Yamaha P-150 Owner's Manual page 19

Connect external line-level components.
4.
If you wish to monitor the P-150's output using an external line-level mixer or
amplifier, connect the input of the external component to the P-150's
[OUTPUT] jacks on the rear panel. (Use both jacks for stereo output; use the
[L/MONO] jack for mono output.)
If you wish to listen to your P-150 using headphones, connect a pair of
stereo headphones to the [PHONES] jack located on the left side of the front
panel. The P-150's internal speakers will automatically be disconnected
whenever headphones are plugged into the [PHONES] jack.
If you wish to monitor the output of an external line-level device (such as a
rhythm programmer, tone generator or synthesizer) via the P-150's internal
speakers, connect the output of the external device to the P-150's [INPUT]
jacks on the rear panel. (Use both jacks for stereo input; use the [L/MONO]
jack for mono input.)
Speaker (L)
Amp
OUTPUT L/MONO OUTPUT R
P-150
P-150
Tone Generator
INPUT L/MONO INPUT R
P-150
Speaker (R)
15
Before connecting the P-150 to any
external device, be sure that the
power switches of all devices are
turned off.