INTRODUCTION
The T L Audio Ivory Series consists of a range of hybrid valve signal processors,
which utilise low noise solid state electronics in conjunction with classic valve
circuitry to produce audio processing units offering very high quality signal paths with
the unique valve audio character. The Ivory Series units offer comprehensive control
facilities, whilst remaining straight-forward to operate, and represent excellent value
for money.
The VP-5051 is a single channel voice processor, combining a high quality
microphone pre-amplifier, four band equaliser, compressor and gate. All parameters of
the compressor are variable, and a link facility enables two units to be operated in
stereo mode. The VU meter may be used to display various input and output levels, or
gain reduction.
The block diagram of the unit is shown in fig.1. There are four input sources -
balanced microphone and line inputs via XLR connectors, plus 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 processor. The line level inputs have a nominal
level switch adjacent to the sockets, to match the balanced and unbalanced inputs to a
wide variety of sources, including the high level from some digital tape systems. The
input stage is a hybrid valve circuit with gain controlled by a centre-detented control,
calibrated for 0dB nominal gain from the line input sources at the centre and providing
up to 60dB of microphone gain. The instrument input is also controlled by the input
gain potentiometer, and will accept a wide range of sources from passive pickups to
active guitars and keyboards. The input stage is monitored by the “Drive” and “Peak”
LEDs. Drive is a variable intensity LED designed to indicate the signal level through
the valve stages, and thus the amount of “warming” applied to the signal. Peak is a
conventional warning that clipping is about to occur, and will illuminate if there is an
excessively high level present at any point in the signal chain.
The equaliser is entirely valve based, with four bands providing up to 12dB of cut and
boost at the selected frequencies. The “EQ PRE” switch positions the equaliser before
(pre) or after (post) the compressor.
The sidechain monitors the both the input (“feedforward”) and output (“feedback”) 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 time constants for attack and release.
The circuit includes a “Hold” facility to reduce the distortion introduced when a very
low frequency signal is heavily compressed with a short release time. Frequency
dependant compression (for example de-essing) may be achieved by placing an
equaliser into the sidechain insertion point.
An independent gain make-up control is provided, to retain the subjective loudness of
a compressed signal, plus an overall master output level control. The output control
