I am putting together a battery powered audio amplifier using the PAM8403 class D chip.


It has an active low STANDBY pin which turns off the circuits bias and brings idle current to <1uA. The PAM8403 will be powered by a 3.7V lipo battery and a DC/DC boost circuit to bring it up to 5V so I'd like to conserve all the microamps I can. I'm thinking of a circuit that checks for voltage or current at the amp inputs and sends the low signal to the STANDBY pin if nothing is detected after x amount of time. Ideally the circuit would have to be low current itself as it would be on so long as the battery is connected.

What are my options here? I tried google and couldn't find anything but I am thinking a small microcontroller might be the best bet?

  • 1
    \$\begingroup\$ If you plan on using a connector for the input you can probably exploit a microswitch on said connector. The input signal is normal line level? Were would it come from? \$\endgroup\$ May 14, 2015 at 17:06
  • \$\begingroup\$ Input signal would be higher than line level as it would be coming from the headphone output of an iPhone or iPod (user can adjust volume on iPhone/iPad AND the volume of the PAM8403 circuit). \$\endgroup\$
    – disorder
    May 14, 2015 at 17:31
  • \$\begingroup\$ earphone level is pretty much line level, but it's capable to drive some 32\$\Omega\$, which is something you don't need or care. What about the connector? \$\endgroup\$ May 14, 2015 at 17:36
  • \$\begingroup\$ I can think of various designs based around comparators and envelope detectors, but @VladimirCravero's suggestion of using a headphone jack that has a switch seems like the simplest to me. \$\endgroup\$ May 14, 2015 at 17:36
  • \$\begingroup\$ aaaand the most power saving one since you can disconnect the whole circuit from the battery, pretty much as in guitar effects pedals \$\endgroup\$ May 14, 2015 at 17:38

2 Answers 2


I have implemented something similar to that described by @Dean to turn on/off my whole house audio system when audio is available.

I used an LM339 dual comparator. I AC couple the input signal to the first comparator with the other input set to a DC voltage representing the smallest signal I wish to turn the system on. The open collector output is arranged to go low when signal goes above the threshold. There is a weak pull-up and a large capacitor to give a time delay of a few minutes.

At the output of the first comparator then goes to the second comparator so that when if the voltage on the capacitor is below a threshold the output signal then enables the audio amplifiers (in my case it actually switches the power supply).

A few minutes after the audio input goes away the amp turns off and it only needs a short duration of signal to enable the amplifiers.



Voltage comparator with digital logic compatible level output (CMOS voltage comparator), use a capacitor and resistor with a long time constant so that the comparator doesn't respond to the actual signal dropping rapidly to zero volts.


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