How do you connect mono speaker into Spresense?
How do I connect two 5W speakers into Spresense? What's built-in amplifier on Spresense?
Personally I used an external amplifier (PAM8406 for example) with a TRRS 3.5mm jack and connected that to the jack on the Spresense extension board.
I just found this in the docs:
4.12. How to use speakers
CXD5247 on Spresense main board has an on-chip Class-D stereo amplifier that can drive a stereo pair of speakers.
This section will describe how to configure the Spresense boards to enable the speaker output when using the main board together with the extension board. The Class-D stereo amplifier can produce up to 400mW into 8Ω impedance speakers.
That means you can't run speakers with a lower impedance without risking damage to the onboard amplifier. However, if they are 8Ω or higher, it will just be quieter the speakers are rated for more power.
To enable the high power output you need to change a few components (Change resistors to onductors and use different capacitor values) on the main board. Unfortunately these components are small and need a very fine soldering iron and a steady hand. Maybe even a microscope.