PFL means Pre-Fade Listen. It's function is to do exactly that — listen to the channel's audio at a point before the fader takes effect. The PFL button is usually located just above the channel fader. In the example, it's the red button (the red LED lights when PFL is engaged).
Note: PFL is often pronounced "piffel".
When you press the PFL button, the main monitor output will stop monitoring anything else and the only audio will be the selected PFL channel(s). This does not affect the main output mix — just the sound you hear on the monitor bus. Note that all selected PFL channels will be monitored, so you can press as many PFL buttons as you like.
PFL also takes over the mixer's VU meters.
PFL is useful when setting the initial input gain of a channel, as it reflects the pre-fade level.
PFL vs Solo
PFL is similar to the solo button. There are two differences:
PFL is pre-fader, solo is post-fader (i.e. the fader affects the solo level).
PFL does not affect the master output but soloing a channel may do so (depending on the mixer).
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