diff --git a/source/6522.cpp b/source/6522.cpp
index 3215fca9..16f7cc8c 100644
--- a/source/6522.cpp
+++ b/source/6522.cpp
@@ -110,7 +110,16 @@ USHORT SY6522::SetTimerSyncEvent(BYTE reg, USHORT timerLatch)
 	if (syncEvent->m_active)
 		g_SynchronousEventMgr.Remove(syncEvent->m_id);
 
-	syncEvent->SetCycles(timerLatch + kExtraTimerCycles + opcodeCycleAdjust);
+	if (m_isMegaAudio)
+	{
+		if (reg == rT1CH && timerLatch == 0x0000)
+			timerLatch = 0xFFFF;	// MegaAudio && T1.LATCH=0: use 0xFFFF (or maybe 0x10000?)
+		syncEvent->SetCycles(timerLatch + kExtraMegaAudioTimerCycles + opcodeCycleAdjust);	// MegaAudio asserts IRQ 1 cycle late!
+	}
+	else
+	{
+		syncEvent->SetCycles(timerLatch + kExtraTimerCycles + opcodeCycleAdjust);
+	}
 	g_SynchronousEventMgr.Insert(syncEvent);
 
 	// It doesn't matter if this overflows (ie. >0xFFFF), since on completion of current opcode it'll be corrected
@@ -260,10 +269,21 @@ bool SY6522::CheckTimerUnderflow(USHORT& counter, int& timerIrqDelay, const USHO
 
 	if (oldTimer >= 0 && timer < 0)	// Underflow occurs for 0x0000 -> 0xFFFF
 	{
-		if (timer <= -2)				// TIMER = 0xFFFE (or less)
-			timerIrq = true;
-		else							// TIMER = 0xFFFF
-			timerIrqDelay = 1;			// ...so 1 cycle until IRQ
+		if (m_isMegaAudio)
+		{
+			if (timer <= -3)				// TIMER = 0xFFFD (or less)
+				timerIrq = true;
+			else							// TIMER = 0xFFFF or 0xFFFE
+				timerIrqDelay = 1;			// ...so 1 or 2 cycles until IRQ
+		}
+		else
+		{
+			if (timer <= -2)				// TIMER = 0xFFFE (or less)
+				timerIrq = true;
+			else							// TIMER = 0xFFFF
+				timerIrqDelay = 1;			// ...so 1 cycle until IRQ
+		}
+
 	}
 
 	return timerIrq;
@@ -272,10 +292,19 @@ bool SY6522::CheckTimerUnderflow(USHORT& counter, int& timerIrqDelay, const USHO
 int SY6522::OnTimer1Underflow(USHORT& counter)
 {
 	int timer = (int)(short)(counter);
-	while (timer < -1)
-		timer += (m_regs.TIMER1_LATCH.w + kExtraTimerCycles);	// GH#651: account for underflowed cycles / GH#652: account for extra 2 cycles
+	if (m_isMegaAudio)
+	{
+		const UINT timerLatch = m_regs.TIMER1_LATCH.w ? m_regs.TIMER1_LATCH.w : 0xFFFF;	// MegaAudio && T1.LATCH=0: use 0xFFFF (or maybe 0x10000?)
+		while (timer < -2)
+			timer += (timerLatch + kExtraMegaAudioTimerCycles);		// MegaAudio asserts IRQ 1 cycle late!
+	}
+	else
+	{
+		while (timer < -1)
+			timer += (m_regs.TIMER1_LATCH.w + kExtraTimerCycles);	// GH#651: account for underflowed cycles / GH#652: account for extra 2 cycles
+	}
 	counter = (USHORT)timer;
-	return (timer == -1) ? 1 : 0;		// timer1IrqDelay
+	return (timer < 0) ? 1 : 0;			// timer1IrqDelay
 }
 
