diff --git a/libs/ardour/ardour/gain_control.h b/libs/ardour/ardour/gain_control.h
index fa43b5d39f..9a79a8046f 100644
--- a/libs/ardour/ardour/gain_control.h
+++ b/libs/ardour/ardour/gain_control.h
@@ -20,7 +20,10 @@
 #define __ardour_gain_control_h__
 
 #include <string>
+#include <list>
+
 #include <boost/shared_ptr.hpp>
+#include <glibmm/threads.h>
 
 #include "pbd/controllable.h"
 
@@ -51,12 +54,18 @@ class LIBARDOUR_API GainControl : public AutomationControl {
 	double lower_db;
 	double range_db;
 
-	boost::shared_ptr<GainControl> master() const { return _master; }
-	void set_master (boost::shared_ptr<GainControl>);
+	void add_master (boost::shared_ptr<GainControl>);
+	void remove_master (boost::shared_ptr<GainControl>);
+	void clear_masters ();
 
   private:
 	void _set_value (double val, PBD::Controllable::GroupControlDisposition group_override);
-	boost::shared_ptr<GainControl> _master;
+	gain_t get_master_gain () const;
+
+	mutable Glib::Threads::Mutex master_lock;
+
+	typedef std::list<boost::shared_ptr<GainControl> > Masters;
+	Masters _masters;
 };
 
 } /* namespace */
diff --git a/libs/ardour/gain_control.cc b/libs/ardour/gain_control.cc
index 5af0e2d397..3021151bdc 100644
--- a/libs/ardour/gain_control.cc
+++ b/libs/ardour/gain_control.cc
@@ -39,10 +39,17 @@ GainControl::GainControl (Session& session, const Evoral::Parameter &param, boos
 double
 GainControl::get_value() const
 {
-	if (!_master) {
-		return AutomationControl::get_value();
+	Glib::Threads::Mutex::Lock sm (master_lock, Glib::Threads::TRY_LOCK);
+
+	if (sm.locked()) {
+		if (_masters.empty()) {
+			return AutomationControl::get_value();
+		}
+		return AutomationControl::get_value() * get_master_gain ();
+	} else {
+		/* could not take lock */
+		return AutomationControl::get_value ();
 	}
-	return AutomationControl::get_value() * _master->get_value();
 }
 
 void
@@ -106,25 +113,31 @@ GainControl::get_user_string () const
 	return std::string(theBuf);
 }
 
-void
-GainControl::set_master (boost::shared_ptr<GainControl> m)
+gain_t
+GainControl::get_master_gain () const
 {
-	double old_master_val;
+	/* Master lock MUST be held */
 
-	if (_master) {
-		old_master_val = _master->get_value();
-	} else {
-		old_master_val = 1.0;
+	gain_t g = 1.0;
+
+	for (Masters::const_iterator m = _masters.begin(); m != _masters.end(); ++m) {
+		g *= (*m)->get_value ();
 	}
 
-	_master = m;
+	return g;
+}
 
-	double new_master_val;
+void
+GainControl::add_master (boost::shared_ptr<GainControl> m)
+{
+	gain_t old_master_val;
+	gain_t new_master_val;
 
-	if (_master) {
-		new_master_val = _master->get_value();
-	} else {
-		new_master_val = 1.0;
+	{
+		Glib::Threads::Mutex::Lock lm (master_lock);
+		old_master_val = get_master_gain ();
+		_masters.push_back (m);
+		new_master_val = get_master_gain ();
 	}
 
 	if (old_master_val != new_master_val) {
@@ -132,3 +145,38 @@ GainControl::set_master (boost::shared_ptr<GainControl> m)
 	}
 }
 
+void
+GainControl::remove_master (boost::shared_ptr<GainControl> m)
+{
+	gain_t old_master_val;
+	gain_t new_master_val;
+
+	{
+		Glib::Threads::Mutex::Lock lm (master_lock);
+		old_master_val = get_master_gain ();
+		_masters.remove (m);
+		new_master_val = get_master_gain ();
+	}
+
+	if (old_master_val != new_master_val) {
+		Changed(); /* EMIT SIGNAL */
+	}
+}
+
+void
+GainControl::clear_masters ()
+{
+	gain_t old_master_val;
+	gain_t new_master_val;
+
+	{
+		Glib::Threads::Mutex::Lock lm (master_lock);
+		old_master_val = get_master_gain ();
+		_masters.clear ();
+		new_master_val = get_master_gain ();
+	}
+
+	if (old_master_val != new_master_val) {
+		Changed(); /* EMIT SIGNAL */
+	}
+}
diff --git a/libs/ardour/vca.cc b/libs/ardour/vca.cc
index 2ba8b8c7c4..b49489dfe7 100644
--- a/libs/ardour/vca.cc
+++ b/libs/ardour/vca.cc
@@ -47,16 +47,11 @@ VCA::get_value() const
 void
 VCA::add (boost::shared_ptr<Route> r)
 {
-	boost::dynamic_pointer_cast<GainControl>(r->gain_control())->set_master (_control);
+	boost::dynamic_pointer_cast<GainControl>(r->gain_control())->add_master (_control);
 }
 
 void
 VCA::remove (boost::shared_ptr<Route> r)
 {
-	boost::shared_ptr<GainControl> route_gain = boost::dynamic_pointer_cast<GainControl>(r->gain_control());
-	boost::shared_ptr<GainControl> current_master = route_gain->master();
-
-	if (current_master == _control) {
-		route_gain->set_master (boost::shared_ptr<GainControl>());
-	}
+	r->gain_control()->remove_master (_control);
 }