Audio analog connection

Precautions for using analog audio with APF27_Dev

Connecting a simple stereo headphone and a separate microphone to the APF27_Dev requires no special precaution.

But there are some more complex configurations which may cause the audio output to refuse functioning with no visible cause, which are described below.

Note: The problems described here do not occur with the APF51_Dev, which has no audio virtual ground.

Electrical background

The output of an audio amplifier powered by a single positive voltage presents normally a quiescent DC voltage of approximately half the power supply.

This DC voltage should never be applied to a loudspeaker, so the classical wiring involves a series capacitor to block this DC voltage (two capacitors for a stereo amplifier).

The capacitance value has to be large enough to preserve the lowest limit of the audio bandwidth (example : 220 uF for a 16 Ohm loudspeaker).

Another solution, avoiding the need for large capacitors, consists of creating a virtual ground for the speaker, the virtual ground being a DC voltage source with the same value as the quiescent voltage of the amplifier output.

In this context, the APF27_Dev board is designed to use the virtual ground available on the TS2101 audio chip.

The consequence is that the ground terminal of the audio output jack is connected to a 1.5V DC source (TS2101 VGND pin) instead of the board real ground.

Warning: It is not permitted to connect this virtual ground to the real ground, this would short-circuit the 1.5V source and cause the TS2101 to enter the "short-circuit protection mode" and stop audio output operation (see P. 28 of the TS2101 data sheet).

Ground loop via mains earth

Here is an example of virtual ground short-circuit caused by the mains earth connection :

• the audio output of the APF27_dev is connected to a home stereo amplifier or any other audio equipement having a mains plug with earth connection
• the APF27_Dev has its ground connected to some equipement having a mains plug with earth connection, for example :
  • the development PC, through the RS232 cable
  • the power supply (AC adaptor) itself

suggested workarounds :

• unplug the RS232 cable
• use a power supply with no earth connection

Note: the Ethernet cable causes no ground loop, because it is isolated by transformers at both ends.

Ground loop via the microphone jack

The audio input connector (microphone jack) of the APF27_Dev has its ground connected to the board's real ground.

Connecting together the grounds of both audio jacks causes a virtual ground short-circuit.

Examples :

• connecting the audio output and input to the same audio equipement, like an audio analog mixer
• connecting the audio output and input to a headset with microphone with common ground

suggested workaround :

• when both jacks are used, connect only the ground of the input jack, leaving the ground of the output jack unconnected
• then insert 2 capacitors in series with the audio outputs