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Preamp Section
Preamp Overview
Each preamp channel includes switches for
phantom power, polarity, and 40 Hz high-
pass filter. Each channel also has a 38-
position gain control switch and a 4-segment
output level meter. Inputs are fully protected
against overloads and phantom power
transients.
Microphone Gain Controls
The gain controls are a unique approach to
the popular switched-resistor network
topology. Each gain switch of the MPA1 can
sweep the entire 74 dB gain range with
continuous rotation. This is achieved with a
'virtual 38-position' switch – a 13-position
switch that triggers automatic adjustment of
the gain-range.
These resistor networks are precision-
matched for inter-channel gain matching
within +/- 0.03 dB. This precision provides
precise control of stereo microphone pairs.
Maximum gain is calibrated for exactly 74 dB.
Minimum gain is 0 dB (unity). The MPA1 does
not attenuate the signal with pads or
otherwise. When the gain is set at 0 dB, the
MPA1 can accept a signal at +27.5 dBu
without clipping.
MPA1 Instruction Manual
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 MPA1 is designed to achieve faithful and
transparent microphone amplification.
Two-Stage RF Filter and CMRR
Each microphone input is precision-trimmed
at 3 frequencies for common-mode rejection.
High-frequency RF interference is blocked by
Benchmark's two-stage RF filter. This filter
maintains the MPA1's outstanding
performance in very difficult RF
environments.
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