 //-----------------------------------------------------------------------------
@@ -349,9 +378,11 @@ BYTE SY6522::Read(BYTE nReg)
 	case 0x08:	// TIMER2L
 		nValue = GetTimer2Counter(nReg) & 0xff;
 		UpdateIFR(IxR_TIMER2);
+		if (m_isMegaAudio) nValue = 0xFF;	// MegaAudio: Timer2 just reads as $00FF
 		break;
 	case 0x09:	// TIMER2H
 		nValue = GetTimer2Counter(nReg) >> 8;
+		if (m_isMegaAudio) nValue = 0x00;	// MegaAudio: Timer2 just reads as $00FF
 		break;
 	case 0x0a:	// SERIAL_SHIFT
 		break;
@@ -370,6 +401,8 @@ BYTE SY6522::Read(BYTE nReg)
 		break;
 	case 0x0e:	// IER
 		nValue = 0x80 | m_regs.IER;	// GH#567
+		if (m_isMegaAudio)
+			nValue &= 0x7F;
 		break;
 	case 0x0f:	// ORA_NO_HS
 		nValue = m_regs.ORA;
diff --git a/source/6522.h b/source/6522.h
index 52a4c23b..fae30944 100644
--- a/source/6522.h
+++ b/source/6522.h
@@ -3,7 +3,7 @@
 class SY6522
 {
 public:
-	SY6522(UINT slot) : m_slot(slot)
+	SY6522(UINT slot, bool isMegaAudio) : m_slot(slot), m_isMegaAudio(isMegaAudio)
 	{
 		for (UINT i = 0; i < kNumTimersPer6522; i++)
 			m_syncEvent[i] = NULL;
@@ -143,4 +143,7 @@ private:
 
 	class SyncEvent* m_syncEvent[kNumTimersPer6522];
 	UINT m_slot;
+	bool m_isMegaAudio;
+
+	static const UINT kExtraMegaAudioTimerCycles = kExtraTimerCycles + 1;
 };
diff --git a/source/Card.cpp b/source/Card.cpp
index 96e104b9..4dab1192 100644
--- a/source/Card.cpp
+++ b/source/Card.cpp
@@ -159,6 +159,8 @@ std::string Card::GetCardName(const SS_CARDTYPE cardType)
 		return VidHDCard::GetSnapshotCardName();
 	case CT_Uthernet2:
 		return Uthernet2::GetSnapshotCardName();
+	case CT_MegaAudio:
+		return MockingboardCard::GetSnapshotCardNameMegaAudio();
 	default:
 		return "Unknown";
 	}
@@ -230,6 +232,10 @@ SS_CARDTYPE Card::GetCardType(const std::string & card)
 	{
 		return CT_Uthernet2;
 	}
+	else if (card == MockingboardCard::GetSnapshotCardNameMegaAudio())
+	{
+		return CT_MegaAudio;
+	}
 	else
 	{
 		throw std::runtime_error("Slots: Unknown card: " + card);	// todo: don't throw - just ignore & continue
diff --git a/source/Card.h b/source/Card.h
index e42b7794..d64320c9 100644
--- a/source/Card.h
+++ b/source/Card.h
@@ -25,6 +25,7 @@ enum SS_CARDTYPE
 	CT_SNESMAX,			// 2 port Nintendo NES/SNES controller serial interface card
 	CT_VidHD,
 	CT_Uthernet2,
+	CT_MegaAudio,		// Soundcard
 };
 
 enum SLOTS { SLOT0=0, SLOT1, SLOT2, SLOT3, SLOT4, SLOT5, SLOT6, SLOT7, NUM_SLOTS, SLOT_AUX, GAME_IO_CONNECTOR };
diff --git a/source/CardManager.cpp b/source/CardManager.cpp
index 27533791..f4af3e47 100644
--- a/source/CardManager.cpp
+++ b/source/CardManager.cpp
@@ -66,6 +66,7 @@ void CardManager::InsertInternal(UINT slot, SS_CARDTYPE type)
 		m_slot[slot] = m_pSSC = new CSuperSerialCard(slot);
 		break;
 	case CT_MockingboardC:
+	case CT_MegaAudio:
 		m_slot[slot] = new MockingboardCard(slot, type);
 		break;
 	case CT_GenericPrinter:
diff --git a/source/CmdLine.cpp b/source/CmdLine.cpp
index 2665c190..9a7f3d0d 100644
--- a/source/CmdLine.cpp
+++ b/source/CmdLine.cpp
@@ -175,6 +175,8 @@ bool ProcessCmdLine(LPSTR lpCmdLine)
 				}
 				if (strcmp(lpCmdLine, "hdc") == 0)
 					g_cmdLine.slotInsert[slot] = CT_GenericHDD;
+				if (strcmp(lpCmdLine, "megaaudio") == 0)
+					g_cmdLine.slotInsert[slot] = CT_MegaAudio;
 				if (strcmp(lpCmdLine, "parallel") == 0)
 				{
 					if (slot == SLOT1)
diff --git a/source/Configuration/PageSound.cpp b/source/Configuration/PageSound.cpp
index 3838c052..46f41b6c 100644
--- a/source/Configuration/PageSound.cpp
+++ b/source/Configuration/PageSound.cpp
@@ -44,6 +44,16 @@ const char CPageSound::m_soundCardChoices[] =	"Mockingboard\0"
 												"SAM\0"
 												"Empty\0";
 
