Boss AC-3 Owner's Manual page 9

When Connecting Both to a Guitar Amp and PA System (or Recorder or Similar Equipment)
9 With Effect On
The CHECK indicator lights, the output from the AC-3: G.AMP OUT jack is muted, and the effect sound is output from the LINE OUT jack.
PA System
Recorder
Guitar Amplifier
Effect sound
Mute
Multiple Effects, etc.
Connections
AC Adaptor
BOSS PSA-series
(sold separately)
Electric Guitar
9