A line audio amplifier is a circuit which is good to have around to adjust a the line or raise the level. This may be necessary during a recording or in a broadcasting system. In addition with some of these amplifiers can be built a mixer line.
Amplifier input can resist to high voltages. The volume potentiometer from input has his “cold” terminal not to the ground but to the first amplifier output. Because the signal is dephased from that at the entrance, amplifier gain negative reaction by P1. In this case amplification is inversely proportional to the size of input signal.
It may be accepted at the entrance even a signal taken directly from the speaker terminals of a power amplifier. Voltage is 24V and a current 4mA.