+// Don't reveal MegaAudio card unless it's been specified from the command line.
+// The reasons being are that:
+// . this card is purely for regression testing against mb-audit
+// . it's confusing to offer this to the end user
+const char CPageSound::m_soundCardChoicesEx[] =	"Mockingboard\0"
+												"Phasor\0"
+												"SAM\0"
+												"Empty\0"
+												"MEGA Audio\0";
+
 const char CPageSound::m_soundCardChoice_Unavailable[] = "Unavailable\0\0";	// doubly-null terminate
 
 INT_PTR CALLBACK CPageSound::DlgProc(HWND hWnd, UINT message, WPARAM wparam, LPARAM lparam)
@@ -105,6 +115,7 @@ INT_PTR CPageSound::DlgProcInternal(HWND hWnd, UINT message, WPARAM wparam, LPAR
 				case SC_PHASOR: newCard = CT_Phasor; break;
 				case SC_SAM: newCard = CT_SAM; break;
 				case SC_EMPTY: newCard = CT_Empty; break;
+				case SC_MEGAAUDIO: newCard = CT_MegaAudio; break;
 				default: _ASSERT(0); break;
 				}
 
@@ -166,28 +177,44 @@ CPageSound::SOUNDCARDCHOICE CPageSound::CardTypeToComboItem(SS_CARDTYPE card)
 	case CT_Phasor: return SC_PHASOR;
 	case CT_SAM: return SC_SAM;
 	case CT_Empty: return SC_EMPTY;
+	case CT_MegaAudio: return SC_MEGAAUDIO;
 	default: _ASSERT(0); return SC_EMPTY;
 	}
 }
 
 void CPageSound::InitOptions(HWND hWnd)
 {
-	const SS_CARDTYPE slot4 = m_PropertySheetHelper.GetConfigNew().m_Slot[4];
-	const SS_CARDTYPE slot5 = m_PropertySheetHelper.GetConfigNew().m_Slot[5];
+	const SS_CARDTYPE slot4 = m_PropertySheetHelper.GetConfigNew().m_Slot[SLOT4];
+	const SS_CARDTYPE slot5 = m_PropertySheetHelper.GetConfigNew().m_Slot[SLOT5];
 
 	bool isSlot4SoundCard = slot4 == CT_MockingboardC || slot4 == CT_Phasor || slot4 == CT_SAM || slot4 == CT_Empty;
 	bool isSlot5SoundCard = slot5 == CT_MockingboardC || slot5 == CT_Phasor || slot5 == CT_SAM || slot5 == CT_Empty;
 
+	bool isSlot4SoundCardEx = slot4 == CT_MegaAudio;
+	bool isSlot5SoundCardEx = slot5 == CT_MegaAudio;
+
+	if (isSlot4SoundCardEx || isSlot5SoundCardEx)
+	{
+		isSlot4SoundCardEx = isSlot5SoundCardEx = true;	// if MegaAudio is visible in either, then make it available in both menus
+		isSlot4SoundCard = isSlot5SoundCard = false;
+	}
+
 	if (isSlot4SoundCard)
 		m_PropertySheetHelper.FillComboBox(hWnd, IDC_SOUNDCARD_SLOT4, m_soundCardChoices, (int)CardTypeToComboItem(slot4));
+	else if (isSlot4SoundCardEx)
+		m_PropertySheetHelper.FillComboBox(hWnd, IDC_SOUNDCARD_SLOT4, m_soundCardChoicesEx, (int)CardTypeToComboItem(slot4));
 	else
 		m_PropertySheetHelper.FillComboBox(hWnd, IDC_SOUNDCARD_SLOT4, m_soundCardChoice_Unavailable, 0);
 
