4 • BX-8x7
© Box 73 Amateurfunkservice GmbH 2014
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Circuit
Figure 2 shows the AF dynamic compres-
sor’s schematic. Its central feature is the
special Analog Devices SSM2165 micro-
phone preamplifier [2]. This IC has a gain
of 18 dB, which is compensated for by the
R10/R11 voltage divider. The level of the
AF signal can be changed, and thereby ca-
librated to the transceiver input when col-
lector resistance in the low-noise pre-am-
plifier stage has been set with potentiome-
ter P1. The Switch S1is used to turn the
compressor function on and off, and along
with potentiometer P2 sets the desired
compression ratio. Three of the four
NAND gates on CMOS chip IC2 form the
AF generator, which can be activated by
simultaneously pressing the PTT and
Down microphone buttons.
The compressor is powered from the inter-
nal 5V DC rail of the connected transcei-
ver. Current consumption is approximate-
ly 10 mA.
Figure 2: Dynamic compressor schematic
