The Audio Amplifier Card (AAMP) uses the Diodes PAM8408 Class D amplifier to give up to 2 Watts of audio power.
- Stereo 2 Watt output
- Power efficiency over 85%
- Total Harmonic Distortion less than 0.2% (1 Watt, 8 ohms)
- 5 Volt input power using rugged screw terminals
- Optional FPGA connection for volume control
- 32 steps of volume control from -80dB to 24dB
- Computer controlled mute
- Industry standard 3.5mm input jack
- Diodes PAM8408
Class D amplifiers have a high frequency (relative to audio frequency) digital output. They use Pulse Density Modulation to make the average signal over a short span correspond to the audio output. The high frequency digital output is low pass filtered by the inductance of the speaker coil. A digital output is what gives Class D amplifiers such great efficiency and such low distortion.
Even if the average is low, the instantaneous output is at full power for brief periods. Underrated speakers can catch fire when driven to full power by a Class D amplifier. This is why you must use a 3 Watt (or higher) magnetic speakers with the AAMP card.
The schematic and layout of the AAMP are shown below. The Eagle schematic, board, and bill-of-materials are available here: aamp.sch, aamp.brd, and aamp.txt.