[sdiy] transformer shielding question

[Dave Kendall]

Hi All.

Am finally getting around to planning a simple 3U fixed architecture  
rack synth with an integral mains power supply. I have a 0 - 230V 50VA  
linear transformer, with 18-0-18 secondaries, and a  
regulator/distribution board which is planned to be run at ±15V.  
Details of very similar Tranny here;

http://www.rapidonline.com/sku/Electrical-Power/Transformers/Chassis- 
Transformers/Chassis-mains-transformer/82716/88-3726

Details of regulator in the first couple of pages here.

http://www.modular.fonik.de/files/EFMforum/orgEFMfiles/3500_doc.pdf

The 2200 uF caps have been replaced with 4700uF caps. I'm guessing it  
should be good for maybe up to 3/4 of an amp, based on looking at  
similar designs.

Big question is, physically how far away from the tranny do sensitive  
circuits have to be, to avoid picking up radiated hum? (mu-metal is too  
expensive for me...)
I've noticed AC adaptors (alesis Quadraverb PSU AC - AC line lump for  
example) radiating loads of hum onto any signal lines that were near  
it.

Is there any way to work out a difference, apart from "suck it and  
see"? Will a simple steel box help isolate the tranny?
I'm hoping susceptibility to hum is guided by some sort of inverse  
square law, as space is relatively tight in order to have good access  
to the PCBs when installed in the rack.

Any thoughts greatly appreciated...

cheers,
Dave