SOUND RECORDING AND AMPLIFYING CIRCUIT

摘要

PURPOSE:To prevent a hum in a power voltage from being mixed into an input signal by comparing a reference voltage extracted from the signal of a ripple filter circuit with the output voltage of a low frequency amplifier and controlling a variable impedance element according to the comparison output. CONSTITUTION:Output voltages of plural low frequency amplifying circuits A are compared by a comparing circuit C with the reference voltage from a reference voltage source B. The voltage source B applies the voltage having a ripple component removed through a ripple filter capacitor C3 and a resistance R11 to the base of a transistor (TR) Q38 whose emitter output voltage is applied to the base of a TRQ32 through the connection point between resistances R20 and R21. The emitter of the TRQ32 is connected to the collector of a constant current source TRQ31, so the reference voltage while is never influenced by variation in power voltage is inputted from the emitter of the TRQ32 to the circuit C. The circuit C compares the reference voltage with output voltages of the circuits A and comparison outputs are amplified by a driving circuit D to control the variable impedance element B by the amplified signal, thereby controlling the output levels of the circuits A automatically.