Index: libhb/deca52.c
===================================================================
--- libhb/deca52.c	(revision 1265)
+++ libhb/deca52.c	(working copy)
@@ -215,7 +215,8 @@
     hb_list_getbytes( pv->list, pv->frame, pv->size, &pts, &pos );
 
     /* AC3 passthrough: don't decode the AC3 frame */
-    if( pv->job->acodec & HB_ACODEC_AC3 )
+    if( pv->job->acodec & HB_ACODEC_AC3 ||
+        w->amixdown == HB_AMIXDOWN_AC3 )
     {
         buf = hb_buffer_init( pv->size );
         memcpy( buf->data, pv->frame, pv->size );
Index: libhb/muxmp4.c
===================================================================
--- libhb/muxmp4.c	(revision 1265)
+++ libhb/muxmp4.c	(working copy)
@@ -303,7 +303,8 @@
         mux_data = malloc( sizeof( hb_mux_data_t ) );
         audio->mux_data = mux_data;
 
-        if( job->acodec & HB_ACODEC_AC3 )
+        if( job->acodec & HB_ACODEC_AC3 ||
+            job->audio_mixdowns[i] == HB_AMIXDOWN_AC3 )
         {
             mux_data->track = MP4AddAC3AudioTrack( 
                 m->file,
Index: libhb/sync.c
===================================================================
--- libhb/sync.c	(revision 1265)
+++ libhb/sync.c	(working copy)
@@ -144,7 +144,8 @@
 
     for( i = 0; i < hb_list_count( title->list_audio ); i++ )
     {
-        if( job->acodec & HB_ACODEC_AC3 )
+        if( job->acodec & HB_ACODEC_AC3 ||
+            job->audio_mixdowns[i] == HB_AMIXDOWN_AC3 )
         {
             free( pv->sync_audio[i].ac3_buf );
         }
@@ -206,7 +207,8 @@
     sync        = &pv->sync_audio[i];
     sync->audio = hb_list_item( title->list_audio, i );
 
-    if( job->acodec & HB_ACODEC_AC3 )
+    if( job->acodec & HB_ACODEC_AC3 ||
+        job->audio_mixdowns[i] == HB_AMIXDOWN_AC3 )
     {
         /* Have a silent AC-3 frame ready in case we have to fill a
            gap */
@@ -637,7 +639,8 @@
     sync   = &pv->sync_audio[i];
     audio  = sync->audio;
 
-    if( job->acodec & HB_ACODEC_AC3 )
+    if( job->acodec & HB_ACODEC_AC3 ||
+        job->audio_mixdowns[i] == HB_AMIXDOWN_AC3 )
     {
         fifo = audio->fifo_out;
         rate = audio->rate;
@@ -799,7 +802,8 @@
             }
         }
 
-        if( job->acodec & HB_ACODEC_AC3 )
+        if( job->acodec & HB_ACODEC_AC3 ||
+            job->audio_mixdowns[i] == HB_AMIXDOWN_AC3 )
         {
             buf        = hb_fifo_get( audio->fifo_raw );
             buf->start = start;
@@ -915,7 +919,8 @@
     job    = pv->job;
     sync   = &pv->sync_audio[i];
 
-    if( job->acodec & HB_ACODEC_AC3 )
+    if( job->acodec & HB_ACODEC_AC3 ||
+        job->audio_mixdowns[i] == HB_AMIXDOWN_AC3 )
     {
         buf        = hb_buffer_init( sync->ac3_size );
         buf->start = sync->count_frames * 90000 / sync->audio->rate;
Index: libhb/work.c
===================================================================
--- libhb/work.c	(revision 1265)
+++ libhb/work.c	(working copy)
@@ -351,7 +351,8 @@
     if ( job->dynamic_range_compression )
         hb_log(" + dynamic range compression: %f", job->dynamic_range_compression);
 
-    /* if we are doing AC3 passthru, then remove any non-AC3 audios from the job */
+    /* if we are doing AC3 passthru (at the codec level, not pass-through), 
+     * then remove any non-AC3 audios from the job */
     /* otherwise, Bad Things will happen */
     for( i = 0; i < hb_list_count( title->list_audio ); )
     {
@@ -389,6 +390,8 @@
         int audioCodecsSupport6Ch =  ((audio->codec == HB_ACODEC_AC3 ||
             audio->codec == HB_ACODEC_DCA) && (job->acodec == HB_ACODEC_FAAC || job->acodec == HB_ACODEC_VORBIS));
 
