12 Nov 2012: Headphone and Speaker Circuit
This variant on a headphone and speaker circuit is much simpler, and requires less pins on the STM32F. The headphone socket is used to switch off the two speakers. An ST LM2822D was found (and then a very low-cost China equivalent). The LM2822D is however only 380mW per output, which is a little on the low side. Potentially two of them could be used in Bridge mode, giving 0.8W per channel. The main benefit was finding a suitable PWM filter circuit which could be connected to this amplifier: there was some concern over the Class D MOSFET H-bridge approach creating too much R.F. even though it is much more efficient than Class AB. The H-bridge also required four pins, and could not be connected to the headphones, requiring a separate circuit. This approach is much simpler.