Context

I am using a USB DAC as the default pulseaudio sound output for my machine under Debian 9. The problem is, sometimes, the default output is lost and switched back to internal chipset, which I do not use.

USB DAC adapter

The USB DAC being used is the HiFimeDIY UAE23 USB DAC, whose pulseaudio sink name is the following:

$ pacmd list-sinks | sed -n 's/^.*name: <([^>]*.usb[^>]*)>$/1/p'
alsa_output.usb-HiFimeDIY_Audio_SA9023_USB_Audio-01.analog-stereo

Of course, my machine is also equipped with a built-in chipset with audio capabilities:

$ pacmd list-sinks | grep -e 'name:'
name: <alsa_output.usb-HiFimeDIY_Audio_SA9023_USB_Audio-01.analog-stereo>
name: <alsa_output.pci-0000_00_1b.0.iec958-stereo>

The first sink name (index 0) is the DAC, the second one (index 1) is the chipset audio output (oddly the digital output despite providing as well analog capabilities).

System details

$ uname -a
Linux hostname 4.9.0-8-amd64 #1 SMP Debian 4.9.130-2 (2018-10-27) x86_64 GNU/Linux

$ lsmod | grep audio
snd_usb_audio 180224 3
snd_usbmidi_lib 28672 1 snd_usb_audio
snd_hwdep 16384 2 snd_hda_codec,snd_usb_audio
snd_pcm 110592 8 snd_hda_intel,snd_hda_codec,snd_usb_audio,snd_hda_core,snd_hda_codec_hdmi
snd 86016 37 snd_hda_intel,snd_hwdep,snd_hda_codec,snd_usb_audio,snd_timer,snd_rawmidi,snd_hda_codec_hdmi,snd_hda_codec_generic,snd_usbmidi_lib,snd_seq_device,snd_hda_codec_realtek,snd_pcm
usbcore 253952 10 usbhid,snd_usb_audio,ehci_hcd,xpad,xhci_pci,snd_usbmidi_lib,ath3k,btusb,xhci_hcd,ehci_pci

$ pulseaudio --version
pulseaudio 10.0

$ pactl info
Server String: unix:/run/user/1000/pulse/native
Library Protocol Version: 32
Server Protocol Version: 32
Is Local: yes
Client Index: 21
Tile Size: 65472
User Name: username
Host Name: hostname
Server Name: pulseaudio
Server Version: 10.0
Default Sample Specification: s16le 2ch 44100Hz
Default Channel Map: front-left,front-right
Default Sink: alsa_output.usb-HiFimeDIY_Audio_SA9023_USB_Audio-01.analog-stereo
Default Source: alsa_input.pci-0000_00_1b.0.analog-stereo
Cookie: 50ff:68bf

$ aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: PCH [HDA Intel PCH], device 0: ALC898 Analog [ALC898 Analog]
 Subdevices: 1/1
 Subdevice #0: subdevice #0
card 0: PCH [HDA Intel PCH], device 1: ALC898 Digital [ALC898 Digital]
 Subdevices: 1/1
 Subdevice #0: subdevice #0
card 0: PCH [HDA Intel PCH], device 3: HDMI 0 [HDMI 0]
 Subdevices: 1/1
 Subdevice #0: subdevice #0
card 0: PCH [HDA Intel PCH], device 7: HDMI 1 [HDMI 1]
 Subdevices: 1/1
 Subdevice #0: subdevice #0
card 1: NVidia [HDA NVidia], device 3: HDMI 0 [HDMI 0]
 Subdevices: 1/1
 Subdevice #0: subdevice #0
card 1: NVidia [HDA NVidia], device 7: HDMI 1 [HDMI 1]
 Subdevices: 1/1
 Subdevice #0: subdevice #0
card 1: NVidia [HDA NVidia], device 8: HDMI 2 [HDMI 2]
 Subdevices: 1/1
 Subdevice #0: subdevice #0
card 1: NVidia [HDA NVidia], device 9: HDMI 3 [HDMI 3]
 Subdevices: 1/1
 Subdevice #0: subdevice #0
card 2: Audio [SA9023 USB Audio], device 0: USB Audio [USB Audio]
 Subdevices: 0/1
 Subdevice #0: subdevice #0
card 2: Audio [SA9023 USB Audio], device 1: USB Audio [USB Audio #1]
 Subdevices: 1/1
 Subdevice #0: subdevice #0

What I've tried so far

• 
  

  Setting up the DAC as the default output sound in CLI by index:

  
  
  

  $ pacmd set-default-sink 0
  

  
  


• 
  

  Setting up the DAC as the default output sound in CLI by sink name:

  
  
  

  $ ADAPTER_SINK_NAME=$(pacmd list-sinks | sed -n 's/^.*name: <([^>]*.usb[^>]*)>$/1/p')
  $ pacmd set-default-sink $ADAPTER_SINK_NAME
  

  
  



• Setting up the DAC as the default output sound using pavucontrol and "Set as fallback"

What's the problem exactly?

When I boot up and I happen to have no sound on the DAC, using pacmd list-sinks, I can see directly that the default sound adapter has changed from the DAC (sink index 0) to the internal built-in output (sink index 1), since it is denoted by the *.

I need to set back the DAC as the default sink, using whatever method I want (pacmd set-default, pavucontrol) to get back the sound on the DAC.

Bonus points

• 
  The problem does not appear after each (re)boot. I cannot explain it.


• It seems that there is a confusion between digital and analog outputs from pulseaudio on the chipset sound capabilities ; I used it without any problem before purchasing the DAC. The chipset provides an analog output and a digital one.

Conclusion

I do not know what to try at this point and I may be missing something. So, I would welcome any hindsight.