Shure SCM268 User manual

SCM268
Four-Channel Microphone Mixer
Shure Brothers Incorporated
222 Hartrey Avenue
Evanston IL 60202-3696 U.S.A.
Model SCM268 User Guide
27A8606 (SC)
E1999, Shure Brothers Incorporated Printed in U.S.A.

1
English –
SHURE SCM268
DESCRIPTION
The Shure Model SCM268 is a transformer-balanced, four-
channel microphone mixer. Its simple, compact design delivers
superior performance and exceptional sound quality with low
noise and a flat frequency response.
Versatile in all types of applications, the SCM268 integrates
transformer-balanced XLR inputs, a switchable microphone/line
level transformer-balanced XLR output, phono jack inputs and
output, phantom power, and an auxiliary input channel. It can
function as a primary or add-on mixer for sound reinforcement,
recording, broadcast, or audio-visual presentation systems.
With the supplied hardware, the mixer’s half-rack chassis
mounts securely in single or dual rackmount installations. For
fixed installations, the SCM268 can be fastened on or below a
shelf, counter, or tabletop.
FEATURES
SFour transformer-balanced microphone inputs
STransformer-balanced output—switchable mic/line level
SFive –10 dB line-level inputs
SSix-segment LED output level meter
SBuilt-in 12-volt phantom power
SInternal power transformer
SBuilt-in low-cut filter on microphone inputs (below 80 Hz)
FRONT PANEL
ÍÎ
ÊËÌ
ÊGain Controls (1–4). Adjusts gain for microphone level in-
puts and auxiliary level inputs 1–4.
ËAuxiliaryChannelGainControl(AUXIN).Adjuststheaux-
iliary channel input gain.
ÌOutputMeter.LEDmeterindicatespeakoutputsignallevel.
ÍMaster Gain Control (MASTER). Adjusts overall output
level.
ÎPower Indicator (POWER). This LED illuminates when the
unit is plugged in and receiving power.
NOTE: The SCM268 does not have a power switch. To turn
the unit off, unplug the power cord or use an external power
strip with a switch. However, it can remain plugged in as it
uses very little power when idle.
REAR PANEL
Ñ
Î
ÊË ÍÌÏ Ð
ÊPower Connector. Accepts 100–120 Vac (SCM268) or
220–240 Vac (SCM268E).
ËOutput Connector (MIC/LINE OUT). Transformer-bal-
anced XLR output connector. Switchable between line and
microphone level.
ÌOutput Level Switch (MIC/LINE OUT). Recessed switch
changes the signal level of the XLR output connector:
In = Microphone Level
Out = Line Level
ÍAuxiliary Output Connector (AUX OUT). Phono jack
feeds consumer-level audio equipment. Not affected by
MIC/LINE switch.
ÎAuxiliaryLevelInputs(AUXLEVEL INPUTS,1–4). Phono
jacks connect to consumer-level audio sources.
ÏAuxiliary Input Channel (AUX IN). A dedicated auxiliary-
level input for the auxiliary channel.
ÐPhantom Power Switch (12V PHANTOM). Recessed
switch turns on phantom power for microphone inputs 1–4.
ÑMicrophone Level Inputs (MIC LEVEL INPUTS). Trans-
former-balanced, microphone-level XLR inputs.

