Audio-Technica AT-MX351 manual Adjusting ”Off” Attenuation, Preamp Outputs, Input Limiters

Adjusting ”Off” Attenuation

In instances when the number of microphones in use is high, it may be necessary to increase the amount of “off ” attenuation per microphone to keep the total ambient noise level low. There are “off ” attenuation adjustments inside the unit. To adjust the “off ” attenuation of channel 1, unplug the unit, remove the top cover and locate the trimpot designated on the circuit board by “VR3” (Figs. 4 and 5, page 11). It is factory set at approximately 8 dB, the attenuation at the middle of the control’s rotation. When the control is turned fully counter- clockwise, “off ” attenuation is approximately 20 dB. Conversely, when the adjustment is turned fully clockwise, “off ” attenuation is approximately 6 dB. VR6, VR9 and VR12 control “off ” attenuation for channels 2, 3 and 4 respectively.

Changing the settings of the switches designated on the circuit board by “SW5,” “SW11,” “SW17” and SW23” will extend the range of “off ” attenuation to –40 dB for channels 1, 2, 3 and 4 respectively.

Preamp Outputs

Each microphone channel has an independent unbalanced preamp output that is separate from the main mixer output. This is helpful when it is necessary to record the output of each channel, whether or not it is the active mixer output (as is required, for example, in some courtroom proceedings). As set at the factory, no gating is applied to these outputs. To gate these outputs, unplug the unit, remove the top cover and locate the switches designated on the circuit board by “SW6,” “SW12,” “SW18” and SW24” (Figs. 4 and 5, page 11). Change the switch position(s) to “Gating” for channels 1, 2, 3 and/or 4 respectively, as desired.

Input Limiters

Independent, adjustable limiters are available on all mic channels. To adjust the limiter threshold of channel 1, unplug the unit, remove the top cover and locate the trimpot designated on the circuit board by “VR2” (Figs. 4 and 5, page 11). It is factory set at approximately 0 dB RMS. When the control is turned fully counter-clockwise, limiter threshold is approximately –10 dB RMS. Conversely, when the adjustment is turned fully clockwise, limiter threshold

is approximately +10 dB RMS. VR5, VR8 and VR11 adjust limiter threshold for channels 2, 3 and 4 respectively.

Changing the settings of the switches designated on the circuit board by “SW4,” “SW10,” “SW16” and SW22” will turn off the threshold-limiting function on channels 1, 2, 3 and 4 respectively.

Force-on/Force-off

To activate force-on/force-off, install a closure between the appropriate pin and Ground Reference on the External Control connector on back of unit.

External Control Connector Pinout

Pin 1	Channel 1 force off	Pin 8	Channel 4 force on
Pin 2	Channel 2 force off	Pin 9	Channel 1 TTL out
Pin 3	Channel 3 force off	Pin 10	Channel 2 TTL out
Pin 4	Channel 4 force off	Pin 13	Ground reference
Pin 5	Channel 1 force on	Pin 14	Channel 3 TTL out
Pin 6	Channel 2 force on	Pin 15	Channel 4 TTL out
Pin 7	Channel 3 force on

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