GB1417794A - Muting circuits - Google Patents

Abstract

1417794 Transistor amplifier circuits SONY CORP 10 May 1973 [27 May 1972] 22345/73 Heading H3T The output at terminal 36 from an amplifier 11, deriving an input signal from terminal 38 is muted during switching on and off of the power from A.C. source 13 in order to avoid switching clicks. The power supply comprises transformer 14, full-wave rectifier 15 and voltage stabilizing circuits 18, 19 in the positive and negative supply lines respectively. Due to the time delay in stabilizing circuit 18, a supply voltage appears at terminal 16 before one at terminal 20; consequently transistor 35 receives a bias which renders it conductive to mute the signal output upon switching on, since transistor 25 is non-conductive until capacitor 32 has charged up over resistors 29, 30, 31. When transistor 25 becomes conductive, transistor 35 is biased to cut off. While the amplifier is operative, transistor 26 receives a negative bias on its emitter, from terminal 21, and so is conductive, to charge capacitor 28 negatively. When the supply is switched off, the emitter of transistor 26 goes to earth potential; the transistor cuts off, thereby biasing transistor 35 conductive and so muting the signal output. The arrangement may be applied to a stereo system (Fig. 2, not shown); also the transistor 35 may be replaced by a field-effect transistor with its sourcedrain path in series with the signal path (Fig. 3, not shown).