2
English –
GAIN CONTROL
Input Gain
Thegaincontrolknobs1–4,locatedonthefrontpanel,adjust
the gain for both microphone and auxiliary-level inputs of chan-
nels 1–4 (see Figure 1). For example, the channel 1 gain control
is used for both the channel 1 microphone input (MIC LEVEL IN-
PUT 1) and the channel 1 auxiliary level input (AUX LEVEL IN-
PUT 1). The auxiliary gain control knob (AUX IN) affects onlythe
auxiliary input (AUX IN).
Output Gain
Themasteroutputgaincontrolknob(MASTER)adjustsgain
toboththeXLRbalancedoutput(MIC/LINELEVEL)andtheaux-
iliary level output (AUX LEVEL).
Channel 1
Auxiliary Channel
Master Output
Channel 2
Channel 3
Channel 4
GAIN CONTROL
Figure 1
OUTPUT LEVEL METER
The six LEDs on the front panel labeled OUTPUT LEVEL
METERilluminatetoreflectthepeaklevelofthemixedoutputsig-
nalfromtheSCM268(inreferencetobalancedlineoutput)indBu
(0 dBu = 0.775 V).
Usethemastergaincontrol(MASTER)toadjustpeaklevels,
asindicated by the LEDs. Thered LED illuminates when the out-
put is 2 dB below clipping.
GREEN—Nominal RED—Clip
Peak Level
in dBu
PHANTOM POWER
When the phantom power switch on the back panel is on
(12V PHANTOM—ON), the SCM268 provides 12 V of phantom
power to each XLR microphone input. The switch is recessed to
prevent accidental engagement. Most
condenser microphones
require phantom power. Use it when connecting these types of
microphones to the SCM268.
NOTE:Phantompowerdoesnotaffecttheoperationofbalanced
dynamicmicrophones.Withphantompoweron,theycanbecon-
nected to the SCM268 in combination with condenser micro-
phones that do use it.
Phantom Power Switch
OFF ON
OUTPUT LEVEL SWITCH
The output level switch on the back panel (MIC/LINE OUT)
setsthe level of the balanced XLR output.When set to MIC,it re-
ducestheoutputbyabout50dB.Settheswitchsothattheoutput
level matches the input level of the device to which you are con-
nectingthe SCM268. The switch is recessed to prevent acciden-
tal engagement.
NOTE:Theoutputlevelswitchdoesnotaffecttheauxiliaryoutput
(AUX OUT) level. Output Level Switch
LINE MIC

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CONNECTIONS
ThefollowingdiagramillustratesafewofthemanytypesofconnectionspossiblewiththeSCM268.Notethatnothingisconnectedto
thechannel 4 microphone input(MIC LEVEL INPUT 4). This is because channel 4 is being used for the consumer-level equipment con-
nected to the channel 4 auxiliary level input (AUX LEVEL INPUT 4). Connecting both auxiliary level and microphone level inputs to a
single channel is not recommended because the SCM268 would not be able to independently mix the two sources.
POWER
AMPLIFIER
POWER SUPPLY
DYNAMIC
MICROPHONE
Á
Á
ÁÁ
ÁÁ
ÁÁ
Á
Á
Á
OR
LOUDSPEAKERS
TAPE DECK
AM/FM RECEIVER
CD PLAYER
OR
OR
OR
70 V P.A.
ÂÂÂ
ÂÂÂ
ÂÂÂ
ÂÂÂ
ÂÂÂ
DYNAMIC
MICROPHONE
AUX IN CHANNEL 4
LINE OUT AUX OUT
CHANNEL 1CHANNEL 2
OR
VCR
CHANNEL 3
CONSUMER-LEVEL
EQUIPMENT CONSUMER-LEVEL EQUIPMENT CONDENSER MICROPHONE
ÁÁ
Á
Á
Á
Á
ÁÁ
ÂÂÂ
ÂÂÂ
ÂÂÂ

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INSTALLATION
Supplied Hardware
S4 rubber feet. For stand-alone installation.
S1rackmountbracket,long. Forhalf-rack(singleunit)instal-
lations.
S1 rackmount bracket, short. For half-rack (single) or dual-
mount installations.
S2 straddle brackets. For dual-mount or fixed installations.
S12bracketscrews,1/4in.(6mm).Forsecuringthebrackets
to the chassis.
S4rackmount screws,1in. (2.5cm).Formountingtheunitin
a rack.
S4 plastic washers. For use with the supplied rackmount
screws.
S4 wood screws, 1/2 in. (1.25 cm). For fixed installations.
Bracket Screw
Rackmount Screw Wood Screw
Rackmount Installation
The SCM268 can be mounted as a single unit or dual-
mountedwith eitheranother SCM268 or anotherShure half-rack
unit such as the SCM262 or DFR11EQ.
1. Attach the rackmount brackets using one of the following
methods:
Single unit (half-rack) installation: Attach the short and
longrackmount bracketsto the SCM268 with eight(8) of the
supplied bracket screws.
Dual-mounted installation:
a) Connect the two units together side-by-side using two
(2) straddle brackets. The brackets should straddle the
recessededgesononthe top andbottomofeachchas-
sis. Fasten them using eight (8) bracket screws.
NOTE: Be sure to use both straddle brackets—one on
the top and one on the bottom.
b) Attach the
short rackmount brackets to the outsides of
the combined units with eight (8) of the bracket screws.
2. After attaching the brackets, mount the unit in an equipment
rackusingthe supplied rackmountscrewsandplastic wash-
ers.
Stand-Alone Installation
Adhere the four (4) supplied rubber feet to the bottom of the
unit at each corner. This will keep it from sliding and protect the
table surface.
Fixed Installation
To permanently affix the SCM262 above or below a table,
shelf, or counter top, use the following steps:
1. Fasten the straddle brackets to the recessed edges of the
chassis using four (4) bracket screws.
Topmount:Fastenthestraddlebracketstoonthebottomof
the unit
Hanging mount: Fasten the straddle brackets to the top of
the unit.
2. Fastenthestraddlebracketstothesurfaceusingthefour(4)
supplied wood screws.
TOP MOUNT
HANGING MOUNT

