Pre-Amplifier example.

This example project uses the EFM32 CMSIS including DVK BSP (board
support package) and demonstrates the use of the ADC/DAC/DMA/PRS/TIMER
in order to implement a very simple pre-amplifier.

- Connect an audio source to the audio in connector
  on the DVK, for instance an MP3 player.
- Connect an active loudspeaker with built-in amplifier
  to the audio out connector on the DVK.

WARNING:
--------
Do not attach or use headphones with this example. Use small loadspeakers
with built in amplification, ensuring volume is at an acceptable level. 
Exposure to loud noises from any source for extended periods of time may
temporarily or permanently affect your hearing. The louder the volume 
sound level, the less time is required before your hearing could be 
affected. Hearing damage from loud noise is sometimes undetectable at 
first and can have a cumulative effect.

The input signal is converted to digital and level
adjusted by the EFM32 core before being converted back
to analog.

The volume level is adjusted with the potentiometer, which
is also read using the ADC. The volume level is indicated
by the 14 rightmost user LEDs.

The example will clip the output signal if exceeding predefined
level. This may occur due to too high input signal and/or to high
volume setting. Clipping is indicated by the leftmost user LED.

Board:  Silicon Labs EFM32-Gxxx-DK Development Kit
Device: EFM32G290F128 and EFM32G890F128
