Audio analog connection
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. |
Contents
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.
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