Operation
The LMPRE
has simple intuitive controls for boosting
the gain of a microphone to drive a line level device.
Both dynamic and condenser type microphones can be
used, as the LMPRE supplies 24 Volt phantom power to
the female XLR input jack. A rear panel switch turns the
phantom power on and off and a front panel LED
illuminates when the phantom is turned on.
Microphone signal amplification
is set using a
combination of the Input Gain and the Output Level Set
potentiometers. There is a 10 dB fixed gain stage
directly at the input and a fixed 6 dB of gain at the
balanced XLR output. The Input Gain pot adds another
39 dB at full CW position. The Output Level Set has a
range of ±15 dB and can boost the maximum gain to 70
dB at full CW position. Setting both pots to the 12 o'clock
position provides approximately 40 dB of gain from XLR
input to XLR output. The RCA output jacks deliver the
same signal, unbalanced, at levels usually required by
consumer equipment such as receivers. The XLR and
RCA outputs can be used simultaneously, if required.
LED metering
provides a visual indication of the output
level of the LMPRE. The output meter consists of two
green and a yellow LED. When the 0 VU green LED is
illuminated, the signal level at the XLR balanced output
is +4 dBu. The level at the RCA outputs is approximately
14 dB lower.
Peak limiting
with the LMPRE can protect audio
systems from signal overload and distortion that can
occur from improper or unexpected microphone usage.
The Input Gain control is used to set the amount of input
level that determines when the limiter will activate.
Increasing the input gain initiates limiting more quickly.
The output level control has no effect on the operation of
the limiter.
For limiter setup
, determine the normal user
microphone placement and adjust input gain to activate
the limiter at the loudest expected level. The red “LIMIT”
LED illuminates when the limiter becomes active. Then
set the system drive level with the Output Level Set
control. The red “LIMIT” LED is not part of the output
metering and it is possible to have the “LIMIT” LED lit
without any output level LEDs being illuminated.
Low and High Tone controls
are tailored to enhance
the intelligibility of voice applications of the LMPRE. The
Low frequency band is centered around 250 Hz and is
used to reduce “muddiness” or increase “warmth”. The
High band centers around 4 kHz and is used to reduce
“harshness” or increase “brightness”. Both are center
detent controls, in which position no tonal changes
occur.
Power Supply
requirements for the LMPRE are 24
VDC at 200 mA. The Whirlwind PS242.1 wall adapter
with a 2.1mm, center positive, barrel plug is included.
With power is applied to the LMPRE, the red “POWER”
LED illuminates.
VU Meter
XLR Output Level
RCA Output Level
Yellow
+16 dBu
+2 dBV
Top green
+4 dBu
-10 dBV
Lower green
-14 dBu
-28 dBV
Output Level Reference
PHTM
MIC INPUT
LIMIT
POWER
VU
MIC TONE
OUTPUT
MONO
+12
0
-18
INPUT
GAIN
LOW
250 Hz
HIGH
4 kHz
LEVEL
SET
