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Sound Activated Switch for Amplifiers
Automatic Switch-On for Amplifiers and Active Speakers
When you have a power amplifier connected after any sort of audio source, you usually need an additional power switch, which makes things inconvenient. It would be better if the amplifier would automatically switch on at the same time as the signal source. This is possible with the small circuit described here, consisting of three transistors, which detects the presence of an audio signal.
Component list
Component list
Resistors
(SMD *)
R1,R2 = 2.2 kΩ
R3 = 10 kΩ trimpot
R4,R9 = 330 kΩ
R5 = 51 kΩ (15 kΩ @ 20 V) *
R6,R8,R10,R13 = 10 kΩ
R7 = 100 Ω
R11 = 10 kΩ (39 kΩ @ 20 V) *
R12 = 1 kΩ (6.8 kΩ @ 20 V) *
R14 = 10 MΩ
RV = 100 kΩ potentiometer, stereo, log (optional)
Capacitors
C1 = 100 µF 10 V (100 µF 25 V @ 20 V) *
C2,C3,C4 = 100 nF ceramic or film
C5 = 2.2 µF 10V (470 nF 25 V @ 20 V) *
C6–C9 = 220 pF ceramic
Semiconductors
D1,D2 = 1N5711 (Schottky)
D3 = omitted (12 V Z diode @ 20 V) *
D4 = LED *
T1,T2 = BC848B
T3 = BSS84
Other
Amplifier module with PAM8403 or PAM8302 *
FB1–FB5 = ferrite bead
J1–J4 *
* See text
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