# Amp & Computer - distance to magnetic interference?



## andguent (Mar 21, 2004)

Hey all,

My audio booth is very space constrained, and so I have possible future problem that I want to avoid. I have bought myself a Raxxess econo more-permanent-than-not cube to mount my rack equipment. I would also like to buy a rack shelf and slide my PC in the shelf on its side. How far away does my hard drive need to be from a 1000w amplifier and not worry about EMI corruption? 

I would assume that with other equipment like an equalizer, wireless mics, and a dvd player in the way, they would shield it, but I hate to assume. I would greatly appreciate advice from anyone experienced with the EMI resistance of an average IDE hard drive. Thanks.


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## DMXtools (Mar 21, 2004)

I'd be a bit leery about putting a PC in the same rack with the audio gear. I'd worry more about digital noise from the PC getting into the equalizers, DVD player and wireless receivers than about the magnetic field from the amp's power transformer erasing the hard drive on the PC. Whether you'd actually have a problem depends on your particular PC, amp, EQ, DVD player and wireless receivers, but if I was in your shoes, I'd look for some other option to solve my space constraints.

John


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## soundman (Mar 21, 2004)

I think you can a matting to sheild from interfence, or some cases come with it http://www.cluboverclocker.com/reviews/cases/casemaxx610-g/


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## Nephilim (Mar 22, 2004)

HDs are fairly resistant to EMI - you need a relatively high power high-frequency fluctuating field to overpower the head signal. Thus the metal casing on most HDs. But, safe side, space it as wide as possible. The only component in a PC that might interfere with other gear is the PSU.


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## BenFranske (Mar 22, 2004)

> HDs are fairly resistant to EMI - you need a relatively high power high-frequency fluctuating field to overpower the head signal.


You do need a strong field to overpower the head signal; however, over time any magnetic field will kill a hard drive. I had a computer at a client's with a fridge magnet on the side that HDs never lasted more than a year and a few months in before dying. I'm now pretty careful to keep even mild, magnetic fields away from HDs.


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## dj_illusions (Mar 22, 2004)

At my work, we have two pc's in the booth for whatever they need to be used for. We ran all the computer gear off a spare power circuit that we had laying about and got some monitor extension leads. We put all the pc hard drives together in the corner, on the opposite side of the box and ran all the extensions around to the monitor and mouses etc. 

this was mainly done cuz of space and someone suggested that it would be easier, but it works great!


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## Mayhem (Mar 22, 2004)

dj_illusions said:


> this was mainly done cuz of space and someone suggested that it would be easier, but it works great!


Good idea and KVM switches are great for this.

I have found that (in the few times I have had a lap top close to sound gear) the interference is from the ?CPU ?crystal - I am not sure how to accurately explain it but it seemed that if the computer was thinking hard there was a burst of noise through the system. Very similar to what happens when a mobile phone is used too close to computer speakers.

VHF wireless systems were especially prone to this.

I hope that this makes sense!


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## Nephilim (Mar 22, 2004)

Not from any crystal, just from the CPU/chipset combination, they're pretty leaky in laptops because there isn't nearly as much shielding in most laptop casings.


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## Mayhem (Mar 22, 2004)

Thanks for clearing that up Nephilim. Anyway - as I have said in a previous post, my personal preference is not to use computers for shows. I just think that there are too many variables that can go wrong. But - each to their own.


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## andguent (Mar 22, 2004)

Well I thank you all for the advice. I think I am going to go ahead and risk it, and just be more persistant about backups. Whether I get noise in the system or not will be a question easily answered.

I am heavily leaning towards the near future purchase of a Carvin 500wX2 amp. It is a bit more power then I need, but it's nice to have flexibility.

Does anyone have recommendations for an online store with rack mounting hardware like shelves? How about EMI shielding material just incase?  Thanks all.


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## Mayhem (Mar 22, 2004)

As I am several thousand miles away I am not going to be much help when it comes to an online shop that will be beneficial to your location. However, I would suggest looking at the accessories and cases the SKB produce http://www.skbcases.com/home/index.html 

I tend to build my own rack mounted shelves and drawers as I find it much more cost effective and not all that difficult. Besides, there are some instances where you can never find exactly what it is that you want.

In many cases you can get the materials free from metal fabrication workshops as the off cuts that they throw away are ideal for what is needed. I am not sure what the situation is in the US but here you can get almost anything done for a case of beer. I often get things bent up and pressed at a local fabricator and the most he has ever charged was a case of beer. 

FWIW – I usually remove the screw on feet of non-rack mounted equipment and then screw thee equipment to the shelf via these existing holes. This saves on having to drill holes (in which you have to be very careful as the metal filings can cause shorts if you don’t clean out the case properly), or messing about with adhesive backed Velcro. 

For drawers – I do is bend up 2 flat strips of metal strap so that it forms a very wide U shape. 

The length of the two upright sections of the U will be the same width as a standard rack mounting tag and the length of the bottom section of the U will be the same as the distance between the outer faces of the rack-mounting strip (front and rear).

This then gives you a flat section of metal that sits flush with the wall of your rack (as most racks have walls that are not flat, you can use the ‘pockets’ created by the extrusion as the space for the screws, bolts, rivets to poke through).

The next step is to fix the runners directly to the flat section and make the draw as you normally would.

For a shelf – I make the same metal strip that I have described above but instead of bending both ends upwards; I bend one up and one down. 

The next step it to cut a length of angle the same width as the rack MINUS 2x the thickness of the metal you are using and any distance that your metal strap sits away from the wall of the rack. Fix this to the 2 strips of metal to form the back support of the shelf. 

The next step will vary depending on what you want to do and what materials you have. Sometimes I will a piece of sheet metal bent up to form the base of the shelf, in which cases the two sides (the are bunt up at 90 degrees to the base) will be fixed to the metal strip and the rear edge fixed to the angle. Some times I have use a thin piece of plywood as the base, in this case I would put a couple of off-cuts of angle on to the metal strip to make supports/mounts. There are a couple of ways to do it and it really does come down to what you want to achieve and the materials you have at your disposal.

Some tips:

1.	Do as much of the drilling that you can do before the material is cut/bent as this will make it easier.
2.	Use Aluminium angle as much as possible as this will reduce the weight and doesn’t rust – you can often pick up off-cuts and short lengths from window/shower screen manufacturers for free.
3.	If using plywood for the bases, you can cut sections out to reduce the weight and improve airflow. I tend to use a 2” hole saw for this.

Hope that this is helpful. It may sound like a lot of messing around but for what we have to pay here for shelves and drawers – it is well worth it!


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