Focusrite Scarlett 18i8 Manual page 38

Controls
The HeadAmp4Pro allows you to control the volume level of each headphone output
independently, as well as to listen to or talkback to individual headphone outputs directly
form the control room.
On/Off
button
XLR input for the
talkback mic – connect
the built-in microphone
in the control room.
Additional Resources
Read the user manual:
https://artproaudio.com/framework/uploads/2018/06/HeadAmp4Prp-v103-6-21-18.pdf
Press and hold to talkback
or listen to ALL outputs.
Talkback mic
volume
Press and hold to talkback or listen
to the corresponding output.
Volume dial for corresponding
headphone output
38