APM SECTION THEORY OF OPERATION
ANALOG INPUTS:
The input to the APM can accept 1.4Vrms input and drive the resultant APM output
to a level that drives the amplifier to full rated power into 8Ω. The differential analog
inputs come from J700 which connects to the MAIN board of the amplifier. The
input XLR, ¼”, and RCA connectors tie directly to this point on the MAIN board.
The following will be for CH2, with CH1 being identical except for the analog Y
input switch (U601).
U600-A,B form a differential to single ended gain stage with a gain of -3.54dB. See
Gain Block Diagrams below. The output of U600B drives the input of U601, which
is a unity gain amplifier that selects either the ‘A’ input (CH2) or ‘B’ input (CH1)
dependent upon the INPUT_Y drive from the microprocessor (Q600 to U601-1).
U601 provides the Input Y audio routing. The output of U601 drives front panel pot
R610 that feeds single ended amplifier section U600-C. This stage has a maximum
gain of +13.86dB as shown.
For CH2, the output of U601-5,6 drives a comparator, U700-B, that lights the front
panel SPI LED when the input signal is >-40dBu. R600/601 provides the DC bias
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