MixPre Microphone Preamplifier
User Guide and Technical Information
Page 6 ©1999-2002 Sound Devices, LLC
vibration, unidirectional proximity effect, or high
ambient noise levels. 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 MixPre high pass
circuit topology is unique in that the filters are
placed immediately after the input transformer,
before
any active gain circuitry. This gives the
MixPre higher headroom with low frequency
signals, as the low frequency signals are not
amplified before they are removed, as in most
other designs. The high pass filter is a single pole
design (slope of 6 dB per octave) and uses high
quality film capacitors for very low distortion. The
high pass filter switch controls both inputs. The
center position of the switch removes the filter
from both audio paths.
When possible, attempt to equalize at the sound
source with microphone selection, use of
windscreens, shock mounts, microphone
placement, and onboard microphone filtering.
Multiple high pass filters (filters on microphones
and on the MixPre) will give an additive effect,
increasing the slope of the filter.
Headphone Monitoring and Tape Return
Headphone monitoring is an essential need in
production audio. The MixPre enables monitoring
of program (mixer) audio or a second audio
source. In normal operation the headphones
monitor the output bus directly. Using the Tape
Return input, a second audio source can be
monitored in the headphones. This is useful to
verify that signal is reaching cameras and tape
machines. The three position Tape Return switch
on the front panel selects the audio source being
monitored. The left and right positions monitor the
tape return audio. Left is locking, and right is
momentary. The center position monitors the Left
and Right outputs of the MixPre. As the MixPre can
drive headphones to very high levels, care should
be exercised when monitoring.
Limiters
The MixPre has two built in peak responding
limiters, one for each input channel. The three-
position LIMITER switch on the input panel
activates both channels limiters. Each limiter in the
MixPre is a two-stage circuit; the first stage keeps
the input gain stage from clipping; the second
stage limits the variable gain stage to the level set
by the Limiter Threshold control. This unique two
stage topology limits the gain stage directly after
the mic input transformer, to make the front end
virtually “unclippable”, but does not change the
input impedance as other “at the mic” limiters do.
The circuit enables the MixPre to limit, when
necessary, in excess of 50 dB, making it very
difficult to clip the unit no matter the gain setting.
The three-position Limiter switch can be set to
operate in either dual mono or stereo linked
operation. Dual mono operation (ON switch
position) allows each input to limit independently,
responding only to its input signal. Stereo linked
operation (LINK switch position) connects both
limiters, ensuring that the limiters control both
channels identically. This linking is important for
stereo operation to maintain a proper stereo
image. When the limiters activate, the Peak/Limiter
LED on the front panel illuminates amber in
proportion to the amount of limiting for each
channel.
Tone Oscillator and Slate Microphone
Two functions are controlled by the single switch
labeled 1 kHz/SLATE. The 1 kHz position (left -
locking) activates a 1 kHz sine wave calibration
tone at a +4 dBu output - sent to all outputs.
Inputs are muted when the tone oscillator is
activated. The tone oscillator is useful during setup
to verify connections and to set nominal levels to
recording and transmission equipment. The SLATE
position (right – momentary) of the switch
activates the built in slate microphone (located
behind the front panel). The Slate Mic circuit
contains an AGC (automatic gain control) to keep
the slate level relatively constant regardless of the
acoustic level. Slate microphones are convenient to
document “takes” right at the mixer location when
microphones are “on talent” or away from the
production mixer. Slate microphone audio is
present at all outputs, and inputs are muted while
the slate mic is active.
Metering
Above each input gain control is a two-color LED
that illuminates red when the input approaches
clipping (3 dB before clipping). This clip circuit
monitors both the front end gain stage and the
variable gain stage. If the audio signal clips
anywhere, this LED will show it. Above the
headphone level control, the headphone clip LED
shows when either channel of the headphone
circuit is nearing clipping level. The headphone clip
LED also shows when either channel of the tape
return audio is nearing clipping level. Between
these three clip indicators, any clipping, anywhere
in the mixer will be seen before it is heard.
