LA Audio MPX10 - Manual (Page 29)

Page 29
6 . 1 I N P U T S
The Mic input has a dedicated amplifier stage which is optimised for low
noise, impedance matching and common mode rejection. This amplifier is
protected against RFI and excessive input levels. The GAIN control for this
stage is ganged to the Line amplifier to ensure maximum signal headroom.
The +48V phantom power is supplied to pins 2 and 3 of the XLR connector
via 6k8 resistors.
The Line input feeds a separate variable gain differential amplifier which
also provides all of the gain for the DI input.
The DI Input is buffered by a high input impedance FET amplifier prior to
being switched into the Line amplifier.
The Phase switch swaps the inputs to the Line amplifier thereby reversing
the phase of the incoming signal. The Mic, Line and DI inputs are all
affected by the Phase switch.
Peak detector circuitry monitors the signal level from the Line amplifier and
is arranged to light 3dB before clipping.
The Filter is a 2nd order (12dB/Oct.) highpass optimised for maximally flat
response above the cutoff frequency. The -3dB frequency is 75Hz.
6 . 1 E X P A N D E R & C O M P R E S S O R
Gain reduction for the Expander and Compressor is provide by a single
VCA (voltage controlled amplifier) which responds to the combined control
voltage produced by the Expander and Compressor sidechains. This
parallel control topology prevents interaction problems commonly
associated with series connected Expanders and Compressors.
After the sidechain insert jack the control signal feeds the DS filter. This is a
high pass filter with a 6dB slope starting around 1kHz. Following this is the
RMS detector which produces a DC output voltage proportional to the RMS
value of the input signal.
E X P A N D E R
Threshold adjusts the reference level for the expander which is compared
with the RMS voltage. For input signals falling below Threshold the
expander sidechain produces an output proportional to the Ratio and the
amount the signal is below Threshold. The EXP led lights when the input
signal is below Threshold ie. at the onset of attenuation.
Attack (<1mS) and Release (0.5S) are fixed.