MixPre User Guide and Technical Information

MixPre
Portable Mixer
User Guide and Technical Information
Sound Devices, LLC
300 Wengel Drive • Reedsburg, WI • USA
+1 (608) 524-0625 • fax: +1 (608) 524-0655
Toll-Free: (800) 505-0625
www.sounddevices.com
support@sounddevices.com
MixPre User Guide and Technical Information
Front Panel Descriptors
1
2
3
4
5
6
7
8
L
-30
MIC
-15
0
+4 +8 +12 +16
METER
1kHz
LCR
POWER
R
2
9
LCR
SLATE
10
1.
Gain Control
Adjusts the input gain via sealed, conductive
plastic rotary potentiometers.
2.
Input Peak/Limiter LED
Two-color LED illuminates red at 3 dB below
clipping; illuminates amber to indicate limiter
activity.
TAPE RTN
11
3.
Slate Microphone
Condenser microphone with AGC controlled
by momentary Slate Microphone Switch. Slate
Microphone output appears at all outputs.
4.
Output Level Meter
Indicates peak output level in dBu appearing
at the left and right outputs.
5.
Headphone Connector
Accepts stereo and mono headphones with
1/4-inch connectors.
6.
Headphone Volume Control
Adjusts the level in the headphones.
7.
Headphone Peak LED
Illuminates 3 dB before clipping of either channel of the headphone circuit. Also illuminates
3 dB before clipping of either channel of Tape
Return audio.
8.
Power LED
Two-color LED illuminates green when the
unit is powered and changes to red when batteries require changing. For external DC, the
power LED is always green.
9.
Pan Switches (Input Assignment)
Three-position switch pans inputs to Left, Center (both left and right) or Right outputs.
12
PHONES
INT EXT
13
10. Tone Oscillator/Slate Microphone Switch
Toggle switch activates a 1 kHz tone oscillator
when switched to the left position and activates the slate microphone when in the right
(momentary) position. Microphone inputs are
muted when tone or slate are activated.
11. Meter Brightness Control
Three-position switch adjusts the intensity of
illumination of the output meter; low illumination, normal illumination, or super-bright
illumination.
12. Tape Return Switch
Toggle switch allows external audio to be
monitored in the headphones. The center position is MixPre program audio. The left and
right positions monitor tape return audio; the
left position is locking, and the right position is
momentary.
13. Power Switch
Three-position switch selects the power source.
The unit is powered from internal batteries when in the left position; powered from
external DC source when in the right position.
Center position is off.
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MixPre User Guide and Technical Information
Input Panel Connectors and Controls
1
3
2
TAPE OUT
TAPE RTN
LEVEL
L
1
80
LINK
12V
160
ON
2
R
L
R
THRESHOLD
LIMITER
MIC INPUTS
PHANTOM
4
4
48V
1.
Mic Inputs
Transformer-balanced XLR inputs accepts
microphone level signals. Pin 2 = hot, pin 3 =
cold, pin 1 = ground.
2.
Tape Output
3.5 mm TRS stereo output (unbalanced) can
be used to feed consumer level DAT, MiniDisc, and CD recorders. Tip = left, ring = right,
sleeve = ground.
3.
Tape Return Level Control
Recessed potentiometers adjust Tape Return
level feeding the Headphone Monitor.
4.
Phantom Power
Three-position switch selects either 48-volt or
15-volt phantom power for both inputs. Center
position turns phantom power off.
5
6
7
5.
High-Pass Filter Switch
Three-position switch selects 80 Hz or 160 Hz
corner frequency filters, 6 dB per octave. Filters
affect both inputs. Center position of switch
removes filters from the signal path.
6.
Limiter Switch
Activates input peak limiter. ON position
functions as a dual mono limiter, with each
input signal controlling its own limiter. LINK
functions as a stereo limiter, with both left and
right inputs controlled simultaneously. Center
position of switch turns limiter off.
7.
Limiter Threshold Level Control
Recessed potentiometers adjust peak level of
limiter activation. Can be independently controlled for each input.
Features and specifications are subject to change. Visit www.sounddevices.com for the latest documentation.
MixPre User Guide and Technical Information
Output Panel Connectors
2
1
3
TAPE RTN
1(L)
2(R)
LINE OUTPUTS
4
1.
Tape Return Input Connector
3.5 mm TRS stereo (unbalanced) connector allows external audio sources to be monitored in
the MixPre Headphone circuit. Tip = left, ring =
right, sleeve = ground.
2.
Left and Right Line-Level Outputs
Active-balanced XLR line-level outputs. +22
dBu peak output level. Pin-2 = hot, pin-3 =
cold, pin-1 = ground. To unbalance, use pin-2
as signal and pin-1 as ground, pin-3 should not
be grounded.
3.
Battery Compartment
MixPre operates on two AA batteries. Insert
positive (+) end of batteries first.
5-18 VDC
-
+
5
4.
Strap Slot
Attachment point for camera straps.
5.
