FUNCTIONALITY

What can you do with this board? Here’s a list that we verified through usage. With further programming, more is likely possible.

1) Play back digital music files via headphones

It acts as a digital music player, in our case using bundled software LXmusic; other players are available. Our version of LXmusic played only stereo FLAC files, up to 24/192 without any difficulty. Volume is digitally controlled by a slider (softvol).

2) Play back digital music files via Line Out.

This is a fixed level 1V maximum output that will feed the hi-fi, through CD or Aux in sockets. Volume is controlled in the hi-fi.

3) Use as external DAC

The board can act as a hi-res audio DAC. The S/PDIF input must be selected for this, alongside the Line Output and/or HeadSet output. Digital signals can be fed in from, say, a portable player. Note that there is no USB, nor an optical input.

4) Use with external DAC

If you want even better hi-res playback quality then the S/PDIF output can be selected, in order to make an electrical connection to an external DAC from, say, Audiolab, Oppo or Chord, all of whom make DACs with greater dynamic range and even better audio quality.

5) Record analogue from Line In.

The input must be selected and then the record file spec etc. It gets complicated, but works well if you have enough card space. Use Ctrl C to stop.

6) Record digital from S/PDIF in. More hanky-panky with user scripts but it works.

7) Record from on-board mics. This is a subject in itself. See the WM5102 Spec Sheet for deep detail on circuitry.  

DRIVING LOUDSPEAKERS

To drive loudspeakers you need to buy an external 5V, 2A power supply with suitable co-axial plug (5.5mm/2.1mm) with centre pole positive; Maplins have a good, smoothed supply of suitable specs and with the necessary co-axial connector. This PSU will run the whole board; the microUSB power line becomes unnecessary. To get up and running means soldering in connecting pins but we soldered in flying leads with insulated croc-clips. The loudspeaker output must also be selected by a screen command at LXTerminal. 

The small Class D amps produce a mass of high frequency switching mush that varied with level and distortion values were in the order of 3-13%. Output moved into overload clipping around 3V, or 1 Watt (into 8 Ohms), so these are 1 Watt amps. A low pass output filter is recommended in the WM5102 datasheet to kill mush – see p318.

What is not said is that there is no software volume control; output is fixed! To change volume I ran the following script:

amixer $1 -Dhw:sndrpiwsp cset name='Speaker Digital Volume' 126

The number at the end – 126 here – sets gain / volume – hardly user friendly. A script could be recalled by using cursor Up key, then modified, to change volume quickly without a complete re-write. But the commands are arcane and tedious to apply.

These little Class Ds are crude so are best not bothered with – hence the lack of connectors etc. LP filters will clear HF mush, but not audio-band mush.

Four through-plated holes, seen at front here, need either pins or wires

soldered in to feed loudspeakers. The vertical gold pins are for external

connection.