The LM4857 is an audio power amplifier and part of the Neo1973 audio subsystem. It's connected to the VB power rail (battery terminal voltage) for power and thus needs to be powered off separately from the main system.
The audio power output may vary as the battery discharges:
- 0.5W per-channel stereo speaker output, used for the main speakers (LLS+- and RLS+-)
- 33mW per-channel stereo headphone output (32 ohms), for the headset (LHP and RHP)
- 43mW mono earpiece output, for the earpiece (EP+-)
- Line-out, unused.
There is a stereo (LIN and RIN) and a mono (MIN) input. There are several sources connected via switches that can cut each source out:
- "Left Mixer Left" and "Right Mixer Right Playback Switch" connect the WM8753.
- "Mono Mixer Left", "Mono Mixer Right Playback Switch"
- XXX bypass, sidetone, voice
Mixer settings starting "Amp" control it via I2C, including:
- Amp Mode: Off, Call Speaker, Stereo Speakers, Stereo Speakers + Headphones, Headphones
- The volume controls Amp Left and Amp Right, and Amp Mono
- Amp Earpiece 6dB toggle amplifies the call speaker output