Mix Bus
Direct access to the mixing bus is provided through the rear-panel MIX BUS phono pin jack. This provision is made primarily to facilitate stacking or ªmultingº M267s to achieve additional input capacity without losing any inputs. With two M267s, for example, the two mixing buses are di- rectly connected, providing two independent master gain controls and two isolated line amplifiers with eight individu- ally controlled inputs. Since the buses are directly paral- leled, a 6 dB drop in gain will occur; and the master or input controls must be increased to compensate. Noise specifi- cations are not adversely affected by this interconnection. Mix bus interconnection can also be made with other Shure mixers, such as the M268, FP42. etc.
VU Meter
The VU meter is factory-calibrated for use with a 600 Ω terminated line. The VU RANGE switch on the rear panel selects either a +4 or +8 dBm output at 0 VU meter indica- tion. (This switch changes the meter indication but does not change the actual output level.) Microphone output levels are 50 dB below line output. The +4 range is recom- mended for normal use to provide approximately 14 dB of headroom from operating level to clipping level.
The VU meter is illuminated by two No. 86 lamps operat- ing well under their normal ratings for a life expectancy of greater than 10,000 hours. The lamps are only lit during ac operation. Consequently, the illumination serves as a visual alarm if the ac is interrupted and the unit has switched to battery.
The VU meter is connected on the primary side of the output transformer to assure protection from any dc level on a telephone line.
Headphones
The headphone outlet appears at the front-panel jack panel designated PHONES. The two-circuit phone jack will accommodate most stereo or mono headphones. The output level is sufficient to provide high volume for use in noisy environments.
Note that the headphone output level is also high enough to use as an auxiliary unbalanced line feed to drive a tape deck or a power amplifier.
The tip and ring connections of the headphone plugs are in phase with pin 3 of all input and output connectors, and with the tip of the MIX BUS jack.
Tone Oscillator
The highly stable, low-distortion tone oscillator provides for line test and level checks. The oscillator is instantly ac- tivated by the front-panel INPUT 1/OSC 1 switch; its level can then be controlled by the INPUT 1 gain control on the front panel. The tone oscillator frequency is 1,000 Hz, and the signal appears on both the line and microphone out- puts, as well as the headphone and mix bus connectors. The oscillator should be switched off (INPUT 1 position) when not in use.
Phantom Power
The rear-panel PHANTOM OFF/ON switch controls the application of phantom power for condenser microphones, such as the Shure SM81 and SM87A, to all inputs. With the switch on and the rear-panel MIC/LINE switches in the MIC positions, +30 Vdc is applied to pins 2 and 3 of each input connection. Series current-limiting resistance is 3.3 kΩ for each input. When using other condenser micro- phones with the M267, verify that the voltage and resis- tance requirements are compatible.
Note that the phantom power cannot normally be ap- plied to the inputs with the MIC/LINE switches in the LINE position.
IMPORTANT: Do not turn the PHANTOM switch on when using unbalanced low-impedance microphones; objec- tionable hum will result. Turn off the PHANTOM switch when condenser microphones are not being used.
Use only high-quality cable. Intermittent shorts between broken shield wires and balanced conductors will cause of- fensive noise transients in the system.
BATTERY OPERATION AND EXTERNAL POWERING
In addition to ac operation, the M267 can be operated from an internal battery pack. Current drain is typically 17 mA at +8 dBm output level and typically 15 mA at + 4 dBm. Battery power is recommended both for remote, on-loca- tion operation, and as an emergency backup source in case of failure of the ac power.
Access to the battery compartment is provided at the bottom of the chassis. Three 9-volt alkaline batteries pow- er the M267 at full rated output. Use alkaline batteries for maximum life. Duracell MN1604A or Eveready 522 are recommended. Battery life is approximately 20 hours at +4 dBm continuos use. Note that battery operation with phan- tom power loads and high level headphone monitoring will increase battery drain.
With batteries in the battery compartment, the M267 will automatically and silently switch to battery operation should the ac voltage fall below a suitable level. If the ac power fails completely, the VU meter lamps will go out, pro- viding a visual indication of line failure.
Battery condition can be determined by using the BATT CHECK switch on the front panel. Activate the BATT CHECK switch and observe the VU meter. A new set of batteries will give about a +2 VU indication. Battery condi- tion is good if the reading is above 0 VU; a lower reading means that new batteries are required for proper opera- tion. Note that the M267 power switch must be turned on to check battery condition.
Telephone Interconnection
When using the M267 connected directly to a telephone line, check to see whether the local telephone company re- quires an interface coupler between the M267 and the tele- phone line. If a coupler is required, make certain the cou- pler selected and the wiring arrangement are in compliance with the telephone company regulations.