EMI FIELDS

Not by merely putting the fabric on the top of the unit. Enclosing an measurement set up in a conductive mesh (called a Faraday cage) is a good solution when dealing with low level signals. In a lab to which I provided engineering services, they measured brain potentials (signals in the microvolt range) inside a Faraday cage - doing so outside the cage in a room lit with flourescent lights was impossible - all you could see on the scope was the noise. To be effective the cage has to enclose the area of interest - not merely laying a sheet on top of the unit. You don't need a Faraday cage around you equipment. As to your reference to static electricity discharge - no the conductive sheet on top of the unit will not do anything. Of course if you are trying to keep the spark down when you walk across the room to touch the unit, you can attach a wrist strap to the chassis of the unit and simply put the wrist strap on before touching the chassis - but that is a different problem - the source of that charge is not on the chassis. Conductive wrist straps are common when working on electronic equipment and you can likely find them at any electronic supply house.

A correction to my post of 2 minutes ago. With regard to the wrist strap, for your application do not attach the wrist strap to the chassis of the unit. Rather attach it to ground at some other point. Of course you really don't need the wrist strap, just discharge yourself by touching ground at some point other than the chassis before touching the chassis (obviously this other point needs to be physically within reach without walking across the carpet again).

What you did was stop the interference by shielding the source of the interference rather than shielding the part affected. This is the preferable solution in the case where you have identified the source. From a practical standpoint, if it works as is - no it doesn't need to be grounded - you have achieved what you set out to do. If you continue to have problems, ground the conductive sheet. The question then becomes how to ground it - at one point or more. In your application grounding at more than one point should not be a problem as you aren't going to be concerned about ground loops. But, since you have solved your problem, I would not go any further. I am assuming that your interconnection between the amp and preamp is not balanced. If you the capability of using balanced interconnects then you may want to do so and thus be able to remove the conductive sheet you are now solving the problem with. If you do so, attach only one end of the shield of the interconnect to ground - at the preamp end.

Imnewton: A good source of information for shielding and grounding is "Noise Reduction Techniques in Electronic Systems" by Henry Ott. I had his first edition on my desk for a decade and bought his second edition after that. It is not a new book (copyright 1988 for the second edition) and I don't know if he has a third edition. It is practical and not math intensive- a basic high school math education with an understanding of logrithms and introductory calc is more than enough to understand the explanations, and even that is not essential.