Ground?

This is a segment of a schematic of a Yamaha receiver:
http://www.tinyuploads.com/images/8LzeOF.jpg
My question is about the 4 diodes in mid screen, 2 pairs, each pair
back-to-back.
What purpose do these diodes serve?
The conductor common to all diodes runs to the left and connects to the shield on all low-level RCA output connectors.
The conductor running to the right of the upper pair of diodes serves as ground for some circuitry, as you can see, and is also connected to ground at the bottom edge of the board, center.
Here's where the ground conductor originates (seen at bottom of previous schematic):
http://www.tinyuploads.com/images/8O0dYH.jpg
The conductor running to the right of the lower pair of diodes runs to the right and connects to a terminal labeled "Earth" (top left corner in the following schematic):
http://www.tinyuploads.com/images/zTnLl1.jpg
These all serve as ground and/or earth and are connected together though these diodes: 3 conductors (analog ground, earth, output connector shield.
Why the separation via diodes? What purpose do these diodes serve and how?
Thanks, Dave
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Amplifier stages may have a separated ground to prevent ground loops in external cabling from inducing hum. The diodes serve to clamp the separated grounds at no more than the diode forward voltage drop (typically 0.6V). There's probably an interesting coupling between the separate grounded stages, possibly using op-amps to cancel out any differential signal between the two grounds.
Difficult to be sure without seeing more of the schematic.
--
Andrew Gabriel
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Why is R346 there ?
Is the mains plug assumed to be polarized ??
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