Reduces the POP noise when switching outputs on the Asus Xonar Essence STX (ST also probably). Linux Kernel Patch.

apply_patch.sh

#!/bin/bash

MAKE_FLAGS="-j8"

<<About
    This script compiles the snd_oxygen_lib/snd_oxygen/snd_virtuoso modules with the patch in this gist and installs them.
    You will need to rerun this every time you use a new kernel.
    Tested on Arch Linux (Kernel 4.4).

    Copyright (C) 2016  kevinlekiller
    
    This program is free software; you can redistribute it and/or
    modify it under the terms of the GNU General Public License
    as published by the Free Software Foundation; either version 2
    of the License, or (at your option) any later version.
    
    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.
    
    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
    https://www.gnu.org/licenses/old-licenses/gpl-2.0.en.html
About

kernel_version=$(uname -r | grep -Po "^[\d.]+")

if [[ -z $kernel_version ]]; then
    echo "Error: Problem detecting current kernel version."
    exit 1
fi

modules_path="/lib/modules/$(uname -r)"

if [[ ! -f "$modules_path/build/Module.symvers" ]]; then
    echo "Error: Can not find the Modules.symvers file for the current kernel."
    exit 1
fi

if [[ ! -f "xonar_pcm179x.c.patch" ]]; then
    wget "https://gist.githubusercontent.com/kevinlekiller/f533f4d1f7318a7cf81a/raw/xonar_pcm179x.c.patch"
    if [[ $? != 0 ]] || [[ ! -f "xonar_pcm179x.c.patch" ]]; then
        echo "Error: Problem downloading kernel patch."
        exit 1
    fi
fi

if [[ ! -f "linux-$kernel_version.tar.xz" ]]; then

    wget "https://cdn.kernel.org/pub/linux/kernel/v$(echo $kernel_version | head -c 1).x/linux-$kernel_version.tar.xz"

    if [[ $? != 0 ]]; then
        echo "Error: Problem downloading kernel source."
        exit 1
    fi

    tar -xf "linux-$kernel_version.tar.xz"

    if [[ $? != 0 ]]; then
        echo "Error: Could not extract kernel."
        exit 1
    fi
else
    tar -xf "linux-$kernel_version.tar.xz"

    if [[ $? != 0 ]]; then
        echo "Error: Could not extract kernel."
        exit 1
    fi
fi

if [[ ! -d "linux-$kernel_version" ]]; then
    echo "Error: Kernel directory not found."
    exit 1
fi

cd "linux-$kernel_version"

make clean

cp "$modules_path/build/Module.symvers" .

zcat /proc/config.gz > .config

sed -i "s/CONFIG_LOCALVERSION=\".*\"/CONFIG_LOCALVERSION=\"$(uname -r | grep -Po "\-.*$")\"/" .config

patch sound/pci/oxygen/xonar_pcm179x.c ../xonar_pcm179x.c.patch

if [[ $? != 0 ]]; then
    echo "Error: Problem patching xonar_pcm179x.c"
    exit 1
fi

make prepare

make "$MAKE_FLAGS" sound/pci/oxygen

make modules_prepare

make "$MAKE_FLAGS" modules SUBDIRS=sound/pci/oxygen

sudo make modules_install SUBDIRS=sound/pci/oxygen

sudo mv "$modules_path"/extra/snd-*.ko.* "$modules_path"/extramodules/

sudo depmod -a

cd ..

sudo rm -rf linux-$kernel_version*
sudo rm -rf linux-$kernel-version
rm -f xonar_pcm179x.c.patch
rm -f "linux-$kernel_version.tar.xz"

echo "Done, reboot to safely reload the compiled modules."

xonar_pcm179x.c.patch

--- linux-4.4.17/sound/pci/oxygen/xonar_pcm179x.c	2016-08-10 05:49:43.000000000 -0400
+++ linux-4.4.17/sound/pci/oxygen/xonar_pcm179x.c	2016-08-18 18:40:53.814312616 -0400
@@ -430,11 +430,14 @@
 	oxygen_set_bits16(chip, OXYGEN_GPIO_CONTROL,
 			  GPIO_INPUT_ROUTE | GPIO_ST_HP_REAR |
 			  GPIO_ST_MAGIC | GPIO_ST_HP);
-	oxygen_clear_bits16(chip, OXYGEN_GPIO_DATA,
-			    GPIO_INPUT_ROUTE | GPIO_ST_HP_REAR | GPIO_ST_HP);
+	// From what I gather, all this does is resets the output to speakers, which clicks a relay for no reason.
+	/*oxygen_clear_bits16(chip, OXYGEN_GPIO_DATA,
+			    GPIO_INPUT_ROUTE | GPIO_ST_HP_REAR | GPIO_ST_HP);*/
 
 	xonar_init_cs53x1(chip);
-	xonar_enable_output(chip);
+	// Do not enable the DATA right now, only the CONTROL, we will enable the DATA later, otherwise we get the POP.
+	//xonar_enable_output(chip);
+	oxygen_set_bits16(chip, OXYGEN_GPIO_CONTROL, data->generic.output_enable_bit);
 
 	snd_component_add(chip->card, "PCM1792A");
 	snd_component_add(chip->card, "CS5381");
@@ -834,9 +837,20 @@
 		gpio = (gpio | GPIO_ST_HP) & ~GPIO_ST_HP_REAR;
 		break;
 	}
+	// Disable the output.
+	oxygen_clear_bits16(chip, OXYGEN_GPIO_DATA, data->generic.output_enable_bit);
+	msleep(data->generic.anti_pop_delay);
+	gpio &= ~GPIO_ST_OUTPUT_ENABLE;
+
 	oxygen_write16(chip, OXYGEN_GPIO_DATA, gpio);
 	data->hp_active = gpio & GPIO_ST_HP;
 	update_pcm1796_volume(chip);
+
+	// Undo the change to gpio for the return, enable the output.
+	gpio &= GPIO_ST_OUTPUT_ENABLE;
+	msleep(data->generic.anti_pop_delay);
+	oxygen_set_bits16(chip, OXYGEN_GPIO_DATA, data->generic.output_enable_bit);
+
 	mutex_unlock(&chip->mutex);
 	return gpio != gpio_old;
 }

Well I tip my fedora to you mister. Been looking for a way to fix the loud pop on STX for roughly 5 years now and now its almost completely fixed.

Worked perfectly on Archlunux Kernel 4.9.