Case / housing material choice - non ferrous?

[woolly]

Everyone seems to be using non ferrous materials for cases and heat sinks. Can someone explain what is the reason for this?

1) It is more expensive and weaker...
2) Heat transfer must be similar to ferrous.
3) its is obviously not an electrical insulator, so must be the non-magnetic thing.
4) Is it to avoid the case behaving like an aerial? surely the signal/noise would be small and most times the case is not electrically connected to anything except gnd.
5) Or is it to absorb noise from the circuitry? wouldn't iron do that too? if not better.

In the process of building a chipamp, mostly wood structure and covering, with plans for some (ferrous/non-ferrous) metal plate to block interference.
thanks in advance for help.

[OzMikeH]

1: yes, but aluminium can be left raw without painting, it is easier to cut and drill, is cheaper to ship because it weighs less. so Steel might not actually be cheaper overall...

2: Copper Bus bar as used in big electrical switchboards makes wonderful heatsinks, aluminium conducts heat much better than steel, and again we come to the weight for shipping. When choosing heat sinks there are 3 things you need to consider, how much heat you are going to put into them, thermal resistance (the result of thermal conductivity and thickness) and surface area, lots of surface area...

3: magnetic fields from transformers etc are no big deal, Unless you have a lot of really high current power cable with variable load nearby you don't need to worry about magnetic interference. only ferrous metals influence magnetic fields.

4: any conductor can be an aerial, I have even seen certain species of trees with a wire stabbed in them used as antennas. A completely closed conductive box that is earthed is called a Faraday cage, it stops radio signals from getting inside. It only works when the wires going in and out have low pass filters on them, otherwise the radio interference still gets inside along the wires. Radio interference is often not much of a problem unless your neighbour is a ham radio freak.

5: now we're talking about FCC and CE standards, commercial equipment should not cause harmful interference with other equipment.

I wouldn't bother with the metal plate if I were you, if you do use any you need to earth it well.
Just use shielded wires for low level signals, route your power supply wires away from your low level signals, twist your DC power supply wires together. twist your speaker wires together (but away form the power supply) and you'll be fine. If your amp is all wood with exposed heatsinks you should still earth your heatsinks, just in case there is a fault with your mains wiring and it touches the heatsink.

[woolly]

thanks very much mike. thats really useful. so its more about practical and cost than anything technical. im gonna go with all wood (i like working with it and more eco too i think) , twisty wires, copper heatsink, and just a metal screen around power supply just so i feel happier knowing there is something to stop any noise.