Abstract: An oscillation-echo preventing circuit has a microphone/speaker unit and a voltage-canceling circuit for canceling voltages of audio receive signals. The microphone/speaker unit has a main body, at least two microphones, and a speaker or an earphone. The microphone seals a first inside space from an outside space. The microphone seals the first inside space from a second inside space. The speaker or the earphone seals the first inside space from the outside space. The voltage-canceling circuit cancels out the voltages of audio receive signals coming from the microphones, respectively, generating an output of minimum magnitude. Thus, the circuit can sufficiently suppress oscillation and echoing.

Abstract: An oscillation-echo preventing circuit according to the present invention has a microphone/speaker unit (12) and a voltage-canceling circuit (13) for canceling voltages of audio receive signals. The microphone/speaker unit (12) has a main body, at least two microphones MIC1 and MIC2, and a speaker SP1 or an earphone. The microphone MIC1 seals a first inside space from an outside space. The microphone MIC2 seals the first inside space from a second inside space. The speaker SP1 or the earphone seals the first inside space from the outside space. The voltage-canceling circuit (13) cancels out the voltages of audio receive signals coming from the microphones, respectively, generating an output of minimum magnitude. Thus, the circuit can sufficiently suppress oscillation and echoing.

Abstract: This oscillation prevention circuit is characterized in that a charging switch circuit and a transfer switch circuit are provided in series between a first amplifier circuit and a second amplifier circuit; and a control circuit turning OFF the transfer switch circuit when the charging switch circuit is ON and also turning OFF the charging switch circuit when the transfer switch circuit is ON is provided to prevent the two switch circuits from being turned ON at the same time.

Abstract: The oscillation prevention circuit according to the present invention is used for two-point interactive communication between two communication equipment connected to each other via a transmission system, and either one of the two communication equipment has a means for preventing the oscillation phenomenon which occurs between the two communication equipment.