Footswitch Board
Principles of Operation
Force FX Electrosurgical Generator C Service Manual 4-19
Meanwhile, the output of U1D is allowed to float, thus removing the variable resistor
divider from the circuit. In this case, the volume control potentiometer becomes a small
resistance in series with the high impedance input from the audio amplifier, negating the
effect of the volume setting.
Audio amplifier U7 and speaker SP1 comprise the final stage in the audio system. The
audio signal is AC coupled to the amplifier by C25 to eliminate the need for well-
controlled input biasing. The voltage gain of U7 is set to about 20 by floating its gain
select pins. Because the U1 output signal is internally biased to Vcc/2, it is necessary to
AC couple the speaker through C27 to prevent the amplifier from DC biasing the
speaker.
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