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This symbol indicates that dangerous voltage constituting a risk of electric shock is present within this unit. |
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This symbol indicates that there are important operating and maintenance instructions in the literature accompanying this unit. |
WARNING: Voltages in this equipment are hazardous to life. No user-serviceable parts inside. Refer all servicing to qualified service personnel. The safety certifications do not apply when the operating voltage is changed from the factory setting.
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.
① Gain Controls (1-4).
Adjusts gain for microphone level inputs and auxiliary level inputs 1-4.
② Auxiliary Channel Gain Control (AUX IN).
Adjusts the auxiliary channel input gain.
③ Output Meter.
LED meter indicates peak output signal level.
④ Master Gain Control (MASTER).
Adjusts overall output level.
⑤ Power Indicator (POWER).
This LED illuminates when the unit is plugged in and receiving power.
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.
⑥ POWER Switch
Country dependent.
① Power Connector.
Accepts 100-120 Vac (SCM268) or 220-240 Vac (SCM268E).
② Output Connector (MIC/LINE OUT).
Transformer-balanced XLR output connector. Switchable between line and microphone level.
③ Output Level Switch (MIC/LINE OUT).
Recessed switch changes the signal level of the XLR output connector: In = Microphone Level. Out = Line Level.
④ Auxiliary Output Connector (AUX OUT).
RCA output feeds consumer-level audio equipment. Not affected by MIC/LINE switch.
⑤ Auxiliary Input Channel (AUX IN).
A dedicated auxiliary-level input for the auxiliary channel.
⑥ Auxiliary Level Inputs (AUX LEVEL INPUTS, 1-4).
RCA inputs connect to consumer-level audio sources.
⑦ Phantom Power Switch (12V PHANTOM).
Recessed switch turns on phantom power for microphone inputs 1-4.
⑧ Microphone Level Inputs (MIC LEVEL INPUTS).
Transformer-balanced, microphone-level XLR inputs.
The gain control knobs 1-4, located on the front panel, adjust the gain for both microphone and auxiliary-level inputs of channels 1-4 (see Figure1). For example, the channel 1 gain control is used for both the channel 1 microphone input (MIC LEVEL INPUT 1) and the channel 1 auxiliary level input (AUX LEVEL INPUT 1). The auxiliary gain control knob (AUX IN) affects only the auxiliary input (AUX IN).
The master output gain control knob (MASTER) adjusts gain to both the XLR balanced output (MIC/LINE LEVEL) and the auxiliary level output (AUX LEVEL).
Figure 1
The six LEDs on the front panel labeled OUTPUT LEVEL METER illuminate to reflect the peak level of the mixed output signal from the SCM268 (in reference to balanced line output) in dBu (0 dBu = 0.775 V).
Use the master gain control (MASTER) to adjust peak levels, as indicated by the LEDs. The red LED illuminates when the output is 2 dB below clipping.
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.
Phantom power does not affect the operation of balanced dynamic microphones. With phantom power on, they can be connected to the SCM268 in combination with condenser microphones that do use it.
The output level switch on the back panel (MIC/LINE OUT) sets the level of the balanced XLR output. When set to MIC, it reduces the output by about 50 dB. Set the switch so that the output level matches the input level of the device to which you are connecting the SCM268. The switch is recessed to prevent accidental engagement.
The output level switch does not affect the auxiliary output (AUX OUT) level.
The following diagram illustrates a few of the many types of connections possible with the SCM268. Note that nothing is connected to the channel 4 microphone input (MIC LEVEL INPUT 4). This is because channel 4 is being used for the consumer-level equipment connected 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.
The SCM268 can be mounted as a single unit or dual-mounted with either another SCM268 or another Shure half-rack unit such as the SCM262 or DFR11EQ. Attach the rackmount brackets using one of the following methods:
Be sure to use both straddle brackets-one on the top and one on the bottom.
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.
To permanently affix the SCM268 above or below a table, shelf, or counter top, use the following steps:
Top mount: Fasten the straddle brackets to the bottom of the unit.
Hanging mount: Fasten the straddle brackets to the top of the unit.
Voltages in this equipment are hazardous to life. No user-serviceable parts inside. Refer all servicing and modifications to qualified service personnel.
