Pre-Amp & Power-Amps chasis

Well, I'm building my pre-amp chasis now and intend to make a power-amp chasis similar when I finish my power-amp... It's basically made from aluminium, is this half descent for sheilding? Does the chasis have to be grounded for sheilding's sake, or just for safety's sake? That's all I need to know for know ;), thanks again to all those that answer my horribly pathetic questions...
Hi JoeBob.
I guess You're talking about RF-shielding.
In that case ;) You shall make the holes in it as small as possible, have a small distance between the screws, and use a good RFI-filter on the powercord.
If You're using ballanced signals, You can use a shield-driver too.
It's certainly not a pathetic question. RFI is a complex science, an a very impredictable one.
Hi JoeBob.

Agree with all my colleagues ;). Al is OK for RF shielding, but is not an efficient shield against stray fields (magnetic coupling). You should prefer steel for this purpose. Futhermore, take care when making electrical connections directly to aluminium, 'cause, in its bare state, it oxydizes very quickly on its surface, and aluminium oxyde is not a conductor at all :). When anodized, just sandpaper the connection location and quickly connect your wires

So, best (for a preamp) is to house the power trafo and the rectifiers/supply capacitors in a separate (steel) box, to only have to deal with DC supply voltages inside the preamp (regulators, additional filtering, etc...)

A lower cost solution is to shield the power trafo with a small steel box.

Hope this helps
I feel quite strongly that the shielding should be
separate from the chassis ground, since the latter
must always be connected to mains earth for
safety. If you have signal ground connected to
mains earth within each box of your system you
open yourself up to hum loops. The shielding is
best connected to the local signal ground through
a low-impedance link.

In my own system I use a separate copper shield
around the high-gain circuitry, and connect this
to the local ground reference, but leave it
insulated from the chassis. For RF shielding you
really need solid copper anyway, rather than
aluminium, since the latter has a rather lower



2000-12-31 9:31 am
Fortunately, rf pickup has not been a big problem for me in audio. However, in my real job it is, and I found copper is much better rf shield than Al however it's a real pain to work with.
You should definitelty figure out from where the rf is leaking in. Try to use a AC line filter and see how things sound. Real rf shielding is very complex, you have to really start talking about wave guides for all your wire connections entering and exiting the box. If you do things right with copper you can obtain rf shielding up to 100 dB (measured from signal averaging).
Well, for my pre-amp, I'm using a "plug-pack" transformer, so it won't be in the case. And I'll be using a seperate box for my power-amp power suply... I'm pretty much finished my pre-amp box, but if it needs more sheilding, would copper foil added to the interior work well? The only metals I have available to me (ie get free) is aluminium and brass. Maybe I should go with brass for my power-amp and power-amp power suply? Brass is an aloy of copper and a few other things if I'm right, but I dunno if it's a good shield...


2000-12-31 9:31 am
brass is good, and easier to work than Cu.
I am intrigued by the 'it needs more shielding' statement especially in conjunction with the separate box for the PS in the power amp. Can you hear the radio in the speakers? Or are you referring to some type of hum you are hearing.
Well, I haven't finished the boxes yet. I'm working on them now. I was just asking that if I could hear hums or interferences (sp?) what would work best to correct it. But I'm hoping for the pre-amp, plain old aluminium will do good enough (for that's what I've started with), but I'll go with brass tomorrow when I build the power-amp and power-amp power suplies then...


2000-12-31 9:31 am
hums will probably be cured by better grounding schemes rather than shielding. I would build one box for the powerAmp, there is just too much power loss in going with high power connections outside the PS box and into the circuit box. Separate PS maybe smarter for the preamp. If you are worried about transformer noise you could do something cool that is to build a brass sub-enclosure for the transformer and stick that into the reg. Al power amp box.