Cambridge Audio 851A Amplifier - Knob Feel Review Well, here is a treat! The Cambridge Audio 851A Amplifier. Fantastic weight here, and lovely texture. Also, a first for me… They’ve clearly thought about the axial skew on the Knob, so have dampened it with what feels like rubber. So although it is present, it’s like a boat on an ocean of rotation. First class Knob Feel here! Bravo!