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INTERNAL MODIFICATIONS
WARNING!
Voltages in this equipment are hazardous to life. No
user-serviceable parts inside. Refer all servicing to
qualified service personnel.
Disassembly
To access the printed circuit board (pc board) for internal
modifications, use the following steps:
1. Unplug the power cord.
2. Remove the knobs and retainer nuts from the front panel
(See Figure 2).
KNOB ASSEMBLY
Figure 2
3. Remove the four screws at each corner of the back panel.
4. Remove the two screws at each bottom corner of the front
panel
5. Slide the back panel and pc board out from the rear of the
chassis.
CAUTION: When reassembling the SCM268, DO NOT OVER-
TIGHTEN the knob retainer nuts. Use a minimal amount of force
tosecurethenut(0.6–0.8N⋅m(5–7in⋅lb)).Damagetotheinternal
components will result if too much force is used.
Low-Cut Filter
To bypass the built-in low-cut filter for a given channel, re-
movethespecifiedresistorandplacea10mFto33mFcapacitorin
thespecifiedpcboardlocation(polaritydoesnotmatter).Referto
the following table:
Channel Remove Resistor
from: Place 10mF to 33mF
Capacitor in:
1 R18 X17
2 R28 X27
3 R38 X37
4 R48 X47
To select a particular corner frequency for the low cut filter,
removetheR18,R28,R38,orR48resistorforagivenchannelas
specified above. Then, in the corresponding pc board location
(X17, X27, X37, or X47), place a capacitor of the specified value
(polarity does not matter). Refer to the following formula for se-
lectingthecorrectcapacitor value forthedesiredcornerfrequen-
cy.
26.5
F
C =
where:
C = value of capacitor in mF
F = corner frequency (–3 bB) for low-cut filter in Hz
The following table lists the low-cut frequency corners for
some of the most common capacitor values:
Capacitor Value (mF) Low-Cut Frequency Corner (Hz)
.033 803
.047 564
.068 390
.1 265
.22 120
.33 80
.47 56
.68 39
1.0 26.5
2.2 12
Phantom Power Disable
To disable phantom power for a given microphone input, re-
move the specified resistor as listed in the following table:
Channel Remove Resistor:
1 R15
2 R25
3 R35
4 R45
Line Pad
To insert a 50 dB line pad for a given microphone input, re-
movethespecifiedresistorandshortthesolderpointsatthespe-
cified pc board locations. Refer to the following table:
Channel Remove Resistors: Short Solder Points:
1R12, R13, R15 X11 and X14
2R22, R23, R25 X21 and X24
3R32, R33, R35 X31 and X34
4R42, R43, R45 X41 and X44
Hot Mic Pad
Some condenser mics have a high output. In order to avoid
overdrivingtheinputstage,theusermayneedtosettheinputpot
lower than desired. To fix this problem, the user can place an 11
dB pad into the input gain stage of a selected channel.
1. Twisttogethertheleadsfromonesideofa15kΩresistorand
a 0.1 µF capacitor: 15 kΩ
0.1 µF
2. Solder the free ends of the resistor-capacitor combination
into the holes at the jumper position indicated by the follow-
ing table and remove the corresponding surface mount re-
sistor.
Channel Remove
Resistor Insert Resistor–Capacitor Combination at Jumper
1 R18 X17
2 R28 X27
3 R38 X37
4 R48 X47

