# Circuit to put digital amplifier into standby when no audio is at inputs?

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

http://www.diodes.com/datasheets/PAM8403.pdf

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?

• 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? – Vladimir Cravero May 14 '15 at 17:06
• 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). – disorder May 14 '15 at 17:31
• 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? – Vladimir Cravero May 14 '15 at 17:36
• 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. – Nick Johnson May 14 '15 at 17:36
• aaaand the most power saving one since you can disconnect the whole circuit from the battery, pretty much as in guitar effects pedals – Vladimir Cravero May 14 '15 at 17:38

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.

kevin

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.