Hello eoin_oc,
The SGTL5000 has two gain stages like show in the picture below. A digital DAC gain stage and an analog headphone amp stage.
So you can control both gain settings. it is alway good to start with the highes gain first. otherwise you just amplify ti noise floor with the second stage. but this is also stated in the Datasheet of the SGTL5000
“HeadphoneStereo headphone outputs are provided which can be used to drive a headphone load or a line level output. The headphone output has its own independent analog volume control with a volume range of -52dB to +12dB in 0.5dB step sizes. This volume control can be used in addition to the DAC volume control. For best performance the DAC volume control should be left at 0dB until the headphone is brought to its lowest setting of -52dB. The register CHIP_ANA_HP_CTRL is used to control the headphone volume.”
This is how you can change both settings: keep in mind that you can also change the gain of you Speaker Amplifier.
HeadPhone gain:
Simple mixer control ‘Headphone’,0
..
DAC gain:
Simple mixer control ‘PCM’,0
…
To change Headphone playback volume:
amixer set Headphone 100
(0..127)
To change DAC playback volume:
amixer set PCM 100
(0..192) (edited)
