Power failure/resumption circuit

The current transients mentioned, depending on magnitude and how well the amplifier circuit is decoupled, may cause dips in V_supply as described.

Some points to address for a solution:

• Most important: Use a freewheeling diode, connected reverse biased, across the speaker leads. The on-to-off transition of the speaker causes a significant reverse spike from an inductive load such as a speaker.
  • Use a power Shottky diode if the current involved is big, else any fast diode with a current rating of at least twice the current expected through the speaker coils in on state would work.
• Use a lot of bulk capacitance at the source of the transients, i.e. across the amplifier circuit, to reduce supply noise propagation: One always tries to eliminate noise at source if possible.
  • Depending on the transient magnitudes, a few hundred to a few thousand microfarads are common in power audio amplifiers. Voltage rating of better than twice the supply voltage would be best.
• Separate out the power and ground planes for the amplifier portion and the microcontroller portion if feasible, interconnecting grounds at a single point.
• Coupling the power plane using a choke coil as mentioned in the question is a good additional measure, but ensure this is a very low resistance high inductance coil, preferably with a core. Tricks with PCB traces will not provide the high inductance needed, an actual inductor would be best.
  • Values from a few hundred microHenries to a milliHenry or so should be good to effectively throttle spike propagation. Use an inductor with the least DC resistance that you can afford. Personal preference is something well under 250 milliOhms if possible.

A simple battery backup IC will do what you want.

Essentially, a comparator samples two voltage inputs, the main power, and a backup power supply (Batteries). Based on that, two cascading inverters paired with two p-channel fets power the circuit at Vo, while two n-channel fets are used as open-drain outputs. Pull them up with a resistor, and they are the perfect interrupt source for your arduino.

The specific one shown is the ICL7673, but it's maximum current is 38mA. It's designed for rtc and ram with a coincell, but an extra pair of P-channels can be used to increase the current.

There are other battery backup or power switches which will do higher current builtin.