Linux ALSA software preamp config

asound.conf

# (I use /etc/asound.conf instead of ~/.asoundrc because I run `shairport-sync` as a system service)

pcm.analog_out {             # Defines a new PCM (Pulse Code Modulation) device named "analog_out". This serves as a direct reference to your physical analog output.
    type plug                # Specifies that this PCM is a "plug" type, which means it can handle format conversions and routing to another PCM.
    slave.pcm "hw:PCH,0"     # Sets the "slave" PCM to the actual hardware device. "hw:PCH,0" refers to your sound card (PCH) and its analog output device (device 0).
}

# ---

pcm.preamp {                 # Defines a new PCM device named "preamp". This is your software pre-amplifier.
    type softvol             # Specifies that this PCM device is a software volume control.
    slave {                  # Defines the "slave" PCM, which is the actual audio output device this softvol controls.
        pcm "analog_out"     # Sets the slave PCM to "analog_out", meaning audio will be routed through your direct analog output after softvol processing.
    }
    control {                # This block defines the mixer control that will be visible in ALSA mixer applications like `alsamixer`.
        name "Pre-amp"       # Sets the user-friendly name of the mixer control that will appear in mixer utilities.
        card 0               # Associates this mixer control with sound card 0 (your "HDA Intel PCH").
    }
    min_dB -50.0             # Sets the minimum volume level for the "Pre-amp" control, in decibels (dB). Allows for significant attenuation.
    max_dB 20.0              # Sets the maximum volume level for the "Pre-amp" control, in decibels (dB). Allows for some amplification.
    resolution 128          # Defines the number of steps (granularity) for the volume control between min_dB and max_dB. A higher number means finer control.
}

# ---

pcm.!default {               # Defines the system's default PCM device. The "!" makes it override any other default setting.
    type plug                # Specifies that this default PCM is a "plug" type, allowing for routing.
    slave.pcm "preamp"       # Sets the default output to your "preamp" PCM. This means any application playing to the default ALSA device will use your software pre-amp first.
}