INTRODUCTION
The T L Audio Indigo Series combines classic valve techniques with low noise solid state circuitry to
produce audio processing units offering very high quality signal paths with comprehensive control facilities
and the unique valve sound.
The C-2021 is a two channel valve compressor. It may be operated as two independent mono channels or
as a full stereo compressor, with ganging of controls and combined control voltages. All parameters of the
compressor are variable, and a bargraph meter may be used to display output level or gain reduction.
The block diagram of the unit is shown in fig.1. Each channel has three input sources - a balanced line level
input via an XLR connector, an unbalanced line level input via a jack socket, and an instrument input also
on an unbalanced jack socket but located on the front panel where it provides a convenient means of
patching a guitar or keyboard directly into the compressor. All three inputs may be used simultaneously.
The gain of the input stages is controlled by a centre-detented control, calibrated for 0dB nominal gain from
the line input sources at the centre and providing +/-20dB of variation. The instrument input has a sensitivity
switch associated with it, to match the gain to a low level pickup (e.g. a passive guitar or microphone) or a
higher output source such as a keyboard or active guitar, followed by the 40dB variation of the input gain
control
After the input stages, the signal is fed to the valve compressor core and the sidechain processing. The
sidechain also monitors the output of the compression stage, and generates a control signal which varies the
attenuation of the compressor. The sidechain processing includes fully variable control of the compressor
threshold and ratio, plus switched fast and slow time constants for attack and release. Frequency dependant
compression (for example de-essing) may be achieved by placing an equaliser into the sidechain insertion
point.
The channel B sidechain controls normally operate independently, but the comprehensive stereo mode
offered by the C-2021 allows the channel A controls to be used for both channels A and B, ensuring
accurate tracking and easy adjustment of the parameters. Stereo mode also combines the compressor cell
control signals, which always applies the same amount of gain reduction to both channels, thereby
preserving the stereo image.
Independent gain make-up controls are provide, to retain the subjective loudness of a compressed signal.
Each channel has a balanced line level output on an XLR socket, and an unbalanced output on a jack
socket. Twin LED bargraph meters are provided, which may be switched to monitor the output signal level
or compression applied.
Please read this manual fully before installing or operating the Compressor.
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