Best way to shield a preamp

[fryingpan]

What about the antiparallel diodes + resistor + cap to ground? This should provide a clear pathway to ground for high frequencies, whatever their amplitude, while low amplitude DC and low frequency signals are basically (partly) blocked by the diode + resistor combo. Why not just connect everything to ground directly?

[Rob Strand]

What about the antiparallel diodes + resistor + cap to ground? This should provide a clear pathway to ground for high frequencies, whatever their amplitude, while low amplitude DC and low frequency signals are basically (partly) blocked by the diode + resistor combo. Why not just connect everything to ground directly?

If you wire to ground you end up with ground loops.   The network presents a higher impedance than the wires.   The signal wires are wired directly as you would expect but the ground is "sort of" broken by the network.  If you get ground loops the hum voltage appears across the network and not between the connections on the signal wires.  If there was no network the hum voltage has to appear in the signal wires and grounds.  As soon as it appears across the signal wires it becomes part of the signal and you hear hum.   The cap allows RF and high frequencies to pass to ground.  The diodes are for safety if there is some kind of mains fault to ground they prevent the ground potential rising to a hazardous level by acting as a clamp.  The diodes should be large enough not to explode and open under a mains fault.   So there's quite a few details under the hood.   Check out the schematics of some professional equipment and you will see this set-up appears quite often.

There are many pieces of equipment which don't hum and don't have that network.   It takes quite bit of care to get that to happen.  Then it takes even more care to connect to other pieces of equipment without hum - I even have doubts it would work hum free all the time.   If you go down this route and you have a problem it usually means a new PCB layout and you have to be sure the new layout is hum free.    When you use off-the-shelf modules you don't have the luxury to change the pcb layout and it becomes a big headache.  (I might add the chassis ground isn't the only problem with off-the-shelf modules.)

The short story is a lot of people who build equipment have had headaches with ground loops.   The reason that configuration is popular is it is quite a good recipe for staying out of trouble.

[fryingpan]

Yeah, all right, but then how large should the rectifier's current rating be? Say, twice the fuse rating?

[Rob Strand]

but then how large should the rectifier's current rating be? Say, twice the fuse rating?

The fault currents are enormous.    A fuse doesn't do that much to *limit* the current.   A small fuse will
just blow sooner.   This type of thing in 25A or 35A is common,

https://www.onsemi.com/pdf/datasheet/gbpc3510-d.pdf

Notice the high surge current rating.   You connect them up with the across the DC terminals
(and yes there will be two diode drops).