+        if( job->audio_mixdowns[i] != HB_AMIXDOWN_AC3 )
+        {
         /* find out what the format of our source audio is */
         switch (audio->input_channel_layout & HB_INPUT_CH_LAYOUT_DISCRETE_NO_LFE_MASK) {
         
@@ -481,6 +484,7 @@
                 }
 
         }
+        }
 
 		/* log the output mixdown */
 		for (j = 0; j < hb_audio_mixdowns_count; j++) {
@@ -496,12 +500,16 @@
 
         audio->config.vorbis.language = audio->lang_simple;
 
-		/* set up the audio work structures */
+        /* set up the audio work structures */
         audio->fifo_in   = hb_fifo_init( 2048 );
         audio->fifo_raw  = hb_fifo_init( FIFO_CPU_MULT * cpu_count );
         audio->fifo_sync = hb_fifo_init( FIFO_CPU_MULT * cpu_count );
         audio->fifo_out  = hb_fifo_init( FIFO_CPU_MULT * cpu_count );
 
+
+        /*
+         * Audio Decoder Thread
+         */
         switch( audio->codec )
         {
             case HB_ACODEC_AC3:
@@ -529,8 +537,13 @@
         
         hb_list_add( job->list_work, audio_w );
 
-        switch( job->acodec )
+        /*
+         * Audio Encoder Thread
+         */
+        if( job->audio_mixdowns[i] != HB_AMIXDOWN_AC3 )
         {
+            switch( job->acodec )
+            {
             case HB_ACODEC_FAAC:
                 w = getWork( WORK_ENCFAAC );
                 break;
@@ -540,10 +553,15 @@
             case HB_ACODEC_VORBIS:
                 w = getWork( WORK_ENCVORBIS );
                 break;
+            }
         }
 
-        if( job->acodec != HB_ACODEC_AC3 )
+        if( job->acodec != HB_ACODEC_AC3 &&
+            job->audio_mixdowns[i] != HB_AMIXDOWN_AC3)
         {
+            /*
+             * Add the encoder thread if not doing AC-3 pass through
+             */
             w->fifo_in       = audio->fifo_sync;
             w->fifo_out      = audio->fifo_out;
             w->config        = &audio->config;
Index: libhb/common.c
===================================================================
--- libhb/common.c	(revision 1265)
+++ libhb/common.c	(working copy)
@@ -43,7 +43,9 @@
   { "Stereo",             "HB_AMIXDOWN_STEREO",    "stereo", HB_AMIXDOWN_STEREO    },
   { "Dolby Surround",     "HB_AMIXDOWN_DOLBY",     "dpl1",   HB_AMIXDOWN_DOLBY     },
   { "Dolby Pro Logic II", "HB_AMIXDOWN_DOLBYPLII", "dpl2",   HB_AMIXDOWN_DOLBYPLII },
-  { "6-channel discrete", "HB_AMIXDOWN_6CH",       "6ch",    HB_AMIXDOWN_6CH       } };
+  { "6-channel discrete", "HB_AMIXDOWN_6CH",       "6ch",    HB_AMIXDOWN_6CH       },
+  { "AC-3 Pass-through",  "HB_AMIXDOWN_AC3",       "ac-3",   HB_AMIXDOWN_AC3       },
+};
 int hb_audio_mixdowns_count = sizeof( hb_audio_mixdowns ) /
                               sizeof( hb_mixdown_t );
 