 	if (isSlot5SoundCard)
 		m_PropertySheetHelper.FillComboBox(hWnd, IDC_SOUNDCARD_SLOT5, m_soundCardChoices, (int)CardTypeToComboItem(slot5));
+	else if (isSlot5SoundCardEx)
+		m_PropertySheetHelper.FillComboBox(hWnd, IDC_SOUNDCARD_SLOT5, m_soundCardChoicesEx, (int)CardTypeToComboItem(slot5));
 	else
 		m_PropertySheetHelper.FillComboBox(hWnd, IDC_SOUNDCARD_SLOT5, m_soundCardChoice_Unavailable, 0);
 
-	bool enableMBVolume = slot4 == CT_MockingboardC || slot5 == CT_MockingboardC || slot4 == CT_Phasor || slot5 == CT_Phasor;
+	bool enableMBVolume = slot4 == CT_MockingboardC || slot5 == CT_MockingboardC
+						|| slot4 == CT_Phasor || slot5 == CT_Phasor
+						|| slot4 == CT_MegaAudio || slot5 == CT_MegaAudio;
 	EnableWindow(GetDlgItem(hWnd, IDC_MB_VOLUME), enableMBVolume ? TRUE : FALSE);
 }
diff --git a/source/Configuration/PageSound.h b/source/Configuration/PageSound.h
index 80d6fb82..53a8bc5c 100644
--- a/source/Configuration/PageSound.h
+++ b/source/Configuration/PageSound.h
@@ -28,7 +28,7 @@ protected:
 	virtual void DlgCANCEL(HWND hWnd){}
 
 private:
-	enum SOUNDCARDCHOICE { SC_MOCKINGBOARD = 0, SC_PHASOR, SC_SAM, SC_EMPTY, _SOUNDCARD_MAX_CHOICES, SC_UNAVAILABLE };
+	enum SOUNDCARDCHOICE { SC_MOCKINGBOARD = 0, SC_PHASOR, SC_SAM, SC_EMPTY, SC_MEGAAUDIO };
 
 	void InitOptions(HWND hWnd);
 	SOUNDCARDCHOICE CardTypeToComboItem(SS_CARDTYPE card);
@@ -42,5 +42,6 @@ private:
 	static const UINT VOLUME_MAX = 59;
 	static const TCHAR m_soundchoices[];
 	static const char m_soundCardChoices[];
+	static const char m_soundCardChoicesEx[];
 	static const char m_soundCardChoice_Unavailable[];
 };
diff --git a/source/Mockingboard.cpp b/source/Mockingboard.cpp
index b94a865e..33a1c022 100644
--- a/source/Mockingboard.cpp
+++ b/source/Mockingboard.cpp
@@ -62,7 +62,7 @@ Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 
 //---------------------------------------------------------------------------
 
-MockingboardCard::MockingboardCard(UINT slot, SS_CARDTYPE type) : Card(type, slot), m_MBSubUnit{slot, slot}
+MockingboardCard::MockingboardCard(UINT slot, SS_CARDTYPE type) : Card(type, slot), m_MBSubUnit{ {slot, type}, {slot, type} }
 {
 	m_lastCumulativeCycle = 0;
 	m_lastAYUpdateCycle = 0;
@@ -206,7 +206,8 @@ void MockingboardCard::WriteToORB(BYTE subunit)
 {
 	BYTE value = m_MBSubUnit[subunit].sy6522.Read(SY6522::rORB);
 
