| 摘要 |
PURPOSE:To prevent the generation of noise owing to external induction of a bias oscillating coil by controlling the operation of a playback amplifier, a sound recording amplifier and a sound recording bias oscillating circuit with one circuit of switch, and short-circuiting the sound recording bais oscillating coil in the stage of playback. CONSTITUTION:A switch S11 is so constituted to change over sound recording and playback by changing the voltage level of the DC bias of a playback amplifier 12 at the same instant when transistors TR1, TR2 are switched. The DC bias terminals 15, 23 of a playback amplifier 12 are usually kept at 1.2V and the amplifier 12 operates when a source voltage is applied on said terminals, but when the voltage level of the terminal 23 is decreased to 0V, the amplifier 12 does not operate any longer even if the source voltage is applied on the terminal. The operation control of the playback amplifier by changing- over of the DC bias of the amplifier 12 and the operation control of a sound recording amplifier 11 and a sound recording bias oscillating circuit 13 by the transistor switch are thus accomplished and at the same instant, the prevention of noise generation owing to external induction in the stage of playback by a coil acting as a sound recording bias oscillator in the stage of sound recording is made possible by one circuit of switch. |
