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Sound Activated Switch for Amplifiers

Automatic Switch-On for Amplifiers and Active Speakers

Sound Activated Switch for Amplifiers
Sound Activated Switch for Amplifiers
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.  
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Materials

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Bill of Materials

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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