Sound Devices MM-1 - Manual (Page 6)

MM-1 User Guide and Technical Information
6
Features and specifications are subject to change. Visit www.sounddevices.com for the latest documentation.
Operational Notes
Transformers
The isolation characteristics of transformers are superior to any other balancing technique,
particularly for the adverse and uncontrolled environments of field production. Transformers
provide galvanic isolation from the driving source, meaning there is no direct electrical connection.
Signals are “transformed” magnetically. Both transformers in the MM-1 use premium magnetic core
material to achieve high signal handling capability (especially at low frequencies) 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.
Both the microphone input and line output of the MM-1 can be balanced or unbalanced without
problems. When unbalancing (either input or output) ground pin 3 to pin 1. There is no change in
gain with an unbalanced connection into or out of the MM-1.
Microphone Powering
Microphones requiring phantom power should use the lowest voltage acceptable to maximize
battery life. Most electret-condenser microphones can operate on phantom voltages from 11-52 volts.
There is typically no performance benefit to using 48-volt; therefore 12-volt phantom is appropriate.
Some microphones which require 48-volt phantom will not operate, or may operate with lower
headroom and increased distortion at 12 volts; therefore use 48-volt phantom. Consult your
microphone documentation for the appropriate voltage.
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 if the microphone cable is suspect. Poor or
incorrectly wired microphone cable can cause audible artifacts in the microphone signal. (Phantom is
an excellent cable tester.)
T-powering is a specific powering topology needed only for T-powered microphones. T-power
electrically is 12 volts applied to pin 2 with respect to pin 3 of the XLR connector. Because of the
voltage differential of T-power, it is incompatible with dynamic or phantom powered microphones
and can permanently damage dynamic and phantom powered mics.
High-Pass Filter
The two positions of the high-pass filter (low-cut) in the MM-1 are useful for removing excess low
frequency energy in the audio signals. The 80 Hz position is appropriate when recording general
speech, music, and ambient sound. The 160 Hz position is useful to enhance speech clarity. The high
pass filter is a single pole design, 6 dB per octave.
When possible, attempt to equalize at the sound source with microphone selection, use of a
windscreen, microphone placement, and onboard microphone filtering. A high-pass filter on the
microphone and a high-pass filter on the MM-1 will give an additive effect, increasing the slope of
the filter.
Limiter
The MM-1 has a built-in peak responding limiter which can be turned on or off by the front panel
switch. The MM-1 limiter is two separate limiters circuits activated by the one switch; the first limiter
keeps the input gain stage from clipping, and the second limiter limits the output to +17 dBu. The
two limiters enable the MM-1 to limit in excess of 50 dB, meaning that it is very difficult to clip the
unit, no matter the gain setting. The Limiter LED on the front panel illuminates amber in proportion
to the amount of limiting.