I agree that they eliminate noise (see previous post), but they also give more headroom...thus the gain settings...setting the amp's gain at 1.8V with a 5V pre-out would waste most of the amp's available power...
KnuKoncepts explained this very well in a thread a couple weeks ago...pretty much the amp will be running at full power when the input voltage and gain settings match...if they are different, your amp is gonna be off kilter..(Gawd, I love the phrase "off-kilter") :p [/QB]
Actually, you've got it reversed.
As long as the HU can produce at least the minimum input of the amp, the amp will be able to produce its maximum output.
For example, let's say your HU only puts out 2V and your amp's input range is 1-4V. Therefore, you set the gain on your amp to 2V and your amp will produce its full potential.
If you set your amp's gain to 1V, like Dave said, you'd be driving it into clipping and risk damaging your equipment.
The gain setting on the amp is ONLY to match the voltage between the HU and the amp. I believe you're referring to the post where Knu was explaining how to set the gains by ear without knowing the actual voltages.
So there! :p