External DC Power Input
Accepts external DC power source (battery
supply or AC to DC transformer) from 5–18
VDC. Locking, 4-pin Hirose pin-4 (+), pin-1 (–)
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MixPre 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. Transformers provide complete galvanic isolation
from the driving source, meaning there is no direct electrical connection. Signals are “transformed”
magnetically. Both the transformers in the MP-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.
Output Circuitry and Tape Output
Each line level XLR output of the MixPre uses an active-balanced output stage. The circuit provides
robust line driving capability. To drive unbalanced inputs, pin-3 should not be connected. When unbalancing the connection the output level will be reduced by 6 dB.
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:
• 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 postproduction processing
of input 1 audio.
Phantom Power
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. .
6
Features and specifications are subject to change. Visit www.sounddevices.com for the latest documentation.
MixPre User Guide and Technical Information
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,
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 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. Because the MixPre can drive headphones to very high levels,
care should be exercised when monitoring and adjusting levels.
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 twostage 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
The switch labeled 1 kHz/SLATE controls two functions. The 1 kHz position (left - locking) activates
a 0 dBu level 1 kHz sine wave calibration tone - sent to both 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.
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MixPre User Guide and Technical Information
Metering
Input Clip LEDs - 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. When the limiter is
activated, the input LED’s illuminate amber in proportion to limiting action.
Headphone Clip LED - 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.
Output Meter - The MixPre has a highly visible, seven-segment LED output meter. The meter is peak
responding-showing the instantaneous output level in dBu. Peak readings are essential when the outputs feed digital devices. The meter uses red and green Gallium Nitride (GaN) LED’s. These premium
LED’s allow for excellent viewing (with minimal current draw) under all lighting conditions, including
direct sunlight. The front panel Meter Brightness switch controls the meter brightness (low, normal, or
super-bright). The meter indicates a signal range of 46 dB. From left to right the first three LED’s are
separated by 15 dB and the next four LED’s are separated by 4 dB. This calibration provides a large
dynamic range with good resolution in the critical signal level range.
Power Sources
The MixPre is designed to operate from two internal AA alkaline cells for approximately 6 hours with
typical signal levels (without phantom power). External DC sources such as AC to DC transformers and
external battery supplies can also power the MixPre. Because of its regulated power supply circuitry,
audio performance does not change depending on battery or external DC voltage.
The external DC input supply is galvanically isolated (floating) from the chassis and the rest of the
circuitry. The isolation provides trouble free interconnection to other pieces of equipment sharing the
same DC power source. The pin-4 of the locking Hirose DC connector is positive, and pin-1 is negative.
As the external DC supply is floating, the positive or the negative can be connected to the chassis with
no adverse effects.
8
Features and specifications are subject to change. Visit www.sounddevices.com for the latest documentation.
MixPre User Guide and Technical Information
Battery Life
Two factors determine battery life - battery power and current draw.
Battery Power
For internal power the MixPre uses two AA (LR6) 1.5-volt batteries. Different batteries have different
power capacities depending on their chemistry, age, operational temperature, and amount of current being drawn. Alkaline cells are a popular and cost-effective battery type. Lithium cells have higher power
per cell with higher current draws and yield significantly longer battery life, at a cost premium. AA
carbon cells and AA nickel-cadmium cells are not recommended battery types in the MixPre since these
batteries have lower power capacity than other types and will result in very short service life. Temperature has a significant effect on the life of batteries. Lithium batteries are significantly better than alkaline
batteries at lower temperatures.
Current Draw
The MixPre can vary significantly in the amount of current it draws. Several functions of the MixPre
directly affect current draw in different ways. The following list highlights the larger current drawing
functions (listed from highest to lowest current draw).
1. Phantom power: the main source of extra MixPre current draw. (See Phantom Power) 48V Phantom
can draw copious amounts of current out of the batteries depending on what model microphone is
used. Two phantom powered microphones draw twice as much current as one. Microphones vary
widely in their current draw depending on type and phantom voltage applied.
2. Headphone output circuit - high headphone output levels increase current draw.
3. Meter illumination intensity - higher meter brightness increases current draw. The meter can be
switched to low for battery conservation.
4. Output drive level - higher output drive levels into low impedance inputs increases current draw.
Experimentation is recommended to determine battery life for each individual setup and application.
The chart below can be used as a starting point to estimate battery life.
Battery Type
Microphone Type
Battery Life
Eveready AA No. L91
(lithium)
2 dynamic handheld microphones, low meter, no headphones
11 hrs.
Duracell AA MN 1500
(alkaline)
2 dynamic handheld microphones, low meter, low headphones
6 hrs.
Duracell AA MN 1500
2 condenser, 12-volt phantom powered microphones,
normal meter intensity, normal headphone drive level
4 hrs.
Duracell AA MN 1500
2 studio condenser, 48-volt phantom powered microphones, super bright meter intensity, cranking headphones
2 hrs.
(Test conditions: 70° F, 42 dB gain with acoustic music source, 600 ohm load, +4 dBu output)
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