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MirrorPan™ Pan Controls
The PRE420 incorporates Benchmark's new
MirrorPan™ constant-power pan control. The
advantages of MirrorPan™ include:
Accurate 3-dB constant-power pan law
•
Accurate balance at center detent
•
Excellent channel separation when fully
•
panned
Ultra-low distortion
•
Ultra-low noise
•
Benchmark's MirrorPan circuit eliminates the
distortion and pan-law errors caused by pan-
pot wiper current. The center detent is
precisely calibrated for an accurate center
pan, and channel separation is excellent when
fully panned.
20 dB Pads
In = Pad Enabled
•
Out = Pad Off
•
The 20 dB Pad allows the connection of line
level signals as high as 27 dBu. Without the
pad, the maximum input level is 7 dBu. The
input impedance of the PRE420 is 1.37k
Ohms with the pad enabled and 8.13k Ohms
when the pad is off. The pad is constructed
from two 750 Ohm series resistors plus one
150 Ohm shunt resistor.
Terminating Unused Inputs
The pad should be used to apply a 150 Ohm
termination to an unused microphone input.
Microphone preamplifiers will amplify the
Johnson noise from the source resistance
connected to the input. Most microphones
have a source resistance of about 150 Ohms.
But, an un-terminated preamplifier sees only
its own input impedance. The input
impedance of the PRE420 is 8.13k Ohms.
The Johnson noise of a 150-Ohm resistor is
-130.8 dBu while the Johnson noise from an
8.13k-Ohm resistor is -113.5 dBu. An un-
terminated microphone input will be about 17
dB noisier than an input that is properly
terminated.
PRE420 Instruction Manual – rev. E
The 20dB pad applies a 150-Ohm resistor
across the inputs of the preamplifier.
Engaging the pad on an unused input
terminates the preamplifier with
approximately the same resistance as a
typical microphone, and drops the EIN of the
preamplifier to its specified value. If a
microphone is not connected, engage the pad
to terminate the input.
TIP: ENGAGE THE PAD ON ANY UNUSED
INPUT AND TURN THE MICROPHONE
GAIN CONTROL TO MINIMUM TO REDUCE
NOISE AT THE MIXER OUTPUTS.
40 Hz High-Pass Filters
In = Filter Enabled
•
Out = Filter Off
•
The 40 Hz high-pass filter is a second-order
filter with a slope of 12 dB per octave. This
filter is intended to remove unwanted
subsonic noise and rumble caused by sources
such as HVAC, air currents, and vibration.
LED Meters --- - Direct Outputs
Each preamp channel has a direct output.
Each direct output is equipped with a two-
segment LED meter.
The green LED shows signal presence. The
intensity of the green LED varies with signal
level over a range of -8 dBu to +24 dBu. The
green LED reaches half brightness at an
output level of about +4 dBu, and is near full
brightness at +24 dBu.
The red LED is factory preset to illuminate at
an output level of precisely +24 dBu, but may
be adjusted to other levels between 20 dBu
and 29 dBu. The red LED is driven by a peak
detection and stretching circuit so that very
short transients can be observed.
The red LED is a warning indicator not a clip
indicator. The direct outputs have 5.5 dB of
remaining headroom when the red LED is just
illuminated. The mix amps have a minimum
of 2 dB of headroom when all channels are
just illuminating the red LEDs.
Page 11
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