Special amplifier circuit which has zero output unless the input level exceeds a chosen threshold level.
"I just wanted to let you know that I received the restoration of my Doc Severinson album, "Tempestuous Trumpet", and I am thrilled with the quality. What a great job you did!!! This album was from the 1960s. I haven't listened to it in years because I no longer have a record player, and it was never released on CD. I can't stop playing it. It ..."