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Preamp Section
Preamp Overview
Each preamp channel includes switches for
phantom power, a 20 dB pad, and a 40 Hz
high-pass filter. Each channel also has a
center-detent MirrorPan™ control, a 41-
detent gain control, a Phantom warning LED,
and a 2-segment output level meter. Inputs
are fully protected against overloads and
phantom power transients.
Input Topology
The first gain stage on each microphone input
is built around precision-matched monolithic
low-noise transistor quads. Bandwidth is 500
kHz, IMD and THD are virtually non-existent.
The PRE420 is designed to achieve faithful
and transparent microphone amplification.
Microphone Gain Controls
These controls are a 41-detent pots that
accurately set the first-stage gain of each
preamplifier. These controls are calibrated for
precise minimum and maximum gain. This
calibration provides accurate channel-to-
channel matching, and allows precise control
of stereo microphone pairs.
Maximum gain is calibrated for exactly 60 dB,
but may be adjusted to any value between 50
and 70 dB using the calibration procedure in
this manual. Minimum gain is 22 dB without
the pad, and 2 dB with the pad enabled.
TIP: ENGAGE THE PAD ON ANY UNUSED
INPUT AND TURN THE MICROPHONE
GAIN CONTROL TO MINIMUM TO REDUCE
NOISE AT THE MIXER OUTPUTS.
PRE420 Instruction Manual – rev. E
Blocking RF, EMI, and Noise
The entire signal path of the PRE420 has a
generously wide bandwidth: 500 kHz. This
creates a significant demand for exceptional
RF filtering and shielding – a demand
Benchmark fulfills.
All signal paths in the PRE420 are
aggressively protected from RF and EMI
(electro-magnetic interference). For example,
the multi-layer printed circuit board uses a
special construction technique that creates a
3-dimensional shield surrounding each mic-
level signal-trace to block RF interference.
Star-quad trace layouts prevent magnetic
interference. Benchmark's innovative
shielding and filtering techniques have given
Benchmark mic-preamps a proven track
record for exceptional performance even in
the most difficult environments.
Common mode signals in the audio band are
removed with a precisely trimmed differential
amplifier. The PRE420 even includes a
subsonic common-mode trim adjustment on
each microphone input that eliminates
sensitivity to +48V ripple caused by power
supply noise and/or the loading of
microphone electronics.
Phantom Power Switches
In = Phantom On
•
Out = Phantom Off
•
Each channel has a +48V warning LED that
will illuminate when +48V is present on the
microphone inputs.
CAUTION: ALWAYS TURN OFF PHANTOM
POWER AND WAIT FOR THE +48V LED TO
GO OUT BEFORE CONNECTING OR
DISCONNECTING MICROPHONES.
Input Protection
Under certain circumstances, phantom power
can cause large voltage transients. The
PRE420 is equipped with high-speed high-
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