Abbingdon Music Research AM-77 Reference Class - Manual (Page 48)

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6.2 HiFi/Pro Switches for Balanced XLR inputs
For the majority of sources, the HiFi/Pro switch (R3) is best left in the ‘HiFi’ position. However, if the source has
a pro-audio style transformer balanced output, the HiFi/Pro switch needs to be set to ‘Pro’.
A transformer balanced output uses a floating secondary from an audio transformer: unlike electronic balanced
outputs of any description, the signal is NOT referenced to ground (Pin 1). For such (rather rare) equipment, the
‘Pro’ switch is engaged to short-scircuit Pins 1 & 3, thus establishing the ground connection.
‘Pro’ switch is for the
few pre-amplifiers that
have floating trans-
former balanced
outputs