Drawmer LX20 Dual Expander Compressor - Manual (Page 4)

LX20 OPERATOR’S MANUAL 2
INSTALLATION
The LX20 is designed for standard 19" rack mounting and occupies 1U of rack space.
Avoid mounting the unit directly above power amplifiers or power supplies that radiate
significant amounts of heat. Fibre or plastic washers may be used to prevent the front
panel becoming marked by the mounting bolts.
INTRODUCTION
Many studios start as a small concern, but quickly expand into serious, productive
environments. Often, this means selling some of the budget equipment, in order to
replace it with equipment of the necessary quality to match the upgraded studio.
Now that you have bought a DRAWMER unit, you have a piece of equipment
which will serve your needs, regardless of the size and quality of the studio.
The compressor is one of the basic tools of a recording studio. Its function is to
reduce or ‘compress’ the dynamic range of a signal so that quiet passages are
audible whilst loud passages do not overload the recording medium, resulting in
excessive amounts of distortion. It performs this function by sensing the level of the
signal and adjusting the gain continuously, so as to give a more constant output.
Essentially, a compressor contains two basic sections: the signal path, which
contains a voltage controlled amplifier/attenuator (VCA) and the ‘side chain’, which
senses the level of the signal and provides the control for the VCA. There are many
ways in which a compressor may be arranged internally, each method producing a
different ‘personality’, thus lending itself to a particular application. A linear
compressor produces the same amount of compression for each increase in signal
level. Threshold, controls the point at which compression occurs, whereas Ratio
adjusts the amount of compression applied to the signal for each increase in level
above the threshold.
An alternative is the ‘soft knee’ characteristic. With this method, the ratio is
increased automatically as the compression increases, which has the advantage of
being easier to use as only one control is required to adjust compression. In some
applications, a ‘soft knee’ characteristic can be more acceptable than linear, but this
would depend on many factors, not least of which is the individual’s opinion.
Because compression effectively increases low level signals, it also increases the
level of background noise, although this is not created by the compressor itself. For
example, compressing vocals will also increase the level of headphone overspill,
breathing etc. This can be overcome by the use of an expander, which is the
opposite of a compressor. Signals below the threshold level are attenuated, so in
the above example, when no vocals are present, the unit will shut off, thus
eliminating the unwanted noise.
THE LX20 EXPANDER/COMPRESSOR
The LX20 is dual channel, ‘soft knee’ compressor with the addition of an expander
and is designed for simplicity of operation. It is suitable for use at system levels of
!10dBu or +4dBu, by changing internal jumper links. Power supply voltages of 115V
or 230V are accommodated via a rear panel switch. The LX20 has a side chain
access point at the rear. This is used to make the compressor section respond in
different ways to various frequencies, which is useful for ‘de-essing’. Another useful
feature of the LX20 is the mute facility. This enables each channel to be shut off