-	if (subunit == 0 && // SC01 only at $Cn00 (not $Cn80)
+	if ((QueryType() == CT_MockingboardC || QueryType() == CT_Phasor) &&	// Not CT_MegaAudio
+		subunit == 0 && // SC01 only at $Cn00 (not $Cn80)
 		m_MBSubUnit[subunit].sy6522.Read(SY6522::rPCR) == 0xB0)
 	{
 		// Votrax speech data
@@ -289,10 +290,17 @@ void MockingboardCard::AY8910_Write(BYTE subunit, BYTE ay, BYTE value)
 					break;
 
 				case AY_READ:		// 5: READ FROM PSG (need to set DDRA to input)
-					if (pMB->isChipSelected[ay] && pMB->isAYLatchedAddressValid[ay])
-						r6522.SetRegORA(AYReadReg(subunit, ay, pMB->nAYCurrentRegister[ay]) & (r6522.GetReg(SY6522::rDDRA) ^ 0xff));
+					if (QueryType() != CT_MegaAudio)
+					{
+						if (pMB->isChipSelected[ay] && pMB->isAYLatchedAddressValid[ay])
+							r6522.SetRegORA(AYReadReg(subunit, ay, pMB->nAYCurrentRegister[ay]) & (r6522.GetReg(SY6522::rDDRA) ^ 0xff));
+						else
+							r6522.UpdatePortAForHiZ();
+					}
 					else
-						r6522.UpdatePortAForHiZ();
+					{
+						r6522.SetRegORA(0x00);		// Reads not supported.
+					}
 
 					if (m_phasorEnable && m_phasorMode == PH_Phasor)	// GH#1192
 					{
@@ -748,10 +756,13 @@ BYTE MockingboardCard::IOWriteInternal(WORD PC, WORD nAddr, BYTE bWrite, BYTE nV
 		WriteToORB(subunit);
 
 #if !DBG_MB_SS_CARD
-	if (nAddr & 0x40)
-		m_MBSubUnit[1].ssi263.Write(nAddr&0x7, nValue);		// 2nd 6522 is used for 1st speech chip
-	if (nAddr & 0x20)
-		m_MBSubUnit[0].ssi263.Write(nAddr&0x7, nValue);		// 1st 6522 is used for 2nd speech chip
+	if (QueryType() == CT_MockingboardC || QueryType() == CT_Phasor)	// Not CT_MegaAudio
+	{
+		if (nAddr & 0x40)
+			m_MBSubUnit[1].ssi263.Write(nAddr & 0x7, nValue);		// 2nd 6522 is used for 1st speech chip
+		if (nAddr & 0x20)
+			m_MBSubUnit[0].ssi263.Write(nAddr & 0x7, nValue);		// 1st 6522 is used for 2nd speech chip
+	}
 #endif
 
 	return 0;
@@ -821,10 +832,10 @@ BYTE MockingboardCard::PhasorIOInternal(WORD PC, WORD nAddr, BYTE bWrite, BYTE n
 
 void MockingboardCard::InitializeIO(LPBYTE pCxRomPeripheral)
 {
-	if (QueryType() == CT_MockingboardC)
-		RegisterIoHandler(m_slot, IO_Null, IO_Null, IORead, IOWrite, this, NULL);
-	else	// Phasor
+	if (QueryType() == CT_Phasor)
 		RegisterIoHandler(m_slot, PhasorIO, PhasorIO, IORead, IOWrite, this, NULL);
+	else	// All other Mockingboard variants
+		RegisterIoHandler(m_slot, IO_Null, IO_Null, IORead, IOWrite, this, NULL);
 
 	if (g_bDisableDirectSound || g_bDisableDirectSoundMockingboard)
 		return;
@@ -932,6 +943,7 @@ int MockingboardCard::MB_SyncEventCallbackInternal(int id, int /*cycles*/, ULONG
 			pMB->sy6522.StartTimer1();
 			if (pMB->sy6522.IsTimer1IrqDelay())
 				return 0x0001;	// T1C=0xFFFF, which is really -1, as there's 1 cycle until underflow occurs
+								// TODO: can also be 0x0002 for MegaAudio
 			return pMB->sy6522.GetRegT1C() + SY6522::kExtraTimerCycles;
 		}
 
@@ -1146,6 +1158,12 @@ std::string MockingboardCard::GetSnapshotCardNamePhasor(void)
 	return name;
 }
 
