An optoisolator device used for channel switching in many modern amplifiers, such as Soldano and Mesa. It is a single package combining an LED and a photoresistor, which changes resistance from very high (essentially an open circuit) to very low (essentially a short circuit) as the current through the LED is turned on and off. It is used as a substitute for relays, to avoid the "clicks" and "pops" that can occur when they are used for channel switching.