JPS57168512A - Automatic level controlling circuit - Google Patents

Abstract

PURPOSE:To prevent the breakage of a tweeter of a speaker system when a high-level signal is supplied, by controlling a gain controlling circuit with the output signal of a detecting circuit that detects the working state of an electric power amplifying circuit. CONSTITUTION:The level is increased for the aural signal which is supplied to an input terminal, and an electric power amplifying circuit 4 works in a saturated region. Thus the output of the circuit 4 is clipped at its peak part. In this case, the negative feedback quantity which is supplied to a voltage amplifying circuit 3 is reduced to less than the level of an input signal via a negative feedback circuit 6. As a result, the output signal of the circuit 3 increases suddenly to be supplied to a working state detecting circuit 7 in the circuit 4. Thus a control signal is obtained at the output side of the circuit 7. This control signal controls an FET2b which forms a gain control circuit 2, and as a result, the level of the signal to be supplied to the circuit 3 is reduced. Thus the circuit 4 becomes active in a linear region to prevent the continuous supply of the electric power higher than the allowable level to a tweeter of a speaker system. Thus the breakage of the tweeter can be avoided.