From the introduction: The circuit presented here was developed to break so called ground loops which cause unwanted hum and other noise to be injected into the audio line between two audio appliances where on each the signal ground has been connected to Earth. Usually rather expensive specially manufactured audio transformers are employed to prevent such ground loops. The optically isolating circuit presented in the following uses only readily available components for the same purpose. The actual optical coupling component IL 300 manufactured by Vishay which is available from many components distributors electrically isolates the connection between two audio appliances. More details in the enclosed zipped pdf file.