PURPOSE: To make it possible to change over signals with no variation in output direct-current level and constant alternating-current gains in terms of monaural, stereophonic and different programs, by the circuit constitution which can change over the operation functions of various input signals easily.

CONSTITUTION: At the time of stereophonic broadcasting, switch unit 8 is connected to open terminal 10. As a result, transistors Trs T10 and T11 turn ON while Trs T5 to T8 turn OFF and the stereophonic matrix circuit is formed of TrS T1 to T4. Applying signal L+R and signal L-R to input terminals 1 and 4, and 2 respectively allows signals -2AR and -2AL to appear at output terminals 5 and 6. Here, A denotes the voltage gain. At the time of monaural broadcasting, on the other hand, switch unit 8 is changed over to terminal 11. As a result, Trs T10 and T11 are OFF, Trs T1 to T8 operate as a differential amplifier, and a monaural signal can be obtained from output terminals 5 and 6 by applying the monaural sound signal to terminals 3 and 4. At the time of different program broadcasting, applying a signal to input terminals 3 and 4 obtains the operation of a cascade amplifier and the signal appears only at output terminals 5 and 6.