Sound Buzz Advice

[pete10uk]

Evening all,

 

I'm currently working on a meeting room install, and have an issue with an audio buzz which is proving illusive. I wonder if anyone could come up with some suggestions?

 

 

The first thing I will say is all the audio cable is van damm screened single pair audio cable.

I have an audio matrix switcher with 6 inputs (5 used all unbalanced) and 6 outputs (4 used all unbalanced). 3 of the outputs go to 3 different amplifiers 1 sited about 2m away from the switch and the other 2 are about 20m away in different directions, the 2 amplifiers furthest away have quite a bad buzz on them and the one closest has a very slight buzz. All the cables are bunched together along on a single run with one of the furthest away amplifiers running a good length with power and 2 of the inputs.

 

The confusing thing is that the buzz is always the same no matter which input is used I.e the local amp's buzz is always quieter than the ones at distance so I had assumed that the interference / problem was on the output of the matrix to the amps? To test the system I removed the matrix and the buzz was still there, I concentrated on the amp with the longest run at the side of power and replaced the cable for one which went no where near the power run, no change. I then tested the amp with a local input from my phone and all was well, no buzz.

 

All power is on the same phase.

 

I'm at a loss as what to try as it seems as if it is the input cable to the amp but I have changed it.

 

Any ideas on what to try or what it could be?

 

Cheers

[boswell]

I think the salient point here is that it is all un-balanced and long runs. See if the amps/mixer will accept balanced or fit Txs at each end.

really need more detail of makes/models before commenting more.

[pete10uk]

I think the salient point here is that it is all un-balanced and long runs. See if the amps/mixer will accept balanced or fit Txs at each end.

really need more detail of makes/models before commenting more.

 

The amps will run on a balanced signal and the switch will accept balanced signals but the sources are all unbalanced, I would insert 8 Di boxes if I were doing a live gig but I'm not familiar with install options of these, I don't have the space or budget to have what I call DI boxes. Is there any small install versions of there?

[Jivemaster]

You can buy transformers to convert un to Balanced for transmission, RS do them and I suspect the other usual suspects do too. However before you buy transformers do some really serious diagnostics.

 

Isolate bits of the system and check for interference in each piece, then possibly swap a DI box in and see whether that improves things.

 

The unbalanced circuits could be the problem, so could interference from dimmed lights, so could odd earth loops or supplies on different phases in different places

[boswell]

If the switch will output balanced and the amps will accept balanced inputs and you are using screened 1 pair mic type cable then convert it to balanced at zero cost.

Then see if the buzz is still there! after that look at fitting a di near the source to convert to balanced to go into the switch.

[pete10uk]

If the switch will output balanced and the amps will accept balanced inputs and you are using screened 1 pair mic type cable then convert it to balanced at zero cost.

Then see if the buzz is still there! after that look at fitting a di near the source to convert to balanced to go into the switch.

 

The switch will not convert the signal, it will only switch either a balanced or un balanced. The unit is a Kramer Vp-66eth, Ive checked with kramer and they have confirmed that it will not convert the signal.

 

It would have been nice though.

 

Any suggestions on the model of converting transformer?

[paulears]

Moderation: Topic title fixed

[Lamplighter]

My first move would be to temporarily run all the amplifiers from the same power socket, as this MIGHT solve the problem without needing to balance the lines. I would always strongly recommend the use of balanced lines, but sometimes you can get away without on an existing installation. It is one of my pet hates coming across long unbalanced circuits , installed by people who should know better.

Brian

[Robin Pratt]

As others have said balancing is most likely to be the key to this, you can either get loose transformer from the likes of RS, CPC, etc. and wire them up yourself, or buy them as ready made boxes with connectors fitted.

 

We sell these http://www.10outof10.co.uk/acatalog/GreenH...ring.html#a7886 which will almost certainly do the job, or there are various other shapes and sizes available from others that all do pretty much the same.

[ramdram]

Unless I missed it, so apologies up front, have you checked each and every cable to ensure the shielding is connected to the connectors properly. I appreciate your use of quality cable but it is the "quality" of the connector/cable joint which sometimes leads to probs.

 

When you say buzz do you mean from 50Hz mains?

 

When you say all the cables are bundled together did you mean the mains and audio are running beside each other?

 

You might consider running the cable not in a bundle.

 

You mentioned the buzz was loudest on the longer runs...longer cable producing louder interference might suggest some sort of increased "pick up" of unwanted signal.

 

What sort of lighting fixtures are in the meeting room? Are there any other electrical items in the room near to your cable? If so does said kit conform to the regs?

 

You might want to try switching off/on/off/on any other kit, including lights, in the room.

 

You may also want to try each item of kit on its own.

 

Some links which might help you:

 

http://answers.yahoo.com/question/index?qi...04123319AAfVCBM

 

http://en.wikipedia.org/wiki/Electromagnetic_interference

 

possibly the best link(?):

 

http://www.soundonsound.com/sos/jan02/articles/faq0102.asp

 

The above will hopefully help you first of all identify the source of the interference and then, even more hopefully, ** laughs out loud **, help you eradicate it.