This applies to the Scarlett 18i20 1st, 2nd & 3rd Gen

The headphone outputs on the 18i20 share their output with some Line Outputs:

• Line Outputs 7/8 for Headphone Output 1 Left/Right
• Line Outputs 9/10 for Headphone Output 2 Left/Right.

This means anything assigned to Line Output 7 in Focusrite Control/MixControl will come out of output 7 on the back of the unit, as well as the left headphone channel for Headphone Output 1. The same goes for Line Output 8. Output 8 on the back and the right headphone channel of Headphone Output 1.

Line Outputs 9 and 10 match the left and right headphone channels of Headphone Output 2.

In the example below, you can see:

• DAW Playback channels 1/2 have been assigned to Line Outputs 7/8, also headphone output 1
• DAW Playback channels 3/4 have been assigned to Line Outputs 9/10, also headphone
  output 2. 

Why is the monitor dial controlling the headphone output level?

In Focusrite Control and Scarlett Mix Control, it is possible to assign the control of the monitor dial to the headphone outputs. This is useful for multiple monitoring purposes but can be annoying if you're using your headphones outputs. 

To stop this from happening:

• In Focusrite Control: Open Settings > Set 'Monitor Controls'* to the monitor outputs you are using (for example, Monitor Controls 1-2 means the monitor dial controls only line outputs 1-2).
• In Scarlett Mix Control: In the Monitor Preset section > Shift + Click* your headphone output channels so they are grey. When they are dark blue, the monitor dial controls them.

*Changing the monitor control will cause your output to jump to full-scale, it will be very loud, so be sure to turn down your headphone dial first.