Rupert Neve 543 - 500 Series Mono Compressor-Limiter - Manual (Page 7)

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The way in which these modes change the dynamic performance can be seen in the above graph -
but the more interesting effects are noted by listening - “Feed-Back” produces a sweeter, warmer
sound but is not as accurate if you need to protect a transmitter, for example.
543 FEATURES
THRESHOLD
Sets level where the compressor may begin to react from -30 dB to +20 dB. Minimal or no compression
is with this control fully clockwise and it gets more sensitive and tends to cause more gain reduction as
the knob is rotated counter-clockwise (which may be counter-intuitive to some).
RATIO
Sets the “slope” of the compression from 1.1:1 (minimal) to 40:1 (Limit) . For example if this knob is set
for 3:1 then if the signal goes 15 dB over the threshold then it attenuates 10 dB and allows the output to
rise 5 dB. In general, low ratios can not damage the music as much as high ratios but high ratios may be
more useful to minimize clipping and OL lights in the recorder.
Excuse the front panel jump and digression, but it is appropriate to mention the GAIN REDUCTION
and OUTPUT meters now because so many of us learned to set up compressors by watching the all-
important GR meter. When setting the Threshold and Ratio it does help to use the meters however, as
always, we should rely on our ears and ye olde “COMP IN” (Bypass Button) most of all. One might try
to interpret the OUTPUT meter as one exercises ye olde “COMP IN” button too, as this might be more
appropriate to your goals than a target of X dB or number of LEDs of compression. Is the goal to make
great sounds or to blink LEDs half way down the meter because you read it somewhere?
COMP IN
The compressor section is engaged with this button in. This may be the most useful control on the
compressor because it is there for “confidence checks”. In particular, exercise it in both the quietest
and the loudest sections of the song. Watch out for the compressor exaggerating noise and room sound
in the quiet sections and chopping off transients and consonants in the loud sections, either of which
suggest maybe too much compressor “action”. Conversely, losing quiet phrases or inadvertent clipping
might suggest that a little more compression is warranted keeping in mind that you can always do a bit
more in the mix but un-doing over-compression is not fun and often not even possible.
FF/FB
These are two very different compressor modes, FEED BACK and FEED FORWARD. Vintage compressors
almost always were FEED BACK designs which means they looked at the output after the VCA (Voltage
Controlled Amplifier) or gain changing element and used that signal to control the attenuation. FEED
FORWARD compressors became popular in the 80’s and read the signal before the VCA and through
more elaborate electronics to control the attenuation because this has some advantages in regards to
the ratio and control timing. Perhaps a more useful generalization is that the FB mode (button in) tends
to sound smoother and often more natural and tends to be quicker to set up. The FF mode can be more
useful for shaping the envelope of the sound and introducing more bounce and pumping in time with
the song, when that is the goal. FF compressors were often used on 80’s dance tracks. Some engineers
prefer FB with lower ratios and FF with higher ratios.