B&K Reference 4420 Owner's Manual page 10

Balanced - XLR type connectors accept line input from the preamplifier's balanced
output connectors. There are two input connectors, one for each
channel, that may be used to connect the amplifier to the preamplifier.
Balanced cable connector
Operation - For balanced
operation, ensure the balanced switch between the balanced inputs on
the rear of the amplifier is in the 'on' (in) position.
Here is a typical input setup:
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