Boulder Amplifiers 1012 DAC Preamplifier - Manual (Page 16)

CONNECTING TO AN UNBALANCED INPUT AMPLIFIER
A special cable is required to make this connection. This cable
connects pin 1 to the shield and pin 2 to the center pin. It leaves the
output pin 3 unconnected.
Connecting the unused output pin (usually pin 3) to ground will
cause excessive ground currents and degrade performance. Use an
ohmmeter or continuity checker to determine how a cable is wired.
GETTING
STARTED
1-10
UNBALANCED OUTPUT CABLE
3-NEG OUTPUT
2-POS OUTPUT
1-GROUND
LINE
PREAMP
INPUT