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

In stereo operation the controls for channels 1 and 2 are independent and
can be set separately. Generally, the channel with the “most processing occurring”
controls the processing for the other channel. For example, if the
same signal is fed to both channels in stereo mode and channel 1 has a lower
threshold setting than channel 2, the channel 1 threshold value is used for
both channels. Similarly, if channel 1 were disabled (using “In” switches),
channel 2 settings would be used because “more processing” occurs with the
channel 2 settings. It's not always so simple though, as in the following cases:
• If you feed the same signal into both channels, you can have a lower threshold
with a lower ratio on one channel, and a higher threshold with a higher
ratio on the other channel. In this case, you will get a double knee, with the
lower ratio being used between the knees, and the higher ratio above both
knees.
• If you feed the same signal into both channels, you can have a lower threshold
with a faster release on one channel, and a higher threshold with a
slower release in the other channel. In this case, you will get a two-stage release
after a transient, with the first channel releasing at the fast rate until
you get down to where the other one is; then the release will continue at the
slower rate.
Gratuitous Question
Is there any reason I would want to use stereo mode and still have the settings
for the two channels different?
Yes. Linking the sidechains simply prevents left-right image shifting. Threshold,
attack, and recovery can be set independently to cause the compressor to be
more sensitive to instruments which are panned to one side or the other. Output
controls can be set separately in order to correct an overall image shift at
the output.
Headroom
Background
The hardware Neve 33609 can accept an analog signal level of approximately
+26dBu before undesirable signal clipping occurs. As the signal increases
up to this point however, desirable audio-path nonlinearities and
“good” harmonic distortion characteristics occur. This musically pleasing
“warmth” at higher levels is what gives the unit much of its revered sonic character.
Because analog mixing consoles can typically output high signal levels,
audio engineers often take advantage of the ability to “push” the hardware
33609 into the colorful arena.
UAD Powered Plug-Ins Manual - 163 - Chapter 17: Neve 33609 Compressor