/etc works.

 

 

To work out why a system hums it's useful to sketch out your system, including your mains connections. Do this, and armed with your new knowledge, you should be able to spot any loops. Including the non-obvious ones.

 

Knowing where the loops are means you can see where to break them.

 

It is NEVER necessary to mess with your mains connections.

 

Instead look at breaking the loop on the signal connections. Start with breaking any loops in the audio connections. Simple transformer isolators like the one used in the picture above are cheap and easy to install. Being passive they are very reliable.

 

Next to tackle are any loops in your video connection and then finally your control connections. Control signal isolators are usually the most expensive; you can even get USB isolators.