UAD Powered Plug-Ins Manual v5.2 - Just Music

This complete pallet of sonic nuance, including the dynamic input response,
is captured in the UAD Neve 33609 model. The plug-in is calibrated internally
so that 0dBFS at its input is equivalent to an input level of approximately
+26dBu on the 33609 hardware, where the coloring is more prominent. The
result is that a typical signal within a DAW will drive the UAD Neve 33609
into these “virtual” higher levels, resulting in fairly high amounts gain reduction.
Headroom Switch
The Headroom switch is provided to accommodate applications where high
amounts of gain reduction are not desired. Headroom simply lowers the internal
operating level so that the plug-in is not “pushed” into gain reduction as
much.
Headroom can be set to 22db, 18db, or 14db. At 22dB, signals will push the
plug-in into gain reduction (and more non-linearity and “good” harmonic distortion)
more easily. Set the switch to a lower value when less gain reduction
and color is desired.
The numbers on the switch indicate where 0dBFS falls relative to nominal
+4dBu. For example, with 22dB of headroom, 0dBFS corresponds to +4dBu
+ 22dB = 26dBu. With 18dB of headroom, 0dBFS corresponds to +4dBu +
18 db = 22dBu. The headroom selected will cause the plug-in to behave as
though it were a hardware 33609 connected to a nominal +4dBu interface
with the selected amount of headroom. Industry standards for most DAW interfaces
are +14dB and +18dB headroom. The +22dB setting approximates
some analog mixing environments, and allows the entire useful dynamic
range of the 33609 to be exercised.
The following settings are application guidelines for the Headroom switch:
22dB
Typical starting point for individual track inserts where maximum gain reduction
is desired. This setting is equivalent to having a real hardware 33609
connected to a +4 interface with +22dB headroom.
18dB
Typical starting point for buses and groups where nominal gain reduction is
desired. This setting is equivalent to having a real hardware 33609 connected
to a +4 interface with +18dB headroom.
UAD Powered Plug-Ins Manual - 164 - Chapter 17: Neve 33609 Compressor