Pre-Fade Listen (PFL)
PFL usually stands for “Pre-Fader Listen”.
PFL is a combined channel and monitoring function. On an analogue mixer with PFL, each channel has a switch which can connect the channel signal path, at a point just before the fader, to the pre-fade bus. This bus is picked up in the monitor module and made available as an alternative signal to the mixer output. On some early mixers the PFL had to be selected by a switch on the monitor module; automatic PFL switching is now almost universal, effected by an additional bus carrying a logic voltage activated by the channel PFL switch. This triggers an electronic switch in the monitor module, selecting the output of the PFL bus to the monitor path whenever PFL is selected on a channel.
Some mixers have a small integral loudspeaker which is dedicated to PFL (or sometimes shared with Reverse Talkback which over-rides PFL via an electronic switch). PFL always appears on this loudspeaker (unless Rev. T.B. is being used) and also on the mixer headphone output, but may not appear on the main monitor loudspeakers until ‘PFL to LS’ is selected.
If the mixer is fitted with meters wired to follow the output of the monitor module, these will show the level of the pre-fader signal. Some mixers have a dedicated meter for PFL which is only active when PFL is selected.
In an analogue stereo mixer, the signal in a mono channel does not become ‘stereo’ until after the panpot, which is usually after the fader; if the panpot is before the fader a stereo fader must be used, which adds to the cost. Stereo PFL also meant additional cost, and was therefore regarded as rather a luxury for some years, with most stereo mixers having a mono PFL bus which used a derived mono feed from any stereo channels.
PFL is used to check the level and quality of a signal before adding it to the mix by ‘fading it up’. It is primarily a broadcasting function, allowing the operator to adjust the channel input gain control, and the equaliser, if necessary, so that a signal which has not been heard during the rehearsal will be at approximately the correct level and of acceptable quality when faded up into the mix ‘on air’. PFL can also be used to pre-hear an Outside Source to check it is “safe” to fade up or to hear a response to a question asked over a Talkback or Cue circuit.