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 designated INPUT 1 through INPUT 4. The inputs are balanced (internal transformer, MUMETAL shielded); pins 2 and 3 are �hot�, and pin 1 is �ground�. For microphone operation, the switches labeled LINE/MIC (directly above the input connectors) must be in the MIC position; for line level inputs, the switches must be in the LINE position. For impedance, clipping and operating signal levels, refer to the SPECIFICATIONS section. Outputs The rear-panel connector labeled OUTPUT is a professional three-pin male XLR audio connector. With the adjacent 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 output with the LINE/MIC switch in either position; pin 1 is ground, pins 2 and 3 are �hot�, 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 calibrated for use with a 600 � line. The line output transformer will operate properly with up to 100 mA dc in the line. This feature permits the use of standard �dialed-up� telephone lines with dc across them. (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 preamplifier output attenuation. As the gain is reduced, the preamplifier input clipping level is increased for that channel. 3
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 individual input controls should be operated at as high a setting 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 fastacting, 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 controls 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 limiter switched OUT and tone oscillator activated, adjust INPUT 1 and MASTER level controls to produce an output 0.5 dB 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 operation with the limiter in, and operates when program levels 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 activated by the LO-CUT IN/OUT switch above each individual 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.