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SPECIFICATIONS
Measurement Conditions (unless otherwise specified): Line
voltage120Vac,60Hz(SCM268)or230Vac,50Hz(SCM268E);
full gain; 1 kHz, one channel activated; source impedances: Mic
150
Ω ,AuxLevel150 Ω ;terminations:Line600 Ω ,Mic600 Ω, Aux
Out 10 k
Ω. 12 V phantom power off.
Frequency Response (Ref 1 kHz, controls centered)
Microphone Inputs: 150 Hz to20 Khz ±2dB (built-in80Hzlow-cut)
Auxiliary Inputs: 20 Hz to 20 kHz ±2 dB
Low-Cut Filter (Microphone inputs only)
3dB down at 80 Hz, 6 dB/octave
Gain (typical, controls full clockwise)
Inp t
Output
Input Mic Line Aux Out
Low-impedance
mic (150 Ω)38 dB 76 dB 65 dB
Aux Level 3 dB 40 dB 29 dB
Inputs
Impedance
Input Designed
for use
with
Actual
(typical) Input Clipping
Level
Mic 19-600 Ω1.2 kΩ–5 dBV
Aux Level ≤2 kΩ21 kΩ>28 dBV
Outputs
Impedance
Output Designed
for use
with
Actual
(typical) Output Clipping
Level
Mic low-Z inputs 0.2 Ω–21 dBV
Line >600 Ω72 Ω+18 dBV
Aux Out >2 kΩ870 Ω+7 dBV
Total Harmonic Distortion
<0.25%at+4dBuoutputlevel (through22Hz-22kHz filter; Input1
and Master centered, all other controls full counterclockwise)
Hum and Noise
Equivalent Input Hum and Noise –124 dBV. . . . . . . . . . . . . . . .
(150 Ωsource; through 22 Hz - 22 kHz filter)
Output Hum and Noise (through 22 Hz to 22 kHz filter; channel
controls full counterclockwise)
Master full counterclockwise –92 dBV. . . . . . . . . . . . . . . . . . . . .
Master full clockwise –70 dBV. . . . . . . . . . . . . . . . . . . . . . . . . . .
Common Mode Rejection
>80 dB at 1 kHz
Polarity
All inputs to all outputs are non-inverting
Overload and Shorting Protection
Shorting outputs, even for prolonged periods, causes no damage.
Microphone inputs are not damaged by signals up to +10 dBV;
Auxiliary inputs by signals up to +36 dBV
Phantom Power
12 Vdc open-circuit through 340Ωseries resistance
Operating Voltage
SCM268: 100–120 Vac rated nominal, 50/60 Hz, 60 mA
SCM268E: 220–240 Vac rated nominal, 50/60 Hz, 30 mA
Temperature Range
Operating –7°to 49°C (20°to 120°F). . . . . . . . . . . . . . . . . . .
Storage –29°to 74°C (–20°to 165°F). . . . . . . . . . . . . . . . . . . .
Overall Dimensions
44 mm H x 218 mm W x 162 mm D
(1.72 x 8.60 x 6.37 inches)
Net Weight
1.20 Kg (2 lbs, 10 oz)
Certifications
SCM268: UL & cUL Listed by Underwriters Laboratories, Inc.
SCM268E: Conforms to European Union directives, eligible to
bear CE marking; VDE GS-Certified to EN 60065; meets Euro-
pean Union EMC Emissions and Immunity Requirements (EN
50081–1: 1992, EN 50082–1: 1992).
Replacement Parts
Knob, Master (blue) 95B8752. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Knob, Channel Gain (white) 95A8752. . . . . . . . . . . . . . . . . . . . . .
Line (Power) Cords:
SCM268: 100–120 Vac (US/Canada) 95A8762. . . . . . . . . . .
SCM268E: 220–240 Vac (EU) 95A8778. . . . . . . . . . . . . . . . .
Fuse, SCM268 (5x20 mm, 250V, 80mA, slow-blow) 80A730. .
Fuse, SCM268E (5x20 mm, 250V, 40mA, slow-blow) 80J258.
Optional Accessories
Line (Power) Cord, 230–240 Vac (UK) 95A8713. . . . . . . . . . . . .