To access the printed circuit board (pc board) for internal modifications, use the following steps:
FIGURE 2
When reassembling the SCM268, DO NOT OVERTIGHTEN the knob retainer nuts. Use a minimal amount of force to secure the nut (0.6-0.8 N m (5-7 in lb)). Damage to the internal components will result if too much force is used.
To bypass the built-in low-cut filter for a given channel, remove the specified resistor and place a 10µF to 33µF capacitor in the specified pc board location (polarity does not matter). Refer to the following table:
| Channel | Remove Resistor from: | Place 0µF to 33µF 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, remove the R18, R28, R38, or R48 resistor for a given channel as 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 selecting the correct capacitor value for the desired corner frequency.
C = 26.5/F
where:
C = value of capacitor in µF
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 (µF) | 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 |
To disable phantom power for a given microphone input, remove the specified resistor as listed in the following table:
| Channel | Remove Resistor: |
|---|---|
| 1 | R15 |
| 2 | R25 |
| 3 | R35 |
| 4 | R45 |
To insert a 50 dB line pad for a given microphone input, remove the specified resistor and short the solder points at the specified pc board locations. Refer to the following table:
| Channel | Remove Resistors: | Short Solder Points: |
|---|---|---|
| 1 | R12, R13, R15 | X11 and X14 |
| 2 | R22, R23, R25 | X21 and X24 |
| 3 | R32, R33, R35 | X31 and X34 |
| 4 | R42, R43, R45 | X41 and X44 |
Some condenser mics have a high output. In order to avoid overdriving the input stage, the user may need to set the input pot lower than desired. To fix this problem, the user can place an 11 dB pad into the input gain stage of a selected channel.
| Channel | Remove Resistor | Insert Resistor-Capacitor Combination at Jumper |
|---|---|---|
| 1 | R18 | X17 |
| 2 | R28 | X27 |
| 3 | R38 | X37 |
| 4 | R48 | X47 |
at 1 kHz
| Mic/Line Switch | 150 Hz to 20 kHz ±2 dB |
| Aux Input | 20 Hz to 20 kHz ±2 dB |
-6 dB/octave below 80 Hz
1 kHz, +4 dBu out, mix output (MASTER) at +0 dB
<0.25%
150Ω balanced source, A-Weighted
-130 dBV
channel controls full counterclockwise, A-Weighted
| Master full counterclockwise | -107 dBV |
| Master full clockwise | -73 dBV |
>80 dB, at 1 kHz
All inputs to all outputs are non-inverting.
Shorting outputs, even for prolonged periods, causes no damage.Microphone inputs are not damaged by signals up to +10 dBV; Line and Aux inputs by signals up to +28 dBV
12 Vdc open-circuit through 680 Ω resistors
| SCM268 | 100–200 V AC, 50/60 Hz, 60 mA |
| SCM268E | 220–240 V AC, 50/60 Hz, 30 mA |
| Operating Temperature | -7° – 35° C (20° –95° F) |
| Storage Temperature | -29° – 74° C (-20° –165° F) |
44 x 218 x 162 mm (1.72 x 8.60 x 6.37 in.)
1.20 kg (2 lbs, 10 oz.
| Line (Power) Cord, 230-240 Vac (UK) | 95A8713 |
| Knob, Master (blue) | 95B8752 |
| Knob, Channel Gain (white) | 95A8752 |
| Line (Power) Cords: SCM268: 100-120 Vac (US/Canada) | 95A8762 |
| Line (Power) Cords: SCM268E: 220-240 Vac (EU) | 95A8778 |
| Fuse, SCM268 (5x20 mm, 250V, 80mA, slow-blow) | 80A730 |
| Fuse, SCM268E (5x20 mm, 250V, 40mA, slow-blow) | 80J258 |
| Hardware Kit | 90AW8100 |
| Link Bars (Bracket) | 53B8443 |
| Single Mount Bracket | 53A8484 |
| Dual Mount Bracket | 53E8484 |
This product meets the Essential Requirements of all relevant European directives and is eligible for CE marking.
The CE Declaration of Conformity can be obtained from: www.shure.com/europe/compliance
Authorized European representative:
Shure Europe GmbH
Headquarters Europe, Middle East & Africa
Department: EMEA Approval
Jakob-Dieffenbacher-Str. 12
75031 Eppingen, Germany
Phone: +49-7262-92 49 0
Fax: +49-7262-92 49 11 4
Email: info@shure.de
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