Basically, the amp has 3 sections. Pre-amp (input section), Output section and the Power Supply. Only the Billet allows you to turn-off/bypass the pre-amp section meaning that the signal goes directly to the output section for the purest amplification.
A car audio amplifier is like an integrated amp in home audio, where there's a pre and power section built in one chassis except the home ones have input selectors to allow multiple sources. Some high-end home audiophiles/purist will upgrade to a dedicated/separate pre and power amplifier or even a CD transport and DAC (just like alpine's new F1).
The Billet amp allows you to make your amplifier a pure amplifier without any ajinamoto added. All pre-amp, crossovers, bass-boost etc. is bypassed. However, this makes your amplifier gain a unity gain (see below in red). The input sensitivity of the Billet with pre-amp on is 400mV-9.5V (most amps only require a minimum of 200mV, in expense of sound quality). With the pre-amp off, the input sensitivity requires between 1V-13.5V. Thus, it is recommended to have at least 5v pre-out from your source/head unit. You can even max out the gain knobs with zero distortion and noise floor.
In electronics, gain is usually taken as the mean ratio of the signal output of a system to the signal input of the system. A gain of five would imply that either the voltage or power is increased by a factor of five. It has wide application in amplifiers.
A gain of factor 1 or (equivalent to 0 dB) where both input and output are at the same voltage level is also known as unity gain.
If you come over to my office, I've a powerpoint presentation that I can present to you lor.
