Wiring of an audio rack

[Rikio]

Firstly, I am not the one considering doing all this, so please no replies along the line of "NO DON'T DO IT IS DANGEROUS". I won't. Don't Worry.

 

Right, When setting up for a cabaret style evening at college today, The music technician was also there setting up some sound equipment for one of the bands. I had set up a mic plugged in to the Halls sound system, which suffers from a hum problem. He commented on it, originally thinking it was a problem with mic positioning. I demonstrated it was not, so he went over to the rack.

 

The halls sound system is a mobile rack with Mic Rx, Amp and CD player etc down one side and a Soundcraft mixer on top. He suggested that an earth loop was present, due to the fact all the units are bolted together in a metal rack. To solve this problem he is going to remove all the earths to the units, and then earth the rack rails, so every device earths via its ears.

 

Personally I think this is a dangerous idea, and may not solve the problem. Only three devices have an earth in the plug (Amp and 2x Feedback Destroyer) so how does he intend to 'lift' the earth on things like the CD player, which are not connected to mains earth?

I would also think a ground loop could still occur on devices that have a stereo connection, via the 2 screens on the cables.

 

I suggested a device I had seen in Canford called 'Humfrees' which are plastic insulators and nylon rack nuts that are used to isolate every item in a rack from the rack strip. He however said that this would not help. I would think that this, with appropriate disconnection of the screens of signal screens would result in a star earthing system, free of ground loops?

 

Under his Idea, if any item was removed, it would have no earth and be dangerous.

 

You advice and comments on this are much welcome, I am worried about the implications of this, and what could happen. Surly such devices would also fail PAT?

 

Many Thanks,

 

Rikio

[paulears]

I think you are saying just the right things. As people say all the time, lifting the protective connection is simply not on. In cases like this, then getting a spare cd to mixer type cable, and carefully stripping back the insulation, cutting it, and then taping up, will let you swap them one by one, hopefully making the hum causing item obvious.

[Simon Lewis]

Rikio,

 

If the system was correctly wired up, and the manufacturers had correctly designed their equipment, then there would be no need for humfrees. Seeing that it's quite possible for the chassis to touch elsewhere, this isn't a good way to "solve" the problem.

 

If the chassis was properly earthed wrt the audio ground (c.f. the "Pin One problem") then it wouldn't matter about the boxes touching each other or the metal rack strip.

 

Your colleague's idea of using the rack strip as the earth is not acceptable since the temporary nature of the rack system makes it difficult to ensure a good contact. Furthermore, the day that somebody decides to "borrow" from the rack just one item that doesn't have a mains earth will be the day they may get a fatal shock...

 

Simon

[Mike C]

I agree with all that has been said,

 

Rack mount rails and the rack mount mounting brackets on most pieces of equipment are plastic coated or painted. This would either partially or totally insulate the unit from the earthed rail. Potentially making the earth less effective due to the added resistance it might be easier for the current to pass through your body if you touch the case.

 

Any unit that is earthed is earthed for a purpose, and should be earthed correctly as not to have issues like this. I cannot at this moment in time think of other reasons for the hum other than the obvious cables etc.

 

Have you ensured that you are keeping power cables and audio cables apart as much as possiable?

[Alec]

Have you ensured that you are keeping power cables and audio cables apart as much as possiable?

 

Also, while it's quite possible it is an earth loop, it's also worth testing for inter-unit interference by removing individual units from the rack while all are powered up.

 

It's always possible that one has a PSU that's generating enough hum to be picked up by another.

 

While I've not been hit by this with rack kit, periodically I've had hum from a PSU sitting right beside a DI box.

[Rikio]

Have you ensured that you are keeping power cables and audio cables apart as much as possible?

 

That is one problem. All the cables run down trunking in one side of the rack.

 

I'm hopefully going to take some of my own cables, and experiment with separating the mains cables from the signals, lifting screens etc, to prove that his drastic measures are not required.

 

Thanks for all replies so far.

 

Rikio

[mal421]

Have you considered the possibility that the source of the hum may be external , a dim pack perhaps ?

[Rikio]

Dimmers are fed by a different feed from the substation, so unlikely.

 

Problem also exists when dimmers turned off.

 

Rikio

[Brian]

It really sounds like it's a case of whoever did the wiring not knowing what they were doing. There is a real science in designing a hum-free audio installation and the bigger they get the harder there are to do right. Somewhere I've got the document that ITN wrote when they were planning the move to their new premises, it runs to over 100 pages. Using balanced audio throughout makes life easier but even that isn't foolproof. Once you introduce unbalanced kit then things just go downhill fast unless you do it properly. One day I will write the book on how to do it properly.

[Simon Lewis]

One day I will write the book on how to do it properly.

 

Hasn't Philip Giddings beaten you to it ? ;-)

 

http://images-eu.amazon.com/images/P/0240802861.01.MZZZZZZZ.jpg

 

Blue Room profiting, clickable image added!

[Brian]

One day I will write the book on how to do it properly.

 

Hasn't Philip Giddings beaten you to it ? ;-)

Bu**er