MPA685 Owner’s Manual
3/2012
11
5. Adjusting the Audio Input Level Settings
After audio connections are made and the audio source is supplying signal, adjust the
“Input” knob until the Overload LED lights infrequently, then, decrease the Input knob by
one step. This maximizes the dynamic range of the audio. For maximum linearity, the
Overload LED should never light. However, because the pre-amp distortion curve
versus input level increases gradually, infrequently lighting the Overload LED with audio
peaks still realizes good quality. As always, your ears are the best guide for how high
the input signal can be set.
The Input section stepped switch functions as a combined attenuator / gain control.
When the switch is adjusted between “-11” and “-1” on the front panel, the switch is
attenuator mode, and the pre-amp gain stage operates with fixed gain. When the
switch is adjusted between “0” and “12” on the front panel, the switch is in gain mode,
the attenuator is removed from the circuit and the pre-amp gain is varied. The following
graph shows the relative changes in total pre-amp gain versus each stepped switch
setting. Relative gain is shown, rather than absolute gain since absolute gain depends
on whether the pre-amp is operated in DI mode or in microphone mode, and which
microphone impedance load is selected.
Sixty five dB adjustment range is provided, while allowing dynamic range to be
maximized with one simple procedure for any signal level.
