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MIC/LINE PREAMP
The SR40•8 uses Mackie’s famous low-noise, high headroom mic/line preamp design. The
circuit has been revised since the version you may be familiar with from the earlier
Mackie products. It’s topology is more elaborate than a standard discrete differential
pair feeding an op-amp and has considerable performance advantages.
INPUT CIRCUITRY
Audio signal can be applied to either the XLR jack or ¼” TRS jack. From the XLR input
R140 and R141 provide fusing protection, C105,107, 108 roll RF, coupling capacitors
C101,106 block DC from the 48V phantom supply or any external source. From the ¼” TRS
input the signal passes through coupling caps C111,112. R114,115 provide a -20dB pad
when using the ¼” TRS input. Both inputs are clamped to within 12V of ground by
D101,102.
PREAMP CIRCUITRY
On the channel board further RF rolloff is provided by C101,102,103 and FB102,103.
Q103,104 form current sources for the differential pair Q101,102. Q105 provides a voltage
reference to U110A,110B that tracks the differential pair’s current source, providing
excellent DC stability and power supply rejection (reduced power-up pop).
Q101,102 function as discrete voltage to current stages for op-amps U110A,110B
respectively. Notice that the collectors of Q101,102 drive current to U110A,110B inverting
inputs, there is no voltage swing on the collectors. Gain is adjusted by R106. FB101 is
added for further RF rejection. The outputs of U110A,110B are then fed to the differential
op-amp U103A.
