Shure M267 Controls and Connectors, Input Gain Controls, Master Gain Control, Low-Cut Filters

CONTROLS AND CONNECTORS

WARNING

This apparatus must be earthed (grounded)! The M267 power supply is energized when the unit is connected to an ac source; disconnect mains (power) plug from supply when not in use.

Inputs

The four inputs are professional three-pin female XLR audio connectors located on the rear panel and desig- nated INPUT 1 through INPUT 4. The inputs are balanced (internal transformer, MUMETAL shielded); pins 2 and 3 are ªhotº, and pin 1 is ªgroundº. For microphone operation, the switches labeled LINE/MIC (directly above the input connectors) must be in the MIC position; for line level in- puts, the switches must be in the LINE position. For im- pedance, clipping and operating signal levels, refer to the SPECIFICATIONS section.

Outputs

The rear-panel connector labeled OUTPUT is a profes- sional three-pin male XLR audio connector. With the adja- cent LINE/MIC switch in the MIC position, the OUTPUT connector is used to feed a low-impedance microphone line or a low-impedance microphone input. With the LINE/ MIC switch in the LINE position, the OUTPUT connector feeds the line-level input of an amplifier, tape recorder, or another mixer. The OUTPUT connector is a balanced out- put with the LINE/MIC switch in either position; pin 1 is ground, pins 2 and 3 are ªhotº, and the connector is in phase with the corresponding pins of the input connectors.

The rear-panel binding-post connector designated LINE OUTPUT is in parallel with the OUTPUT connector and can be used as a line-level output feed simultaneously with the OUTPUT connector. The terminals are numbered 2 and 3 and are in phase with the corresponding pins of the input connectors. While the line outputs can be used to drive various impedance lines, the VU meter is cali- brated for use with a 600 W line.

The line output transformer will operate properly with up to 100 mA dc in the line. This feature permits the use of standard ªdialed-upº telephone lines with dc acrossthem. (Since a slight distortion increase may occur at high output levels with maximum dc current, operation with the VU RANGE switch at +4 dBm is recommended.)

Input Gain Controls

The front-panel controls designated 1 through 4 are the individual active gain controls for correspondingly numbered inputs. Note that the input connectors are located on the rear panel directly behind their corresponding gain control. The controls set the preamplifier gain and provide preampli- fier output attenuation. As the gain is reduced, the preampli- fier input clipping level is increased for that channel.

Input control 1 serves an additional function as the level control for the tone oscillator when the INPUT 1/OCS 1 switch is in the OSC 1 position.

IMPORTANT: For optimum signal-to-noise ratio, the indi- vidual input controls should be operated at as high a set- ting as possible, consistent with maintaining adequate control range and input clipping level.

Master Gain Control

The front-panel control designated MASTER is the master gain control which sets the overall output level of the mixed sources (including signals applied to the MIX BUS input).

Limiter

The front-panel LIMITER IN/OUT switch turns on a fast- acting, peak-responding limiter circuit that cuts overload distortion during loud program intervals without affecting normal program levels. When the LIMITER switch is IN (operating), the mixer output is limited to approximately +15dBm. Increasing the individual or MASTER gain con- trols will increase the average output and the amount of limiting. The limiter threshold can be reset to any other output between ±4 and +18 dBm if desired. With the limit- er switched OUT and tone oscillator activated, adjust IN- PUT 1 and MASTER level controls to produce an output

0.5dB higher than desired. Switch the limiter IN and set the LIMITER THRESHOLD ADJUST control (accessible though the bottom of the chassis) for the desired level.

The front-panel PEAK LED indicator shows limiter op- eration with the limiter in, and operates when program lev- els approach overload with the limiter out. The indicator is much faster than a meter and will be activated by the shortest transient peak, but it remains on long enough to provide easy recognition.

Low-Cut Filters

The low-cut filters provide a low-frequency rolloff to the response curve as shown in Figure 2. The filters are acti- vated by the LO-CUT IN/OUT switch above each individu- al input gain control and can be used individually with each control to reduce wind noise or undesirable low-frequency signals such as from condenser microphones or turntable rumble.

0
±10
±20
20	50	100	200	500	1,000

LOW-CUT FILTER ACTION

FIGURE 2

3