API Audio 1608 Recording Console - Manual (Page 13)

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The controls for the 548B Preamp section function as follows:
GAIN: Provides up to +65dB of gain for the Microphone and +45dB for the Instrument Inputs
Line Input: The gain control does not affect the Line input
48V: Provides 48 Volt Phantom Power to the MIC INPUT XLR connector on the rear panel
 Illuminates in red when engaged
PAD: Inserts a -20 Pad for the Microphone or a -6dB pad for Line inputs
 Illuminates when engage
Ǿ: Inserts a Polarity Inverter (Phase Reverse) for the Microphone and Line inputs
 Illuminates when engaged
No Buttons Engaged: Line Input selected
MIC: Selects the Microphone Preamp as the input for the channel
 The MIC button must be engaged for the Instrument Input to
be used
 Illuminates in blue when engaged
 The MIC button provides a peak Indicator by turning red
whenever the peak threshold is crossed
Note: The signal from the Microphone Input is replaced when a
1/4” plug is inserted in the Instrument Input jack.