+std::string MockingboardCard::GetSnapshotCardNameMegaAudio(void)
+{
+	static const std::string name("MEGA Audio");
+	return name;
+}
+
 void MockingboardCard::SaveSnapshot(YamlSaveHelper& yamlSaveHelper)
 {
 	if (QueryType() == CT_Phasor)
@@ -1153,7 +1171,11 @@ void MockingboardCard::SaveSnapshot(YamlSaveHelper& yamlSaveHelper)
 
 	//
 
-	YamlSaveHelper::Slot slot(yamlSaveHelper, GetSnapshotCardName(), m_slot, kUNIT_VERSION);
+	std::string cardName = GetSnapshotCardName();
+	if (QueryType() == CT_MegaAudio)
+		cardName = GetSnapshotCardNameMegaAudio();
+
+	YamlSaveHelper::Slot slot(yamlSaveHelper, cardName, m_slot, kUNIT_VERSION);
 
 	YamlSaveHelper::Label state(yamlSaveHelper, "%s:\n", SS_YAML_KEY_STATE);
 
diff --git a/source/Mockingboard.h b/source/Mockingboard.h
index 36cb8815..23aa1ca0 100644
--- a/source/Mockingboard.h
+++ b/source/Mockingboard.h
@@ -68,6 +68,7 @@ public:
 
 	static std::string GetSnapshotCardName(void);
 	static std::string GetSnapshotCardNamePhasor(void);
+	static std::string GetSnapshotCardNameMegaAudio(void);
 
 private:
 	enum MockingboardUnitState_e { AY_NOP0, AY_NOP1, AY_INACTIVE, AY_READ, AY_NOP4, AY_NOP5, AY_WRITE, AY_LATCH };
@@ -83,7 +84,7 @@ private:
 		bool isAYLatchedAddressValid[NUM_AY8913_PER_SUBUNIT];
 		bool isChipSelected[NUM_AY8913_PER_SUBUNIT];
 
-		MB_SUBUNIT(UINT slot) : sy6522(slot), ssi263(slot)
+		MB_SUBUNIT(UINT slot, SS_CARDTYPE type) : sy6522(slot, type == CT_MegaAudio), ssi263(slot)
 		{
 			nAY8910Number = 0;
 			// sy6522 & ssi263 have already been default constructed
@@ -95,7 +96,7 @@ private:
 			nAYCurrentRegister[0] = nAYCurrentRegister[1] = 0;	// not valid
 			state[0] = state[1] = AY_INACTIVE;
 			isAYLatchedAddressValid[0] = isAYLatchedAddressValid[1] = false;	// after AY reset
-			isChipSelected[0] = type == CT_MockingboardC ? true : false;
+			isChipSelected[0] = type == CT_Phasor ? false : true;	// Only Phasor is false, all other MB variants are true
 			isChipSelected[1] = false;
 		}
 	};
diff --git a/source/MockingboardCardManager.cpp b/source/MockingboardCardManager.cpp
index e3eeff5a..7c4741a0 100644
--- a/source/MockingboardCardManager.cpp
+++ b/source/MockingboardCardManager.cpp
@@ -42,7 +42,7 @@ Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 bool MockingboardCardManager::IsMockingboard(UINT slot)
 {
 	SS_CARDTYPE type = GetCardMgr().QuerySlot(slot);
-	return type == CT_MockingboardC || type == CT_Phasor;
+	return type == CT_MockingboardC || type == CT_Phasor || type == CT_MegaAudio;
 }
 
 void MockingboardCardManager::ReinitializeClock(void)
diff --git a/source/Windows/AppleWin.cpp b/source/Windows/AppleWin.cpp
index 726a3116..77589c8f 100644
--- a/source/Windows/AppleWin.cpp
+++ b/source/Windows/AppleWin.cpp
@@ -775,6 +775,11 @@ static void RepeatInitialization(void)
 			GetCardMgr().Insert(SLOT3, g_cmdLine.slotInsert[SLOT3]);
 		}
 
+		if (g_cmdLine.slotInsert[SLOT4] != CT_Empty)
+		{
+			GetCardMgr().Insert(SLOT4, g_cmdLine.slotInsert[SLOT4]);
+		}
+
 		if (g_cmdLine.slotInsert[SLOT5] != CT_Empty)
 		{
 			if (GetCardMgr().QuerySlot(SLOT5) != CT_Disk2)	// Ignore if already got Disk2 in slot 5