Operational Notes; Input Circuitry - Sound Devices MixPre User Manual

MixPre Microphone Preamplifier
User Guide and Technical Information

Operational Notes

Input Circuitry

The isolation characteristics of transformers are
superior to other balancing techniques for the
adverse and uncontrolled environments of field
production. Input transformers provide complete
galvanic isolation from the source, meaning there
is no direct electrical connection. Signals are
"transformed" magnetically. The MixPre features
expensive Lundahl input transformers, which are
unparalleled in sound quality. Lundahl input
transformers use premium magnetic core material
and advanced winding techniques to achieve high
signal handling capability and flat frequency
response while keeping distortion to a minimum.
Because of their inherently high common mode
impedance, transformers are unrivaled by any
other type of input for common-mode noise
rejection.
Output Circuitry and Tape Output
Each line level XLR output of the MixPre uses a
discrete, six-transistor, "impedance balanced"
output stage. The circuit uses oversized, high
speed transistors to achieve low-distortion, robust
line driving capability. Pin 2 is driven with signal,
and pin 3 is not. [A common misunderstanding is
that a "balanced" output refers to equal and
opposite signals on pins 2 and 3. Correctly,
"balanced" refers to equal
signal (pin 2 and pin 3) to ground,
voltage
.] To drive unbalanced inputs, pin 3 can be
grounded or left unconnected. There is no change
in signal level driving balanced or unbalanced
inputs.
The Tape Output is designed to feed consumer
level devices, such as computer sound cards, DAT
or MiniDisc recorders, and cassette recorders. A
cable wired with 3.5 mm TRS to 3.5 mm TRS or,
3.5 mm TRS to Left/Right phono (RCA) connectors,
is useful when directly connecting the MixPre to
portable consumer audio electronics.
The "Mix" in MixPre
The MixPre pan switches assign each input channel
to an output bus. With this feature, the MixPre can
be used as a two-channel microphone preamplifier
and as a simple mixer. As a mixer, many
applications are possible, such as:
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impedances
from each
not the signal
• Quickly reversing the stereo image of a
recording without reconnecting inputs or
outputs.
•
During set up, checking for mono compatibility
by assigning both inputs to the same output.
• Recording dialog in mono.
By routing input 1 to Center and input 2 to Right, a
summed mono signal of both inputs appears at the
Right output while only input 1 appears at the Left
output. This is useful for post production
processing of input 1 audio.
Phantom Power / Input Gain Switch
Microphones requiring phantom power should use
the lowest voltage acceptable to maximize MixPre
battery life. Condenser microphones that can
operate on phantom voltages from 11-52 volts will
not have a performance benefit with 48-volt
phantom; therefore 15-volt phantom is
appropriate. The 15-volt setting will increase
battery life versus the 48-volt setting.
Microphones requiring 48-volt phantom will not
operate, or may operate with lower headroom and
increased distortion at the 15-volt setting;
therefore use 48-volt phantom for these mics.
Consult your microphone documentation.
Dynamic microphones do not require phantom
power. A properly connected balanced, dynamic
microphone will not be affected by the presence of
phantom power nor will it draw any current.
However, it is good practice to turn phantom
power off when not needed. Poor or incorrectly
wired microphone cable can cause audible artifacts
in microphone signals. Some wireless receivers
outputs are adversely affected by the presence of
phantom power, therefore, consult the wireless
receiver documentation.
The MixPre phantom power switch reduces the
input circuit gain by 10 dB when switched to either
phantom power setting. As condenser mics are
typically 10 dB or more sensitive than dynamic
mics, the 10 dB pad keeps the input stage from
clipping and helps normalize front panel gain
settings.
High-Pass Filter
The two-position high pass (low cut) filter in the
MixPre is useful for removing excess low frequency
energy in audio signals. This excess energy can be
caused by multiple sources, including wind noise,
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