@@ -224,13 +226,22 @@
     {
         /* Audio data */
         int abitrate;
-        if( job->acodec & HB_ACODEC_AC3 )
+        if( job->acodec & HB_ACODEC_AC3 ||
+            job->audio_mixdowns[i] == HB_AMIXDOWN_AC3)
         {
+            /*
+             * For AC-3 we take the bitrate from the input audio
+             * bitrate as we are simply passing it through.
+             */
             audio = hb_list_item( title->list_audio, job->audios[i] );
             abitrate = audio->bitrate / 8;
         }
         else
         {
+            /*
+             * Where we are transcoding the audio we use the destination
+             * bitrate.
+             */
             abitrate = job->abitrate * 1000 / 8;
         }
         avail -= length * abitrate;
Index: libhb/common.h
===================================================================
--- libhb/common.h	(revision 1265)
+++ libhb/common.h	(working copy)
@@ -211,6 +211,8 @@
 // A52_FORMAT of A52_3F2R | A52_LFE        = 23   = 0x17
 // discrete channel count of 6
 
+#define HB_AMIXDOWN_AC3                         0x20000000
+
 /* define some macros to extract the various information from the HB_AMIXDOWN_XXXX values */
 #define HB_AMIXDOWN_GET_DCA_FORMAT( a ) ( ( a & HB_AMIXDOWN_DCA_FORMAT_MASK ) >> 12 )
 #define HB_AMIXDOWN_GET_A52_FORMAT( a ) ( ( a & HB_AMIXDOWN_A52_FORMAT_MASK ) >> 4 )
Index: macosx/Controller.mm
===================================================================
--- macosx/Controller.mm	(revision 1266)
+++ macosx/Controller.mm	(working copy)
@@ -2922,6 +2922,16 @@
                     maxMixdownUsed = MAX(maxMixdownUsed, hb_audio_mixdowns[4].amixdown);
                 }
 
+                /* do we want to add an AC-3 passthrough option? */
+                if (audio->codec == HB_ACODEC_AC3) {
+                    NSMenuItem *menuItem = [[mixdownPopUp menu] addItemWithTitle:
+                        [NSString stringWithCString: hb_audio_mixdowns[5].human_readable_name]
+                        action: NULL keyEquivalent: @""];
+                    [menuItem setTag: hb_audio_mixdowns[5].amixdown];
+                    if (minMixdownUsed == 0) minMixdownUsed = hb_audio_mixdowns[5].amixdown;
+                    maxMixdownUsed = MAX(maxMixdownUsed, hb_audio_mixdowns[5].amixdown);
+                }
+
                 /* auto-select the best mixdown based on our saved mixdown preference */
                 
                 /* for now, this is hard-coded to a "best" mixdown of HB_AMIXDOWN_DOLBYPLII */
@@ -2975,7 +2985,10 @@
     {
         case HB_ACODEC_FAAC:
             /* check if we have a 6ch discrete conversion in either audio track */
-            if ([[fAudTrack1MixPopUp selectedItem] tag] == HB_AMIXDOWN_6CH || [[fAudTrack2MixPopUp selectedItem] tag] == HB_AMIXDOWN_6CH)
+            if ([[fAudTrack1MixPopUp selectedItem] tag] == HB_AMIXDOWN_6CH || 
+                [[fAudTrack2MixPopUp selectedItem] tag] == HB_AMIXDOWN_6CH || 
+                [[fAudTrack1MixPopUp selectedItem] tag] == HB_AMIXDOWN_AC3 || 
+                [[fAudTrack2MixPopUp selectedItem] tag] == HB_AMIXDOWN_AC3)
             {
                 /* FAAC is happy using our min bitrate of 32 kbps, even for 6ch */
                 minbitrate = 32;
@@ -3041,7 +3054,10 @@
     }
 
     /* select the default bitrate (but use 384 for 6-ch AAC) */
-    if ([[fAudTrack1MixPopUp selectedItem] tag] == HB_AMIXDOWN_6CH || [[fAudTrack2MixPopUp selectedItem] tag] == HB_AMIXDOWN_6CH)
+    if ([[fAudTrack1MixPopUp selectedItem] tag] == HB_AMIXDOWN_6CH || 
+        [[fAudTrack2MixPopUp selectedItem] tag] == HB_AMIXDOWN_6CH ||
+        [[fAudTrack1MixPopUp selectedItem] tag] == HB_AMIXDOWN_AC3 || 
+        [[fAudTrack2MixPopUp selectedItem] tag] == HB_AMIXDOWN_AC3)
     {
         [fAudBitratePopUp selectItemWithTag: 384];
     }