So in all audio, there is "error". Error which is correlated with the signal is called distortion. Error which is not correlated with the signal is called noise

So in all audio, there is "error". Error which is correlated with the signal is called distortion. Error which is not correlated with the signal is called noise. Amplifiers use negative feedback to cancel distortion out ( because it's correlated ). Some folks, like at Pass Labs, tend to avoid neg. feedback for philosophical reasons, and people like their amps. They use part selection and careful design instead. Adding two uncorrelated signals together, you end up with a louder, uncorrelated signal. There is "single-ended noise reduction" ( the version I use is from CoolEdit 96 and CoolEdit 2000 ) but it leaves artifacts. It's still wonderful for things like single-coil hum, things with a limited spectrum.