3 files changed, 21 insertions, 8 deletions
diff --git a/src/drivers/gpios.cpp b/src/drivers/gpios.cpp
index e5560665..5c255204 100644
--- a/src/drivers/gpios.cpp
+++ b/src/drivers/gpios.cpp
@@ -37,13 +37,13 @@ static const uint8_t kPortADefault = 0b00111110;
 // 0 - 3.5mm jack detect (active low)
 // 1 - headphone amp power enable
 // 2 - sd card detect
-// 3 - NC
-// 4 - NC
+// 3 - amplifier unmute (revisions < r8)
+// 4 - amplifier mute (revisions >= r8)
 // 5 - NC
 // 6 - NC
 // 7 - NC
 // Default inputs high, amp off.
-static const uint8_t kPortBDefault = 0b00000101;
+static const uint8_t kPortBDefault = 0b00001101;
 
 /*
  * Convenience mehod for packing the port a and b bytes into a single 16 bit
diff --git a/src/drivers/i2s_dac.cpp b/src/drivers/i2s_dac.cpp
index 4d29f917..e206bd7a 100644
--- a/src/drivers/i2s_dac.cpp
+++ b/src/drivers/i2s_dac.cpp
@@ -88,13 +88,21 @@ I2SDac::I2SDac(IGpios& gpio, i2s_chan_handle_t i2s_handle)
                                                        I2S_SLOT_MODE_STEREO)) {
   clock_config_.clk_src = I2S_CLK_SRC_APLL;
 
+  // The amplifier's power rails ramp unevenly, with the negative rail coming
+  // up ~5ms after the positive rail. Ensure that headphone output is muted
+  // during this to avoid a loud pop during power up.
+  gpio_.WriteSync(IGpios::Pin::kAmplifierMute, true);
+  vTaskDelay(pdMS_TO_TICKS(1));
+
   gpio_.WriteSync(IGpios::Pin::kAmplifierEnable, true);
 
   // Reset all registers back to their default values.
   wm8523::WriteRegister(wm8523::Register::kReset, 1);
+
+  // Wait for DAC reset + analog rails ramp.
   vTaskDelay(pdMS_TO_TICKS(10));
-  wm8523::WriteRegister(wm8523::Register::kPsCtrl, 0b0);
 
+  wm8523::WriteRegister(wm8523::Register::kPsCtrl, 0b0);
   // Use zero-cross detection for volume changes.
   wm8523::WriteRegister(wm8523::Register::kDacCtrl, 0b10000);
 }
@@ -103,6 +111,8 @@ I2SDac::~I2SDac() {
   Stop();
   i2s_del_channel(i2s_handle_);
 
+  gpio_.WriteSync(IGpios::Pin::kAmplifierMute, true);
+  vTaskDelay(pdMS_TO_TICKS(1));
   gpio_.WriteSync(IGpios::Pin::kAmplifierEnable, false);
 }
 
@@ -119,11 +129,13 @@ auto I2SDac::Stop() -> void {
 
 auto I2SDac::SetPaused(bool paused) -> void {
   if (paused) {
-    gpio_.WriteSync(IGpios::Pin::kAmplifierUnmute, false);
+    gpio_.WriteSync(IGpios::Pin::kAmplifierUnmuteLegacy, false);
+    gpio_.WriteSync(IGpios::Pin::kAmplifierMute, true);
     set_channel(false);
   } else {
     set_channel(true);
-    gpio_.WriteSync(IGpios::Pin::kAmplifierUnmute, true);
+    gpio_.WriteSync(IGpios::Pin::kAmplifierUnmuteLegacy, true);
+    gpio_.WriteSync(IGpios::Pin::kAmplifierMute, false);
   }
 }
 
diff --git a/src/drivers/include/gpios.hpp b/src/drivers/include/gpios.hpp
index fe4b1c4c..55486be7 100644
--- a/src/drivers/include/gpios.hpp
+++ b/src/drivers/include/gpios.hpp
@@ -55,8 +55,9 @@ class IGpios {
     kPhoneDetect = 8,
     kAmplifierEnable = 9,
     kSdCardDetect = 10,
-    kAmplifierUnmute = 11,
-    // 12 through 15 are unused
+    kAmplifierUnmuteLegacy = 11,
+    kAmplifierMute = 12,
+    // 13 through 15 are unused
   };
 
   /* Nicer value names for use with kSdMuxSwitch. */