Connections; Microphone Inputs; Balanced Outputs - Overview - Benchmark PRE420 Instruction Manual

Connections

Microphone Inputs (1-4)

The balanced transformerless microphone
inputs use locking Neutrik™ gold-pin female
XLR jacks with both pin 1 (Ground) and the
XLR shell directly bonded to the chassis. This
direct bonding is important for maximum RF
shielding and for immunity to hum due to
shield currents.
XLR pin 2 = + Audio In
•
XLR pin 3 = - Audio In
•
XLR pin 1 = Cable Shield – bonded to
•
chassis
XLR shell – bonded to chassis
•
The microphone inputs have a wide operating
range. With the pad enabled, maximum input
level is +28 dBu. With the pad off, maximum
input level is +8 dBu.
All inputs are equipped with +48V phantom
power, and each input has its own phantom
power switch. Under certain circumstances,
phantom power can cause large voltage
transients. The PRE420 is equipped with
high-current input protection circuits that
protect the sensitive low-noise input
transistors from the worst-case phantom-
induced transients. Large transients are
produced when a microphone is connected or
disconnected while phantom power is on. The
worst possible transients occur when a
microphone cable has an intermittent short to
ground from either pin 2 or pin 3. The
PRE420 is very well protected against both of
the above. For the protection of your
microphones (and your ears), we strongly
recommend shutting off phantom power
several seconds before connecting or
disconnecting a microphone.
PRE420 Instruction Manual – rev. E
The first generation of PRE420's (with five-
digit serial numbers) featured a common-
mode choke for RF immunity. These choke
filters were designed for balanced microphone
or balanced line-level inputs. When using the
PRE420 with unbalanced sources, the user
should insert the internal jumpers that bypass
the choke (J5-J12). The frequency response
of the PRE420's is 500 kHz, but the choke will
limit the frequency response of an un-
balanced input to 26 kHz if the choke is not
bypassed. This difference is because of the
nature of common-mode chokes. RF
interference signals tend to be common-mode
signals, and the choke filter has excellent
common-mode rejection at ultrasonic and RF
frequencies. The common-mode rejection of
the choke filter is 60 dB at 1 MHz.
The second generation (with eight- or nine-
digit serial numbers) features a new RF and
EMI filtering mechanism (the same used in
the Benchmark MPA1 microphone
preamplifier). The RF performance of the
second generation is nearly identical to the
first generation PRE420, with the added
advantage that the new design is compatible
with unbalanced sources without the need to
use jumpers. The newer design works
equally with balanced and unbalanced
sources. It features star-quad signal
distribution between the microphone input
jacks and the preamplifier circuit. This star-
quad signal path is immune to magnetic
interference that may be caused by adjacent
equipment in a tightly packed rack.
Balanced Outputs --- - Overview
The PRE420 has 8 balanced outputs including
four direct outputs, two control room outputs,
and two main outputs. All outputs use gold-
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