Downstream Volume Control

Plugins
3

I’ve built a Volumio plugin that solves exactly the same problem you’re describing — keep the Volumio volume UI fully functional, but redirect all volume changes to an external hardware volume controller while keeping the digital path bit-perfect.

In my case the “external DAC” is a Genelec GLM adapter. I’ve reverse-engineered the USB protocol so I can set system volume directly from the Raspberry Pi, without the GLM PC software. This means the audio stream stays completely unaltered end-to-end (Volumio → digital out → Genelec DSP/room correction → finally the analog stage in the monitors applies gain). No double volume control, no digital attenuation, no clipping losses.

The key insight:
You don’t need to hack ALSA or create a fake mixer file. Volumio already supports mixer overrides cleanly.

My plugin works by:

  1. Registering itself as a volume override via
    volumioUpdateVolumeSettings({ volumeOverride: true, ... })
  2. Providing its own alsavolume() implementation, which catches every incoming volume command (e.g. +, -, absolute values) and never touches ALSA.
  3. Emitting proper volume state objects back to Volumio, so the UI slider, REST API, WebSocket events, and plugins keep working as if nothing changed.
  4. Redirecting the volume value to my external device (in my case by issuing a USB command to the GLM network adapter).

Your situation with AD5234 digital pots, I2C gain elements, or multi-channel analog gain blocks fits the same pattern perfectly.

Here’s the relevant pattern from my plugin:

  • initVolumeSettings() sets up a dummy mixer with volumeOverride: true
  • Volumio now calls my plugin for all volume actions
  • My alsavolume() method updates my hardware instead of ALSA
  • getVolumeObject() reports the resulting state back to Volumio
  • No ALSA mixertype, no MPD softvol, no double attenuation

No UI elements break, no configuration files are write-protected, no system logs fill with ALSA errors, and everything survives updates.

So yes — what you’re trying to do is possible without hacks. You just need to register your own mixer override and implement the volume interface inside a plugin. Volumio is perfectly happy to delegate all volume responsibilities to an external controller as long as you provide the correct callback functions.

I used this plugin as reference for my own custom plugin: