Thermionic Culture THE PHOENIX SB Stereo valve compressor - Manual (Page 4)

©Thermionic Culture Ltd, July 2011 3
1 Introduction
The Phoenix is a stereo compressor which can be used as a
pair of mono compressors. It has a ‘soft knee’ or ‘variable
mu’ characteristic in that the compression ratio increases
with the amount of compression being used. Initial
compression is 1.2:1 increasing to 5:1 at 15dB compression.
The Phoenix is being updated continually and this unit is the
‘2011/12’ version (suffix QSB) with sidechain filter and
standby switch.
The sidechain works by filtering out bass frequencies from
the compressor's detector circuit and this makes the
compression less sensitive to the low frequencies present in
the audio signal.
It is due to the nature of bass frequencies that this is a
desirable effect. Bass frequencies tend to contain a lot of the
energy present in a full band signal and large fluctuations in
the amplitude of these frequencies can tend to dominate the
performance of the compressor.
A classic example is found when using a compressor over
the mix buss. If the music contains a bass instrument that is
not in all the way through the mix, it is noticeable that the
overall volume of the other instruments in the mix can go
down when the bass instrument is introduced. This is
especially noticeable when using heavy compression.
The sidechain can also be useful on single instrument
sources, perhaps a vocal that needs to be dynamically
controlled in the mid-range frequencies for example.
Another useful effect of the control is that it is possible to
make more use of the faster attack times available on the
Phoenix. When the sidechain control is activated the
Phoenix will be responding to the higher frequencies. As
these frequencies are commonly associated with transient
responses it is now possible to speed the attack time of the
Phoenix up so that it predominantly compresses these
transients.